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GSCE - OCR - Physics Combined Sciences - Pcom1: Matter - Pcom1.1 The Particle Model

<h3>P1-1-1 Structure of an atom</h3>In 1897 J.J. Thompson discovered the electron is much lighter than the atom, and hence realised the atom is not the smallest particle. So he proposed the Plum Pudding model:<figure class="image"><img style="aspect-ratio:400/400;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_1a_Plum_pudding_atom_svg_99b0e23a89.png" alt="P1.1a Plum_pudding_atom.svg.png" width="400" height="400"></figure>Figure 1 Plum Pudding model,&nbsp;<a href="https://commons.wikimedia.org/wiki/File:Plum_pudding_atom.svg">source</a>.&nbsp;The pudding is positive, and the plums in it are negative!&nbsp;JJ Thompson did not think of neutrons!&nbsp;Nuclear Model (Rutherford Model):&nbsp;&nbsp;In 1909 Geiger and Marsden fired some alpha particles towards a thin sheet of gold (alpha particle is made of two protons and two neutrons, so it has a positive charge!).Because they thought material is made of atoms that are like a plum pudding and hence it is soft like a pudding, they expected all of the alpha particles to pass through the metal sheet. But they saw some alpha particles bounce back! So their supervisor, Ernest Rutherford, said there is a positive nucleus at the centre of the atom which repels the positive alpha particles. He said electrons randomly orbit around the nucleus.&nbsp;<img src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_1b_alpha_scattering_experiment_d2f132ec9e.jpg" alt="P1.1b alpha scattering experiment.jpg">Figure 2 alpha scattering experiment (Rutherford), source:&nbsp;<a href="https://commons.wikimedia.org/wiki/File:Rutherford_Scattering.svg">Link</a><figure class="image"><img style="aspect-ratio:272/334;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_1c_Rutherford_Model_3365d00a9c.jpg" alt="P1.1c Rutherford Model.jpg" width="272" height="334"></figure>Figure 3 Rutherford Model - Image source:&nbsp;<a href="https://commons.wikimedia.org/wiki/File:Atomo_de_Rutherford_con_neutrones.png">Link</a>.Bohr model:It was shown if the negative electrons orbit randomly around the nucleus (as Rutherford suggested), they would be attracted and absorbed by the positive nucleus. So in 1913 Bohr said electrons only orbit in specific circles called energy levels!&nbsp;<figure class="image"><img style="aspect-ratio:282/391;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_1d_Bohr_Model_of_Lithium_6bfe3c469c.jpg" alt="P1.1d Bohr_Model_of_Lithium.jpg" width="282" height="391"></figure>Figure 4: Bohr model,&nbsp;<a href="https://commons.wikimedia.org/wiki/File:Bohr_Model_of_Lithium.jpg">source</a>&nbsp;P1.1bLater Rutherford discovered proton in 1920, and neutron by James Chadwick 1932.The Bohr’s model is the accepted theory for the model of an atom today: proton and neutron at the centre forming the nucleus, with electrons orbiting the nucleus. Almost all of the mass of an atom is in its nucleus.&nbsp;In general a theory becomes accepted when it can explain results of different experiments.&nbsp;&nbsp;&nbsp;&nbsp;P1.1cSome numbers to remember:Radius of an atom: 10-10 m.Radius of nucleus: <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACoAAAAhCAIAAAAgZq7PAAAAAXNSR0IArs4c6QAAAAlwSFlzAAASdAAAEnQB3mYfeAAAAQ1JREFUWEftVssNgzAMTTpL20PVCcI+yTC9wCacYAMmQBxIdqFxU34irlKnEkWKT+jJjrHzHD8+DAPbz04RqU2tMq7qiBMYMb0pVKYeZdvE5IZY23yy6VwwWZHDbSCx+tiix/iUntpJoztLvd5Q4+nUA9ItTOSaRkB+3Gcnpunv2MT8HzTxoEck5u94cWnwUvOZVzeCpONgmSq8exVzCMbtxtO5FFJKu0LXws1JOVil8OVZqphDOD5LzY1urOT8P978mMMX+OfBE7eL4+X5emdNpzckxRxC8b+e+6lg07dsqmjRA8whGB81mn7d2EqzAeIIh1APcwjHgXq4bqxgHMCEm4CNYQ6heFq4O775T/BPlEzr6FaUAAAAAElFTkSuQmCC" width="42" height="33"> of radius of atomRadius of smallest molecule is almost the same as an atom, e.g. radius of water molecule: 2.8 x 10-10<h3>P1-1-2 Density</h3>Density: is the mass of unit volume of a substance. <img src="data:image/png;base64,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" width="47" height="31"> unit for density: kg/m3 (or g/cm3)<img src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_1e_Density_2a3b12297c.jpg" alt="P1.1e Density.jpg">If material A is denser than B, it means molecules of A are packed closer together than B.Solids usually are denser than liquids, and liquids are usually denser than gases. Because molecules in a gas are more spread out compared to liquid, and same goes for liquid compared to solid (usually! One exception is water: water is denser than ice!)Description of three states of matter:Solid: has a fixed shape and volume. Particles cannot move from a point to another, they just vibrate in their position. The bond between particles (atoms or molecules) is strong.Liquid: has fixed volume, but not a fixed shape, it takes shape of the container. Intermolecular bonds is less strong than in a solid.Gas: no fixed shape or volume! They have no spine! Particles move randomly and fill up the container. There is almost no intermolecular bond. Particles collide with each other and with walls of the container.&nbsp;<figure class="image"><img style="aspect-ratio:800/190;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_1f_Solid_liquid_gas_376186d612.jpg" alt="P1.1f Solid, liquid, gas.jpg" width="800" height="190"></figure>Figure 5: Solid, liquid, gas; source:&nbsp;<a href="https://commons.wikimedia.org/wiki/File:Solids_liquids_and_gases_-_particle_model.jpg">wikimedia</a>Example 1: (P1.1f):Q1. Density of air is 1.3 kg/m3. Calculate the mass of air in a room with dimensions of 2 m x 3 m x 5 m.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Answer: Volume of the air in the room =<img src="data:image/png;base64,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" width="145" height="24"><img src="data:image/png;base64,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" width="325" height="39">Q2. Discuss how density of the air in tyres of a lorry changes when the vehicle is fully loaded.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Answer: Density increases, because mass remains the same, but volume decreases.&nbsp;<figure class="image"><img style="aspect-ratio:560/232;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_1h_Practice_Q2_84ab24e4dd.jpg" alt="P1.1h Practice Q2.jpg" width="560" height="232"></figure>Source of the icon image: <a href="https://pixabay.com/illustrations/waves-particles-atom-physics-7854066/">Source</a>.

The Particle Model

GSCE - OCR - Physics Combined Sciences - Pcom1: Matter - Pcom1.2 Changes of State

<h3>P1.2-1 Solid, Liquid, Gas</h3>It is the bond between the particles that decides which state of the material we have: solid, liquid or gas.&nbsp;In a solid strong bond keeps particles fixed in their positions. Particles can only vibrate in their position.If the solid receives heat energy, particles vibrate more and move further from each other (expansion). If this continues the bonds, holding the particles in place, break and solid turns into liquid (melting).&nbsp;If liquid receives heat energy same thing happens and particles escape into the atmosphere and material turns into gas (boiling). During all this process, mass remains constant, and molecules (or atoms) do not change (i.e. it is a physical change, not chemical), and they all are reversible.<figure class="image"><img style="aspect-ratio:852/747;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2a_Phase_changes_of_matter_75996843b5.jpg" alt="P1.2a Phase changes of matter.jpg" width="852" height="747"></figure>Figure 6: Phase changes of matter,&nbsp;<a href="https://commons.wikimedia.org/wiki/File:Estados_de_agregaci%C3%B3n_y_sus_cambios.svg">source</a>Difference between vaporisation and evaporation:&nbsp;Evaporation: a liquid turns into gas. This can happen at any temperature (liquid doesn’t need to be boiling)! Only molecules at the surface of the liquid escape to the atmosphere. Evaporation cools the material, because faster molecules evaporate and leave behind the slower ones in the liquid.&nbsp;Vaporisation: means both boiling and sublimation! If a liquid is boiling all of its molecules can escape to atmosphere!&nbsp;A little thinking:Q1. Why does our body cool down when we sweat?Q2. Why is a steam burn worse than being burned by boiling water?Q3. Why wet clothes do not dry if left in a pile, and why we hang them to dry?&nbsp;<h3>P1.2-2 Internal Energy</h3>The heat energy received by the material is spent on two things: 1. vibration of particles (temperature); 2. Breaking the bonds between particles (change of state).<img src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2b_Internal_Energy_5b311a64bf.jpg" alt="P1.2b Internal Energy.jpg">The heat energy spent on temperature is kinetic energy, and the one spent on breaking the bonds is potential energy. The sum of the two is called internal energy of the material.&nbsp;When kinetic energy changes, potential does not; and vice versa! This means when material is changing state its temperature does not change!<figure class="image"><img style="aspect-ratio:980/487;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2c_Temperature_vs_Time_70fa91e5ad.jpg" alt="P1.2c Temperature vs Time.jpg" width="980" height="487"></figure>Figure 7: Temperature vs. time;&nbsp;<a href="http://www.ricksci.com/psc/pscr_phase_change_notes.htm">source</a> (Attempted to contact for authorization, website was not accessible)&nbsp;Question: How does adding ice cubes to a glass water, cools it down?&nbsp;&nbsp;Answer: The faster moving molecules of water, transfer heat energy to the ice. The KE of water decreases and cools down, and the KE of ice increases and it gets warmer. When water and ice reach the same temperature (0 oC) there is no more exchange of thermal energy between them.&nbsp;<h3>P1.2-3 Specific Heat</h3>Specific Heat Capacity: amount of heat energy needed to change the temperature of 1 kg of a material by 1 degree.&nbsp;<figure class="image"><img style="aspect-ratio:880/375;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2d_Specific_Heat_Capacity_f94ab8dcc1.jpg" alt="P1.2d Specific Heat Capacity.jpg" width="880" height="375"></figure>Compare some specific heat capacities:<figure class="table"><table><tbody><tr><td style="border:1.0pt solid windowtext;padding:0in 5.4pt;vertical-align:top;width:149.15pt;">Material</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:197.35pt;">Specific heat capacity J/kgoC</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:149.15pt;">Water</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:197.35pt;">4200</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:149.15pt;">Ice</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:197.35pt;">2100</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:149.15pt;">Aluminium</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:197.35pt;">880</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:149.15pt;">copper</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:197.35pt;">380</td></tr></tbody></table></figure>&nbsp;Little Practice:Q1:&nbsp;252 kJ of energy was transferred to 2 kg of water to raise its temperature from 20 to 50 oC. Find the specific heat capacity of water?<figure class="image"><img style="aspect-ratio:1188/163;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2e_Question1_Ans_8e4ef6051a.jpg" alt="P1.2e Question1 - Ans.jpg" width="1188" height="163"></figure>Specific latent heat (SLH): amount of energy needed to change the state of 1 kg of a material, without any change in temperature. There are two types if it:<ol><li>Specific Latent Heat of fusion: amount of energy to melt 1 kg of a solid (also to freeze a liquid).</li></ol>&nbsp; &nbsp;2. Specific Latent Heat of Vaporisation: Amount of energy to vaporise 1 kg of a liquid (also to condense a gas). &nbsp; &nbsp;SLH of vaporisation is larger than SLH of fusion, because in vaporisation all of the bonds between the particles &nbsp; &nbsp;should be broken and separate them totally.&nbsp;<figure class="image image_resized" style="width:73.04%;"><img style="aspect-ratio:845/342;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2f_Specific_Latent_Heat_000c835f28.jpg" alt="P1.2f Specific Latent Heat.jpg" width="845" height="342"></figure>Little Practice:Q1. An electric heater has a power output of 580 W. It is used to heat ice at temperature of -20 oC to water of temperature 90 oC. If this process takes 13 minutes, calculate the mass of the ice which was initially put in the heater.&nbsp;Specific heat capacity of ice: 2.1 kJ/kg oCSpecific latent heat of fusion of ice: 334 kJ/kgSpecific heat capacity of water: 4.2 kJ/kg oC&nbsp;<figure class="image"><img style="aspect-ratio:1454/1684;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2g_Q1_answer_5f692d2140.jpg" alt="P1.2g Q1 answer.jpg" width="1454" height="1684"></figure>Q2. Figure below shows temperature-time graph for melting paraffin wax.&nbsp;<ol style="list-style-type:lower-alpha;"><li>What is the melting point of paraffin?</li><li>What is happening between 7 and 15 minutes?</li></ol><figure class="image"><img style="aspect-ratio:387/297;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2h_Q2_graph_paraffin_a7b3a36ade.jpg" alt="P1.2h Q2 graph paraffin.jpg" width="387" height="297"></figure>&nbsp;Answers:&nbsp;<ol style="list-style-type:lower-alpha;"><li>Melting point is about 50 oC;</li><li>Between 7 and 15 minutes, the temperature does not change and that means the paraffin is melting.</li></ol>&nbsp;Q3. A cube of brass at 0 oC is put into a container of hot water of temperature 80 oC, until they reach thermal equilibrium at 70.7 oC.&nbsp;The brass cube has a mass of 150 gr and the water in the container has mass of 250 gr. Specific heat capacity of water is 4200 J/kgoC. Calculate the specific heat capacity of the brass. Ignore any loss of thermal energy loss to the surroundings.Answer: thermal equilibrium means both material will reach the same temperature eventually (70.7 oC), and the decrease in energy store of water is equal to increase in energy store of brass.<figure class="image"><img style="aspect-ratio:1163/667;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2i_Q3_answer_b0f24e72e3.jpg" alt="P1.2i Q3 answer.jpg" width="1163" height="667"></figure><h3>P1.2-4 Pressure</h3>Particles in a gas move randomly and freely. The hotter the gas the faster the particles move and gain kinetic energy.&nbsp;Particles in a gas collide with each other and the walls of the container. This applies a force on the wall, which causes the pressure of the gas!&nbsp;<figure class="image"><img style="aspect-ratio:695/205;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P1_2j_Pressure_formula_d27d2a5c7f.jpg" alt="P1.2j Pressure formula.jpg" width="695" height="205"></figure>Imagine a gas in a sealed container being warmed up, the particles collide the walls of the container more forceful and more regularly (because they have more KE!), this causes the gas pressure to go up!&nbsp;If the container is not sealed, the gas escapes (volume and mass are not constant) and pressure does not change! (Or at least would not change by the same amount, if the container was sealed).

Changes of State

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GSCE - OCR - Physics Combined Sciences - Pcom2: Forces - Pcom2.1 Motion

Motion

GSCE - OCR - Physics Combined Sciences - Pcom2: Forces - Pcom2.2 Force and Newton’s Laws

<h3>P2.2-1 Force</h3>Force: is either a pull or a push applied on an object by another object, and has a unit of newtons (N). And there are 2 types: contact forces and non-contact forces.&nbsp;<figure class="table"><table style=";"><tbody><tr><td style="border:1.0pt solid windowtext;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Non-contact forces</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Contact forces</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Weight (force of gravity on a mass)</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Tension in a cable</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Electrostatic force (between two charges)</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Friction</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Magnetic force (between magnets)</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Drag (air resistance)</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">&nbsp;</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Normal reaction (normal contact force)</td></tr></tbody></table></figure>&nbsp;Force diagrams:&nbsp;<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:1961_Cessna_172B.svg"><img style="aspect-ratio:1221/411;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2a_Thrust_and_Drag_and_FBD_on_a_plane_39c59491af.jpg" alt="P2.2a Thrust and Drag and FBD on a plane.jpg" width="1221" height="411"></a></figure><figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Book_stock_image.png"><img style="aspect-ratio:1208/582;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2b_Weigth_and_Reaction_and_FBD_3e4c4ef663.jpg" alt="P2.2b Weigth and Reaction and FBD.jpg" width="1208" height="582"></a></figure>&nbsp;Note: when drawing force diagrams, the size of the arrow shows the magnitude of the force, and direction should be correct too! (Forces are vectors after all!)Free Body Diagram: Just draw the forces and ignore the shape of the object.&nbsp;&nbsp;<h4>P2.2-1-1 Resultant force</h4>Resultant force: or the net force, is a single force that is equivalent to all forces applied on an object.&nbsp;The apple is hanging from the tree. It means it is stationary. It means tension in the stalk is equal to the weight and as it is in opposite direction the resultant force is zero!<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Fall_apple_from_tree3.jpg"><img style="aspect-ratio:1435/550;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2c_forces_on_an_apple_and_FBD_d7daaa6c00.jpg" alt="P2.2c forces on an apple and FBD.jpg" width="1435" height="550"></a></figure>&nbsp;The net force on the car is 500 N to the left&nbsp;à the car is accelerating to the left.<figure class="image"><img style="aspect-ratio:1507/493;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2d_net_force_on_car_and_FBD_26013b757c.jpg" alt="P2.2d net force on car and FBD.jpg" width="1507" height="493"></figure>If the thrust decreases to 1000 N, the resultant force = zero&nbsp;à the car is moving with a constant velocity!&nbsp;&nbsp;P2.2e (Higher Tier)<h4>P2.2-1-2 Scale Diagrams</h4>Find resultant of two perpendicular forces:<ol><li>Draw the free body diagram and choose a scale: e.g. 1 cm = 1 N</li><li>After drawing the first force, start the second one, at the end point of the first one</li><li>Connect beginning of the first to end of the last, this is the resultant force!</li><li>Measure the length (magnitude) of the force with a ruler, and the angle that it makes with the horizontal (Θ), with a protractor.</li></ol><figure class="image"><img style="aspect-ratio:1343/343;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2e_find_the_resultant_43b4441343.jpg" alt="P2.2e find the resultant.jpg" width="1343" height="343"></figure>Resolving a force into two perpendicular components:Now we have a force and want to see what two perpendicular forces would add up to give the first one:<ol><li>Draw the force starting from the origin of x-y axes with a proper scale e.g. 10 N = 1cm;</li><li>From the end point of the vector draw perpendicular lines to x and y axis (as if you were trying to find the x and y coordinates of end point of the vector);</li><li>Draw vectors from the origin to the points on x and y from the previous step (Fx and Fy);</li></ol>Example: find the two perpendicular components of a force of 50 N which makes an angle of 30o to the horizontal:<figure class="image"><img style="aspect-ratio:1462/465;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2f_resolving_a_vector_2393c9bbea.jpg" alt="P2.2f resolving a vector.jpg" width="1462" height="465"></figure><h3>P2.2-2 Newton’s First Law</h3>Newton’s first law: if the resultant force on an object is zero:<ol style="list-style-type:lower-alpha;"><li>The object is moving with constant velocity;</li></ol>or<ol style="list-style-type:lower-alpha;" start="2"><li>It is stationary.&nbsp;</li></ol>Note we say constant velocity, not speed! Constant velocity&nbsp;means constant speed plus no change in direction! As velocity is a vector (refer to&nbsp;P2.1d and P2.1h).&nbsp;Circular motion (Higher Tier):The resultant force on an object moving in a circular path with constant speed, is not zero! There is always a force on this object towards the centre of the circle. Hence the velocity is not constant and the object is accelerating.&nbsp;Terminal velocity (Higher Tier):There are two forces applied on a falling object: weight and drag. Drag depends on the speed, and as the object falls, speed increases so does the drag. There will be a point where drag will get equal to the weight of the object, hence resultant force will be zero and the object will stop accelerating downwards and it will continue with a constant velocity, which we call the terminal velocity!&nbsp;&nbsp;<h3>P2.2-3 Newton’s 2nd Law</h3><figure class="image image_resized" style="width:71.85%;"><img style="aspect-ratio:1137/353;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2i_Newtons_2nd_Law_a0acbb5405.jpg" alt="P2.2i Newtons 2nd Law.jpg" width="1137" height="353"></figure>Which means if the resultant force is not zero, the object accelerates, and the rate of acceleration depends on the mass.<h4>P2.2-3-1 Inertia (Higher Tier)</h4>&nbsp;Inertia: natural reluctance of objects to change velocity.&nbsp;Inertial mass: In our universe, the value of inertial mass is the same as gravitational mass (the mass that you always worked with!). But they are different concepts.&nbsp;Force of gravity between two objects exists, because they have mass! This is the gravitational mass! But inertial mass tells you how reluctant an object is to change velocity.<figure class="image image_resized" style="width:57.92%;"><img style="aspect-ratio:951/323;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2j_Inertial_mass_7787fb2797.jpg" alt="P2.2j Inertial mass.jpg" width="951" height="323"></figure><h4>P2.2-3-2 Momentum&nbsp; (Higher Tier)</h4>Momentum is mass times velocity, and it is a vector.<figure class="image image_resized" style="width:68.29%;"><img style="aspect-ratio:1049/300;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2k1_Monetum_79542ab9f2.jpg" alt="P2.2k1 Monetum.jpg" width="1049" height="300"></figure>Change in momentum:If momentum changes, assuming mass does not change, it means velocity has changed. Change in velocity means acceleration, and that means resultant force on the object is not zero! (Newton’s 2nd Law):<figure class="image"><img style="aspect-ratio:1263/702;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2k2_Change_in_momentum_aa63423d95.jpg" alt="P2.2k2 Change in momentum.jpg" width="1263" height="702"></figure>If we increase time of collision, the force applied on objects involved in the collision decreases! All safety features in car intend to do this!&nbsp;<figure class="image image_resized" style="width:76.39%;"><a href="https://commons.wikimedia.org/wiki/File:Lamborghini_Veneno,_Car_Zero_(profile).jpg"><img style="aspect-ratio:1062/700;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2k3_crumple_zones_of_a_car_3c08e4b624.jpg" alt="P2.2k3 crumple zones of a car.jpg" width="1062" height="700"></a></figure>Crumple zones: increase the time from the first impact and car stopping;Seat belts stretch a little and increase the time the passenger’s momentum change;Airbags both increase the time passengers head stops and protects the head against hitting the steering wheel.Example 1:A car moving at a speed of 12 m/s and mass of 800 kg hits a wall and comes to a stop. If the force of impact on the car is 3200 N, find the time it takes for the car to stop after hitting the wall.<figure class="image image_resized" style="width:78.66%;"><img style="aspect-ratio:1370/756;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2k4_Example_1_9a1cc6d779.jpg" alt="P2.2k4 Example 1.jpg" width="1370" height="756"></figure>Note: Force, velocity, and momentum are all vectors and their negative sign means they are in the reverse direction that we have assumed positive. In the question above we assume positive direction to the right. So u is positive and force applied on the car (F) is negative.&nbsp;Principal of conservation of momentumThe total momentum before a collision is equal to the total momentum after the collision.&nbsp;Example 2:A toy car of mass 2 kg moving at 8 m/s collides with a stationary toy car of mass 6kg. Both objects move together after the collision. Determine their velocity after the collision.<figure class="image"><img style="aspect-ratio:1572/796;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2k5_Example_2_170432cf55.jpg" alt="P2.2k5 Example 2.jpg" width="1572" height="796"></figure>Example 3:&nbsp;A cricket ball of mass 0.15 kg has a velocity of 30 m/s before hitting a bat and leaves the bat with the same speed but in the reverse direction. If the ball is in touch with the bat for 0.3 s, find the force applied on the ball by the bat.&nbsp;<figure class="image"><img style="aspect-ratio:1462/702;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P2_2k6_Example_3_beb6c17cb3.jpg" alt="P2.2k6 Example 3.jpg" width="1462" height="702"></figure>&nbsp;

Force and Newton’s Laws

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GSCE - OCR - Physics Combined Sciences - Pcom3: Electricity and Magnetism - Pcom3.1 Static electricity and Charge

<h3>P3.1-1 Charge</h3>A neutral atom or object, has equal number of protons and electrons.&nbsp;<figure class="image image_resized" style="width:38.92%;"><img style="aspect-ratio:570/286;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_1a_Electric_charges_21ccb12079.jpg" alt="P3.1a Electric charges.jpg" width="570" height="286"></figure>Only electron moves! Proton is fixed in the nucleus.&nbsp;Unit of electric charge is Coulombs (C).&nbsp;&nbsp;Conductors (like most metals) allow electrons to flow through them, while insulators (plastic glass, wood) don’t; they hold on to the charge (static electricity)!If you rub a polythene&nbsp;rod with a cloth, electrons gain energy because of friction, and move from the cloth to the rod (it becomes negatively&nbsp;charged). The cloth is left with positive protons.&nbsp;If you rub an acetate (or Perspex) rod with a cloth, the acetate becomes positive&nbsp;and cloth negative.&nbsp;Like charges repel each other and opposite charges attract.&nbsp;Rubbing a balloon on your hair, will make the balloon negatively charged. Bringing the charged balloon close to wall will cause the electrons on the wall surface to go deep into the wall thickness and protons remain on the surface. So now the balloon sticks to the wall because of the attractive force between them.A boy with long hair touching a Van de Graaff will get charged all over, including every strand of his hair, which now because have like charges repel each other and try to get as far as possible from each other!<figure class="image image_resized" style="width:38.05%;"><a href="https://commons.wikimedia.org/wiki/File:Electro-Static_Generator.jpg"><img style="aspect-ratio:640/640;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_1b_Van_De_Graaff_Electro_Static_Generator_99dee6e093.jpg" alt="P3.1b Van De Graaff Electro-Static_Generator.jpg" width="640" height="640"></a></figure><h3>P3.1-2 Current and PD</h3>Electric current: Flow of negative charge (electrons) in a conductor in one second with unit of ampere (A or amps).<figure class="image image_resized" style="width:73.15%;"><img style="aspect-ratio:1287/398;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_1c_Electric_Current_formula_86248263e9.jpg" alt="P3.1c Electric Current formula.jpg" width="1287" height="398"></figure>Current is measured by an ammeter in a circuit.&nbsp;Ammeter is connected in series (see P3.2a).&nbsp;Ideal ammeter has zero resistance.&nbsp;A Single closed loop circuit:<figure class="image image_resized" style="width:80.82%;"><img style="aspect-ratio:1242/637;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_1d_Single_Loop_circuit_e113b58b15.jpg" alt="P3.1d Single Loop circuit.jpg" width="1242" height="637"></figure>In a single closed loop, the current only has one path to flow, so the value of current is the same at any point in such circuit.Thanks to a historic convention, we call the positive terminal of a cell the high potential and the negative end, the low potential. So the “conventional direction of current” is from positive to negative. BUT! Electrons actually flow from negative to positive! But we do not use this direction (this is called direction of flow of electrons)!For current to flow between two points, there need to be a difference in potential energy between the points (plus a closed loop). Just like water falling from a waterfall! The cell in circuit creates this potential difference (PD or voltage) at its two terminals.&nbsp;<figure class="image image_resized" style="width:65.48%;"><a href="In a single closed loop, the current only has one path to flow, so the value of current is the same at any point in such circuit. Thanks to a historic convention, we call the positive terminal of a cell the high potential and the negative end, the low potential. So the “conventional direction of current” is from positive to negative. BUT! Electrons actually flow from negative to positive! But we do not use this direction (this is called direction of flow of electrons)! For current to flow between two points, there need to be a difference in potential energy between the points (plus a closed loop). Just like water falling from a waterfall! The cell in circuit creates this potential difference (PD or voltage) at its two terminals. "><img style="aspect-ratio:810/722;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_1e_waterfall_d492e43958.jpg" alt="P3.1e waterfall.jpg" width="810" height="722"></a></figure>Potential Difference: the amount of energy placed on each unit of charge (Q = 1 C):<figure class="image image_resized" style="width:83.41%;"><img style="aspect-ratio:1138/450;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_1f_PD_Formula_e0cb6f1cc2.jpg" alt="P3.1f PD Formula.jpg" width="1138" height="450"></figure>PD is measured with a voltmeter.Voltmeter is connected in parallel (see P3.2a).Ideal voltmeter has infinite resistance.&nbsp;When in a cloud small pieces of ice collide with each other, they charge up the cloud; and as the potential difference between the cloud and the earth reaches big enough value (about 35 MV), a huge park (lightning) transfer electrons from the cloud to the earth. Air normally is an insulator, but due to huge charge accumulated on the cloud, molecules in air break down and create negative and positive ions, which can transfer the current from the cloud to the ground. Without the air acting as a conductor here, lightning cannot happen.

Static electricity and Charge

GSCE - OCR - Physics Combined Sciences - Pcom3: Electricity and Magnetism - Pcom3.2 Simple Circuits

Simple Circuits

GSCE - OCR - Physics Combined Sciences - Pcom3: Electricity and Magnetism - Pcom3.3 Magnets and magnetic fields

Pole of a magnet is where magnetic force is stronger. Usually at both ends of a bar magnet.Magnets have two poles: North and SouthLike poles repel and unlike ones attract.The attraction and repulsion can be shown by using two suspended bar magnets.&nbsp;Iron, nickel, cobalt and some steel can be magnetised if left in a magnetic field.&nbsp;<figure class="image"><img style="aspect-ratio:1488/543;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3a_Permanent_and_induced_magents_0780bdaa8f.jpg" alt="P3.3a Permanent and induced magents.jpg" width="1488" height="543"></figure><h3>P3.3-1 Magnetic Field</h3>&nbsp;Magnetic field of magnet is a region around it where the magnet will apply a force on another magnet if it is placed in that region. In fact both magnets apply equal and opposite forces on each other (opposite in direction! – Newton’s 3rd law!).&nbsp;Direction of a magnetic field is the same as direction of the force applied on a single North pole placed in the field; so away from the North and towards the South.&nbsp;To show the magnetic field around a magnet we draw magnetic field lines. These lines never cross or meet. We draw arrows on the lines to show direction of the field: from North to South. The closer the lines, the stronger the field! We call the field strength: magnetic flux density.<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:VFPt_cylindrical_magnets_attracting.svg"><img style="aspect-ratio:800/600;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3b1_magnetic_field_lines_N_S_VF_Pt_cylindrical_magnets_attracting_svg_9c4aef3654.jpg" alt="P3.3b1 magnetic field lines N-S VFPt_cylindrical_magnets_attracting.svg.jpg" width="800" height="600"></a></figure><figure class="image"><a href="https://commons.wikimedia.org/wiki/File:VFPt_cylindrical_magnet.svg"><img style="aspect-ratio:600/600;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3b2_magnectic_field_lines_bar_magent_VF_Pt_cylindrical_magnet_svg_d6e96cf4a1.jpg" alt="P3.3b2 magnectic field lines bar magent VFPt_cylindrical_magnet.svg.jpg" width="600" height="600"></a></figure><figure class="image"><a href="https://commons.wikimedia.org/wiki/File:VFPt_cylindrical_magnets_repelling.svg"><img style="aspect-ratio:800/600;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3b3_magnetic_field_lines_N_N_VF_Pt_cylindrical_magnets_repelling_svg_8c1a21d02d.jpg" alt="P3.3b3 magnetic field lines N-N VFPt_cylindrical_magnets_repelling.svg.jpg" width="800" height="600"></a></figure>&nbsp;Plotting compass is small bar magnet. It can be used show magnetic field of another magnet or the Earth.<figure class="image image_resized" style="width:32.58%;"><a href="https://commons.wikimedia.org/wiki/File:Magnetic_field_near_pole.svg"><img style="aspect-ratio:300/500;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3d1_plotting_compass_showing_magnetic_field_Magnetic_field_near_pole_svg_4c847b3c0a.jpg" alt="P3.3d1 plotting compass showing magnetic field Magnetic_field_near_pole.svg.jpg" width="300" height="500"></a></figure>The magnetic south pole of the Earth is in the northern hemisphere. Because the north pole of a compass is attracted to a magnetic south pole which shows the arctic region; and vice versa for the magnetic north pole!<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:VFPt_Earths_Magnetic_Field_Confusion_overlay.svg"><img style="aspect-ratio:320/300;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3d2_geographic_and_magnetic_poles_of_the_earth_VF_Pt_Earths_Magnetic_Field_Confusion_overlay_svg_d09f33175e.jpg" alt="P3.3d2 geographic and magnetic poles of the earth VFPt_Earths_Magnetic_Field_Confusion_overlay.svg.jpg" width="320" height="300"></a></figure>Magnetic poles of the Earth flip every 200 000 years!&nbsp;<h3>P3.3-2 Current Carrying Wire</h3>When a charged object or particle (like an electron) is stationary, there is only an electric filed around it; but once it moves a magnetic field is created around it too.When there is electric current in a wire, it means electrons are moving in there, so there will be a circular magnetic field around the wire.&nbsp;<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Long-wire-right-hand-rule.svg"><img style="aspect-ratio:632/681;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3e_magnetic_field_current_carrying_wire_e642c96353.jpg" alt="P3.3e magnetic field current carrying wire.jpg" width="632" height="681"></a></figure>&nbsp;Direction of this field is found with right-hand rule: thumb shows the direction of current. Then curl other fingers and they show the direction of the circular field.&nbsp;&nbsp;Why material can be magnetised:&nbsp;When magnetic material are left in a magnetic field, the electrons in the atoms start to spin in the same direction which makes the material magnetised. Before this, the number of electrons spinning in reverse direction is the same and they cancel each other’s magnetic field!&nbsp;&nbsp;The strength of the magnetic field depends on the distance from the wire, and the amount of current in the wire.&nbsp;SolenoidSolenoid is a coil of wire which is spread out, with its length much larger than its diameter.&nbsp;The magnetic field around a solenoid is similar to a bar magnet.&nbsp;By looking at direction of current at both ends of the solenoid we can determine the magnetic poles of the solenoid:Look at the direction of current from both ends of the solenoid:<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Openstax_college-physics_22.40_solenoid-Bfield.jpg"><img style="aspect-ratio:1385/591;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3g_Magnetic_field_Solenoid_cd74d7c210.jpg" alt="P3.3g Magnetic field Solenoid.jpg" width="1385" height="591"></a></figure><h3>P3.3-3 The Motor Effect (Higher Only)</h3>When two objects with magnetic field are placed close to each other they apply an equal and opposite force on each other (Newton’s 3rd law).We can determine the direction of this force when a current carrying wire is placed in the magnetic field by Fleming Left-Hand Rule:<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Fleming-left-hand-rule.png"><img style="aspect-ratio:858/733;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3i_Fleming_Left_Hand_Rule_96f3de3e36.jpg" alt="P3.3i Fleming Left Hand Rule.jpg" width="858" height="733"></a></figure>We can calculate the magnitude of the force on the wire placed at right angles in a magnetic field with this formula:<figure class="image"><img style="aspect-ratio:1157/323;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3j1_F_BIL_force_on_current_carrying_wire_formula_2d15f3ee08.jpg" alt="P3.3j1 F BIL force on current carrying wire formula.jpg" width="1157" height="323"></figure>B shows the strength of the magnetic field (also called magnetic flux density) with unit of tesla (shown by T).&nbsp;Example 1Diagram below shows a wire carrying a current of 0.5 A, is placed in the magnetic field of a permanent magnet. If magnitude of the force applied on the wire is 0.0225 N, and the length of the wire in the magnetic field is 15 cm; determine the: a) magnetic flux density and, b) the direction of the force on the wire.<figure class="image"><img style="aspect-ratio:626/375;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3j2_Q1_Question_4c529bfbb6.jpg" alt="P3.3j2 Q1 Question.jpg" width="626" height="375"></figure>Solution:<figure class="image"><a href="https://sketchfab.com/3d-models/left-hand-rule-58daddcfd4674b5a9d65c851b07f8f3c"><img style="aspect-ratio:1207/847;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3j3_example_1_f758a9066b.jpg" alt="P3.3j3 example 1.jpg" width="1207" height="847"></a></figure>So the wires moves upward (direction that the thuMb shows!)P3.3k Electric Motor Diagram-aElectric motorsElectric motors are used in anything that moves with electricity! E.g. food processor, drill, and Tesla cars!&nbsp;<figure class="image"><img style="aspect-ratio:1178/792;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3k1_Electric_Motor_Diagram_a_5e628284c1.jpg" alt="P3.3k1 Electric Motor Diagram-a.jpg" width="1178" height="792"></figure>Fleming left-hand rule&nbsp;à force on the A-B section of the coil is upwards, and on C-D is downwards.&nbsp;This will turn the coil clockwise. Until A-B comes to where C-D is now:<figure class="image"><img style="aspect-ratio:1092/687;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P3_3k2_Electric_Motor_Diagram_b_25e82316d2.jpg" alt="P3.3k2 Electric Motor Diagram-b.jpg" width="1092" height="687"></figure>If the current remains in A-B as it was before, the coil will turn anticlockwise this time!&nbsp;But the split-ring commutator changes the direction of the current before this happens and the motor continues in a circular motion.There is no force applied on the A-C or B-D part of the coil.&nbsp;The magnitude of the force on the coil in a motor depends on:<ol><li>Strength of the magnetic field;</li><li>Amount of current in the coil;</li><li>Number of turns in the coil</li></ol>&nbsp;The dynamoIts structure is very similar to the motor. But here the coil is rotated by us, and we get direct current from it. A direct current (DC) is produced as the coil cuts through the magnetic field lines and a PD is induced in the two ends of the coil.Each half term the split-ring commutator is connected to the other part of the circuit, so the direction of PD reverses, but the current’s flow is consistent!&nbsp;

Magnets and magnetic fields

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GSCE - OCR - Physics Combined Sciences - Pcom4: Waves and Radioactivity  - Pcom4.1 Wave behaviour

Wave behaviour

GSCE - OCR - Physics Combined Sciences - Pcom4: Waves and Radioactivity  - Pcom4.2 Electromagnetic Spectrum

All waves can be reflected or refracted (change speed when entering a new medium). This how we know EM spectrum are also waves.All EM-waves are transverse. And they all travel with the same speed in vacuum (3x108 m/s), and they do not need a medium to transfer energy.&nbsp;&nbsp;<figure class="image image_resized" style="width:88.59%;"><a href="https://commons.wikimedia.org/wiki/File:EM_Spectrum_Properties_edit.svg"><img style="aspect-ratio:640/379;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_2d_EM_Spectrum_89e65dbff7.jpg" alt="P4.2d EM Spectrum.jpg" width="640" height="379"></a></figure>&nbsp;We should remember the order of the EM waves.We should also remember some of their wavelength:<figure class="table"><table><tbody><tr><td style="border:1.0pt solid windowtext;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">EM Wave</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Wavelength</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Radio, microwave, and TV</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">105 to 10-3</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Infrared</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">10-3 to 10-7</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Visible range (red to violet)</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">10-7</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Ultraviolet (UV)</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">10-7 to 10-8</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">X-ray</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">10-8 to 10-10</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">Gamma ray (γ)</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:261.4pt;">10-10 to 10-12</td></tr></tbody></table></figure>&nbsp;The higher the frequency of the EM wave, the more energy it has (and more dangerous it is).&nbsp;<h3>P4.2-1 Applications of EM waves</h3>Radio waves: TV and radio communications. Transmitter sends out the signal, and receiver receives it! The transmitters are put on top of high towers not to be stopped by large buildings and other obstructions.&nbsp;Higher tier only: radio waves can be produced by oscillations in electrical circuits. In radio transmitters current changes constantly and this changes the electric field produced by the current which in turn produces radio signals.&nbsp;In receivers the radio wave induces oscillations in the electric circuit, which can be converted to sound in a radio for example.Radio waves are refracted in ionosphere, where the frequency determines the amount of refraction.&nbsp;&nbsp;<figure class="image image_resized" style="width:87.4%;"><a href="https://commons.wikimedia.org/wiki/File:Ionosphere-ref.png"><img style="aspect-ratio:907/508;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_2i_ionosphere_e4af16130e.jpg" alt="P4.2i ionosphere.jpg" width="907" height="508"></a></figure>&nbsp;The lower the frequency of radio wave, the higher the refraction.&nbsp;Microwaves have higher frequency than radio waves, and hence are not refracted by ionosphere, so they can be used for satellite communication.&nbsp;&nbsp;Microwave: transmit mobile phone signals, heating food and water. Microwaves used for cooking have higher frequency than mobile phones. Microwave vibrate molecules of water and fat in food and heat it up.&nbsp;&nbsp;Infrared: any warm object emits infrared radiation. Traditionally we cook food by infrared, which heats the surface of food, and then heat is transferred deeper by conduction. TV remote, and thermal imaging cameras also use infrared.&nbsp;Ultraviolet: is emitted from very hot objects (4000 oC and above) like the Sun. UV is used in fluorescent lights, tanning beds, disinfecting theatres in hospitals, security strip in banknotes.Gamma-rays: kill cancer cells, or reduce size of a tumour (radiotherapy), medical imaging with gamma tracer and camera.&nbsp;X-rays: imaging bones, computerised tomography (CT) scans used to image body’s soft tissue, radio therapy for cancer treatment, security in airports.&nbsp;Higher tier: both gamma and x-rays have high frequency and energy, penetrative, and highly ionising, which can cause mutation of DNA and create cancerous cells. But they can be used to kill cancerous cells as well! Gamma-ray is produced from nucleus of radioactive atom, but X-ray is produced by colliding high speed electrons with metals.&nbsp;&nbsp;<h3>P4.2-2 Reflection and refraction</h3>Reflection:&nbsp;When a wave reaches the boundary between two materials, it may be reflected (bounce back), transmitted, or absorbed; and these all may happen at the same time, this depends on two things: wavelength and the medium the wave enters. Like when you look out the window: you see what is outside (light is transmitted), but what you see is not as clear as if you were outside (as some light is absorbed by the glass), and you also can see a little reflection of yourself in the glass too (reflection!).Higher tier:&nbsp;If surface of a reflector is flat and smooth (e.g. mirror), all light rays will be reflected with the same angle that came into the surface. In this case the reflection is called specular and an image can be seen from this reflection. If the surface is rough (e.g. clothes) it causes a diffuse reflection and an image cannot be seen from this.&nbsp;&nbsp;<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Specular_v_Diffuse_reflection.png"><img style="aspect-ratio:746/315;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_2j_Specular_and_Diffuse_reflection_49ffc1e8ed.jpg" alt="P4.2j Specular and Diffuse reflection.jpg" width="746" height="315"></a></figure>&nbsp;Refraction: happens when a wave is transmitted from one medium to another, it changes speed and may change direction. The shorter the wave length, the more refraction happens.&nbsp;<figure class="image image_resized" style="width:65.48%;"><a href="https://commons.wikimedia.org/wiki/File:Refraction_photo.png"><img style="aspect-ratio:800/532;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_2j_Refraction_87ade02823.png" alt="P4.2j Refraction.png" width="800" height="532"></a></figure>&nbsp;Higher tier: A wavefront is a line we draw to show all the points that move together on a wave. The shadows that you see on the screen below the ripple tank are like that!&nbsp;&nbsp;<figure class="image image_resized" style="width:79.74%;"><a href="https://commons.wikimedia.org/wiki/File:Refraction_animation.gif"><img style="aspect-ratio:1007/478;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_2k_wave_front_with_points_AB_70b4376c33.jpg" alt="P4.2k wave front with points AB.jpg" width="1007" height="478"></a></figure>&nbsp;When water wave in the ripple tank goes to a shallower part, it is refracted so it changes speed and hence direction. Wave front hits the boundary at point A first, and this part of the wave slows down first, and point B is the last place to hit the boundary. This is why the wave changes direction (provided that the wave hits the boundary at an angle in the first place!). If the wave hits the boundary at right angles, it slows down but does not change direction!Note: in diffraction wave speed and wavelength change, but not the frequency! (v = λ f).When light slows down it gets closer to the normal. The normal is line we draw perpendicular to the boundary between the two materials (in image below i2 &lt; i1).<figure class="image image_resized" style="width:66.24%;"><a href="https://commons.wikimedia.org/wiki/File:RefractionLoi.svg"><img style="aspect-ratio:776/671;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_2k_the_normal_be445317fe.jpg" alt="P4.2k the normal.jpg" width="776" height="671"></a></figure>&nbsp;

Electromagnetic Spectrum

GSCE - OCR - Physics Combined Sciences - Pcom4: Waves and Radioactivity  - Pcom4.3 Radioactivity

<h3>P4.3-1 Atomic structure</h3>There are particles in an atom: electrons, protons, and neutrons. The two last ones are in the nucleus and are called nucleon.&nbsp;Proton is positive, electron is negative, and neutron is neutral.&nbsp;Number of protons in an atom determines what chemical element it is.Isotopes of an element have different number of neutrons.&nbsp;Element symbols:&nbsp;<figure class="image image_resized" style="width:70.56%;"><img style="aspect-ratio:1132/635;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3a_element_symbol_c1b1a591ae.jpg" alt="P4.3a element symbol.jpg" width="1132" height="635"></figure>Number of neutrons in an atom = A- ZIn a neutral atom the number of electrons and protons are equal.&nbsp;Ion: when an atom gains or loses electrons.&nbsp;If it gains it is negatively charged (or negative ion), and if loses it is positive.&nbsp;Note: protons are fixed in the nucleus, if they could move the element would be change!&nbsp;Bohr showed electrons are arranged in distinct energy levels around the nucleus. Electrons can move up and down the energy levels thanks to Electromagnetic (EM) radiation:&nbsp;<figure class="image image_resized" style="width:70.88%;"><img style="aspect-ratio:1206/792;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3b_electron_moving_down_058ac5876a.jpg" alt="P4.3b electron moving down.jpg" width="1206" height="792"></figure><figure class="image image_resized" style="width:74.84%;"><img style="aspect-ratio:1217/757;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3c_electron_moving_up_1598b8319b.jpg" alt="P4.3c electron moving up.jpg" width="1217" height="757"></figure>&nbsp;&nbsp;The difference between energy levels of electrons determines the wavelength of the EM wave.&nbsp;<h3>P4.3-2 Radioactive Decay</h3>Radioisotope: an unstable atom that undergoes radioactive decay to become more stable.&nbsp;Radioactivity is random, meaning we don’t know when a particular nucleus will decay. Radioactivity does not depend on temperature.&nbsp;Activity: number of nuclei (plural of nucleus) that decay per second – unit: Becquerel (Bq) or count per second.There are four decay radiations: alpha, beta, neutron, and gammaAlpha Decay: alpha is made of two protons and two neutrons (helium nucleus). When an alpha particle is released from the nucleus of an atom the atomic number decreases by 2, but the mass number decreases by 4. The decay product is a different element.Beta Decay: a neutron in the nucleus changes to a proton and an electron, the electron is ejected from the nucleus becomes the beta radiation. After beta decay atomic number increases by 1; the mass number does not change! Result is a new element.&nbsp;Gamma Decay: gamma is a high energy EM wave, without any mass, or charge (gamma is not a particle like electrons or protons). They are produced when an electron falls into a lower energy level in the atom. Mass number and atomic number is unchanged.&nbsp;&nbsp;<h4>P4.3-2-a Radioactive Decay Equations</h4>For these equations, the total of mass number, atomic number and charge should be the same on both sides of the equation (equation should be balanced). The atomic number can be used as charge as well because it shows the number of protons.&nbsp;&nbsp;Alpha decay equation:&nbsp;<figure class="image image_resized" style="width:84.49%;"><img style="aspect-ratio:1315/567;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3d_alpha_decay_equation_89f5593f5d.jpg" alt="P4.3d alpha decay equation.jpg" width="1315" height="567"></figure>Beta decay equation&nbsp;<figure class="image image_resized" style="width:91.18%;"><img style="aspect-ratio:1308/513;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3d_beta_decay_equation_c4f86330e8.jpg" alt="P4.3d beta decay equation.jpg" width="1308" height="513"></figure>&nbsp;&nbsp;We normally don’t write an equation for gamma radiation, as it just happens when an electron falls to a lower energy level, so there are no changes in mass or atomic number of the element.&nbsp;&nbsp;<h4>P4.3-2-b Background radiation</h4>It is ionising radiation that around us almost everywhere!&nbsp;It comes from rocks (like granite used in kitchen tops), cosmic rays coming from the space, nuclear weapon testing, and some from medical applications (like X-Ray imaging for bones). Even we all are a little radioactive because of the radioisotopes in our body.Background radiation from our activities are from different sources such as: waste from nuclear power plants, army, and hospitals.The unit for radiation dose is Sieverts (Sv).&nbsp;<h4>P4.3-2-c Half-life</h4>&nbsp;It is the time it takes for half of the radioactive nuclei (plural of nucleus) to decay.OrThe time for the activity of the radioactive material to half!&nbsp;Or the time for count rate to half!&nbsp;Graph gets very close to the x-axis (time) but will never touch it!Smoke alarms use a radio isotope with a long half-life (americium-241), because otherwise the detector would not be reliable and needed replacing often.&nbsp;Some radioisotopes are used inside human body to trace biological process or cancerous cells. These should have very short half-life to reduce the danger to the patient from the ionising radiation.&nbsp;&nbsp;Higher only:Calculating Half-life:<ol><li>The background radiation should be deducted from any reading of the Geiger-Muller Tube</li><li>Draw the graph of half-life: activity vs. time</li><li>By finding the time it takes the activity to half, show this time on several points of the graph, e.g. from 100 counts/minute to 50; and from 80 to 40 etc. these values will be different due to random nature of radioactive decay;</li><li>Calculate the average of the values found in the previous step.</li></ol>&nbsp;<figure class="image image_resized" style="width:81.68%;"><a href="https://commons.wikimedia.org/wiki/File:Exponential-decay-half-life.svg"><img style="aspect-ratio:850/517;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3e_half_life_59fd8570d3.jpg" alt="P4.3e half-life.jpg" width="850" height="517"></a></figure>&nbsp;&nbsp;Example 1:Half-life of Iodine-131 is 8 days. A sample of iodine-131 has an activity of 1600 Bq.&nbsp;<ol style="list-style-type:lower-alpha;"><li>Calculate the count rate after 40 days;</li><li>Calculate the net decline of activity.</li></ol><figure class="table"><table><tbody><tr><td style="border:1.0pt solid windowtext;padding:0in 5.4pt;vertical-align:top;width:89.65pt;">Count rate</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:89.35pt;">1600</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:87.4pt;">800</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:87.4pt;">400</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:87.45pt;">200</td><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:none;border-right-style:solid;border-top-style:solid;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:81.55pt;">100</td></tr><tr><td style="border-bottom-style:solid;border-color:windowtext;border-left-style:solid;border-right-style:solid;border-top-style:none;border-width:1.0pt;padding:0in 5.4pt;vertical-align:top;width:89.65pt;">Days</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:89.35pt;">now</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:87.4pt;">8</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:87.4pt;">16</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:87.45pt;">32</td><td style="border-bottom:1.0pt solid windowtext;border-left-style:none;border-right:1.0pt solid windowtext;border-top-style:none;padding:0in 5.4pt;vertical-align:top;width:81.55pt;">40</td></tr></tbody></table></figure>&nbsp;Net decline is ratio of count rate now over the original: in this case 40/1600 = 1/40 = 0.025.&nbsp;Example 2: (higher only)Determine the half-life of carbon-14 from the graph below:&nbsp;<figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Radioactive_decay_of_Carbon-14_SVG.svg"><img style="aspect-ratio:640/480;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3e_example_1_Radioactive_decay_of_Carbon_14_SVG_12eed9c411.png" alt="P4.3e example 1- Radioactive_decay_of_Carbon-14_SVG.png" width="640" height="480"></a></figure>&nbsp;Solution:&nbsp;<figure class="image image_resized" style="width:80.06%;"><img style="aspect-ratio:1113/783;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3f_example_1_Solution_Radioactive_decay_of_Carbon_14_SVG_660e29ff27.jpg" alt="P4.3f example 1 - Solution- Radioactive_decay_of_Carbon-14_SVG.jpg" width="1113" height="783"></figure>&nbsp;From the vertical axis we find when the amount of active carbon-14 nuclei becomes half, and then from the point on the graph we find the time it takes for that to happen.In this case it is about 5500 years!If the vertical axis shows the activity in Bq (or another unit such as counts per minute) same approach is taken.&nbsp;<h4>P4.3-2-d Penetration of radiations&nbsp;</h4><figure class="image"><a href="https://commons.wikimedia.org/wiki/File:Penetrating_power_of_different_types_of_radiation_-_alpha,_beta,_gamma_and_neutrons.svg"><img style="aspect-ratio:640/293;" src="https://effortless-creativity-dd2bae31d9.media.strapiapp.com/P4_3g_Penetrating_power_of_different_types_of_radiation_alpha_beta_gamma_and_neutrons_svg_1c2e762f09.png" alt="P4.3g Penetrating_power_of_different_types_of_radiation_-_alpha,_beta,_gamma_and_neutrons.svg.png" width="640" height="293"></a></figure>&nbsp;Alpha:&nbsp;<ul style="list-style-type:disc;"><li>Least penetrative, can be stopped by sheet of paper;</li><li>Used in smoke detectors as it can be stopped by few centimetres of air;</li><li>Least dangerous outside our body; most dangerous inside the body;</li><li>Most ionising because has the largest mass and biggest charge.</li></ul>&nbsp;Beta:&nbsp;<ul style="list-style-type:disc;"><li>Can be stopped by sheet of metal (3 mm of aluminium);</li><li>Medium ionising, and penetration.&nbsp;</li></ul>&nbsp;Gamma:&nbsp;<ul style="list-style-type:disc;"><li>Most penetrative, can be stopped by several metres of concrete or several centimetres of lead;</li><li>Least ionising.&nbsp;</li></ul>&nbsp;&nbsp;&nbsp;<h4>P4.3.2-e Contamination and irradiation</h4>Contamination means the undesirable presence of radioactive material.<ul style="list-style-type:disc;"><li>Radioactive material can enter human body, through eating, breathing, or entering via a wound, or can be absorbed via skin.&nbsp;</li><li>Contamination with these material can cause mutation if genes, and cancer.</li></ul>The level of contamination depends on:<ol><li>penetrative level of radiation;</li><li>level of ionisation of it</li></ol>This is why alpha is most dangerous when inside the body! It is the most ionising but least penetrative. It cannot much damage when it is outside the body.&nbsp;Medical tracers are radioactive isotopes that injected or eaten by a patient, to monitor the function of their internal organs, or check if their veins are blocked. The radiation should be penetrative enough to be detectable by scanner outside the patient’s body, and it should have a short half-life, so that it does not expose the patient and people around to too much radiation! It is advised that these people stay away from babies and pregnant women for several days after taking the radioisotope!&nbsp;&nbsp;Irradiation is the exposure of an object to radiation.&nbsp;Sources of radiation: food, sun, earth, building material, nuclear waste from hospitals, power plans, or army.&nbsp;Gamma rays is used to sterilise food by super markets or hospitals.&nbsp;Irradiated DNA of a cell, may cause damaged nucleus which causes mutation and can cause cancer, or the cell may die straight away. Damaged genes of people exposed to radiation may continue to mutate in future generations and cause birth defects.It is important that effects of radiation are published in scientific journals so that other scientists are aware of the research. If a scientific research is peer reviewed it means its results can be repeated by other researchers.&nbsp;

Radioactivity

A-Level - Physics - OCR