Physics - EDEXCEL IGCSE - Magnetism and Electromagnetism

Section 6 - Magnetism and Electromagnetism:

Magnetically hard material - retains its magnetism for a long time. It is also hard to demagnetize.

Magnetically soft material - loses its magnetism almost as soon as it leaves the magnetic field.

Magnetic field lines represent the direction (through the direction of the lines) and magnitude (through the density of lines) of a magnetic field on a single North Pole.  

The three pictures above show the magnetic field lines present in the presence of bar magnets.

The two pictures shown to the left show the magnetic field lines of a straight wire and a solenoid. The direction of the magnetic field lines can be shown by using the right-hand grip rule shown on the first picture to the left.

A coil of wire carrying current acts like a magnet and as such has filed lines as shown to the left. 

The way to test for the magnetic field lines and its direction is by using a compass. The compass will point always in the direction of the magnetic field line. You can then draw dots at the end points of the compass and connect the dots to show the magnetic field lines.

To increase the strength of the magnetism you can:

1. Increase current
2. Increase the number of coils in the wire
3. Adding a magnetic soft material in the solenoid

An electromagnet is created by wrapping a soft magnetic material with a current carrying wire. These are then used in relays and circuit breakers.

How do Electric Motors work:

1. A wire in a magnetic field feels a force and so turns.
2. This turns the split ring commutator.
3. The commutator reverses the current every half turn.
4. The wire continues to spin until the current is switched off.

How do Loud Speakers work:

1. A wire is pushed forwards and then moves back and forwards hundreds of times a second.
2. This pushes the cone / diaphragm.
3. Noise is created.

These both work because of the motor effect. When there is a magnetic field and current then a force will be created. Fleming’s left-hand rule is used to see which direction the force is in.

The picture below shows Fleming’s left-hand rule:

The motor effect can be increased by:

1. Increasing the amount of coils, the magnetic field strength or current strength.
2. Add a magnetically soft material in the centre.

Moving a wire back and forth across a field will induce a voltage.

Ways to increase the voltage are to increase the field strength, quicken the movement or increase the number of coils in the wire.

How do transformers work:

1. An AC current passes through a coil wrapped around a soft iron core (electro-magnet).
2. This induces a magnetic field.
3. The magnetic field makes a second wire move back and forth, creating voltage.

Step up transformers - this has fewer turns in the first wire compared to the second thereby creating a higher voltage and lower current as the output.

Step down transformers - this has more turns in the first wire compared to the second thereby creating a lower voltage and higher current as the output.

How is this used:

Transformers are used to change the voltage and current of electricity. Below shows one example of how transformers are used to deliver electricity to our homes safely:

power plant —> step up transformer (to avoid heat from high current) —> power lines —> step down transformer —> distribution lines —> homes

input (primary) voltage / output (secondary) voltage = primary turns / secondary turns

Vp * Ip = Vs * Is

primary voltage * primary current = secondary voltage * secondary current

The above only occurs, however, if the transformer is 100% efficient