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 South

Like poles repel and unlike ones attract.

The attraction and repulsion can be shown by using two suspended bar magnets.

Iron, nickel, cobalt and some steel can be magnetised if left in a magnetic field. 

P3.3-1 Magnetic Field

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 3^rd law!). 

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. 

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.

Plotting compass is small bar magnet. It can be used show magnetic field of another magnet or the Earth.

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!

Magnetic poles of the Earth flip every 200 000 years!

P3.3-2 Current Carrying Wire

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. 

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. 

Why material can be magnetised: 

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! 

The strength of the magnetic field depends on the distance from the wire, and the amount of current in the wire.

Solenoid

Solenoid is a coil of wire which is spread out, with its length much larger than its diameter. 

The magnetic field around a solenoid is similar to a bar magnet. 

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:

P3.3-3 The Motor Effect (Higher Only)

When two objects with magnetic field are placed close to each other they apply an equal and opposite force on each other (Newton’s 3^rd 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:

We can calculate the magnitude of the force on the wire placed at right angles in a magnetic field with this formula:

B shows the strength of the magnetic field (also called magnetic flux density) with unit of tesla (shown by T). 

Example 1

Diagram 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.

Solution:

So the wires moves upward (direction that the thuMb shows!)

P3.3k Electric Motor Diagram-a

Electric motors

Electric motors are used in anything that moves with electricity! E.g. food processor, drill, and Tesla cars! 

Fleming left-hand rule à force on the A-B section of the coil is upwards, and on C-D is downwards. 

This will turn the coil clockwise. Until A-B comes to where C-D is now:

If the current remains in A-B as it was before, the coil will turn anticlockwise this time! 

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. 

The magnitude of the force on the coil in a motor depends on:

1. Strength of the magnetic field;
2. Amount of current in the coil;
3. Number of turns in the coil

The dynamo

Its 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!