Static electricity and Charge

P3.1-1 Charge

A neutral atom or object, has equal number of protons and electrons. 

Only electron moves! Proton is fixed in the nucleus. 

Unit of electric charge is Coulombs (C). 

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 rod with a cloth, electrons gain energy because of friction, and move from the cloth to the rod (it becomes negatively charged). The cloth is left with positive protons. 

If you rub an acetate (or Perspex) rod with a cloth, the acetate becomes positive and cloth negative. 

Like charges repel each other and opposite charges attract.

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!

P3.1-2 Current and PD

Electric current: Flow of negative charge (electrons) in a conductor in one second with unit of ampere (A or amps).

Current is measured by an ammeter in a circuit. 

Ammeter is connected in series (see P3.2a). 

Ideal ammeter has zero resistance.

A Single closed loop circuit:

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. 

Potential Difference: the amount of energy placed on each unit of charge (Q = 1 C):

PD is measured with a voltmeter.

Voltmeter is connected in parallel (see P3.2a).

Ideal voltmeter has infinite resistance.

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.