Uses of magnetism

Welcome to the World of Electromagnetism!

In this chapter, we are going to explore how magnetism is used to make things happen. From the tiny motor in your phone that makes it vibrate to the massive generators that power our homes, magnetism is the hidden hero of modern life.
Don't worry if some of this feels a bit like "magic" at first—we'll break it down step-by-step. By the end of these notes, you'll understand how we turn electricity into movement, and movement back into electricity!

1. The Motor Effect

The motor effect happens when we put a wire carrying an electric current into a magnetic field. Because the current has its own magnetic field, it interacts with the field of the permanent magnets. This interaction creates a force that pushes the wire.

Fleming’s Left-Hand Rule

To figure out which way the wire will move, we use Fleming’s Left-Hand Rule. This is a classic "physics trick" that helps you visualize 3D directions.
Hold your left hand so your thumb, first finger, and second finger are all at right angles to each other (like a toy gun).

• First finger: Points in the direction of the Field (North to South).
• SeCond finger: Points in the direction of the Current (+ to -).
• Thumb: Points in the direction of the Thrust (the Force/Movement).

Memory Aid: The "FBI" Rule
Think of the FBI (the Federal Bureau of Investigation):
Force (Thumb), B-field/Magnetic Field (First Finger), I-current (Second Finger).

Calculating the Force

We can actually calculate exactly how much force is produced using this equation:
\( F = B \times I \times l \)

• \(F\) = Force (measured in Newtons, N)
• \(B\) = Magnetic flux density (a fancy way of saying "field strength", measured in Tesla, T)
• \(I\) = Current (measured in Amps, A)
• \(l\) = Length of the wire inside the field (measured in metres, m)

Quick Review Box:
To get the biggest force, you can:
1. Use a stronger magnet (increase B).
2. Put more current through the wire (increase I).
3. Use a longer wire in the field (increase l).

Key Takeaway: A current-carrying wire in a magnetic field experiences a force. The direction is found using Fleming's Left-Hand Rule.

2. Electric Motors

An electric motor uses the motor effect to create constant rotation. Instead of just one wire, we use a coil of wire. One side of the coil is pushed up, while the other side is pushed down, making it spin.

The Split-Ring Commutator

The trickiest part of a motor is keeping it spinning in the same direction. Without a split-ring commutator, the coil would just flip 180 degrees and stop.
The commutator reverses the direction of the current every half-turn. This ensures the force on the left side always pushes up and the force on the right side always pushes down, keeping the motor spinning smoothly!

Did you know?
Electric motors are everywhere! They are in your hair dryer, your washing machine, and even the cooling fan in your computer.

3. Electromagnetic Induction (Higher Tier Only)

Electromagnetic induction is the opposite of the motor effect. Instead of using electricity to make movement, we use movement to make electricity.

How it works

If you move a wire through a magnetic field (or move a magnet into a coil of wire), you "cut" the magnetic field lines. This induces a potential difference (voltage) across the ends of the wire. If the wire is part of a complete circuit, a current will flow.

The Golden Rule of Induction:
The induced current always creates its own magnetic field that opposes the change that made it. This is why it takes effort to turn a generator—you are fighting the magnetic forces you are creating!

Alternators vs. Dynamos

• Alternators: Use slip rings to produce Alternating Current (a.c.). This is the type of current that changes direction constantly.
• Dynamos: Use a split-ring commutator to produce Direct Current (d.c.). This current only flows in one direction.

Common Mistake to Avoid:
Students often confuse slip rings (Alternators/A.C.) with split rings (Dynamos/D.C.). Remember: S-p-l-i-t rings s-p-l-i-t the connection to keep the current flowing the same way (D.C.).

Key Takeaway: Moving a magnet near a wire "induces" electricity. This is how power stations generate the electricity we use at home.

4. Transformers (Higher Tier Only)

A transformer is a device that can change the size of an alternating potential difference (voltage). They consist of two coils of wire (primary and secondary) wrapped around an iron core.

How they work

1. An alternating current flows through the primary coil.
2. This creates a changing magnetic field in the iron core.
3. The changing field passes through the secondary coil.
4. This induces an alternating potential difference in the secondary coil.

Important Point: Transformers only work with alternating current (a.c.). They will not work with direct current (d.c.) because d.c. doesn't create a changing magnetic field.

The Transformer Equation

The ratio of the voltages depends on the ratio of the number of turns on the coils:
\( \frac{V_p}{V_s} = \frac{N_p}{N_s} \)

• \(V_p\) = Potential difference in primary coil
• \(V_s\) = Potential difference in secondary coil
• \(N_p\) = Number of turns on primary coil
• \(N_s\) = Number of turns on secondary coil

• Step-up Transformer: Increases voltage (more turns on the secondary coil).
• Step-down Transformer: Decreases voltage (fewer turns on the secondary coil).

Key Takeaway: Transformers change voltage by using a changing magnetic field to transfer energy between two coils.

5. Microphones and Loudspeakers (Higher Tier Only)

These two devices are perfect examples of the two concepts we've learned.

Microphones (Induction)

A microphone turns sound waves into electrical signals.
1. Sound waves hit a flexible diaphragm.
2. The diaphragm vibrates, moving a coil attached to it.
3. The coil moves back and forth over a permanent magnet.
4. This induces an electrical current that matches the sound wave pattern.

Loudspeakers (Motor Effect)

A loudspeaker turns electrical signals back into sound.
1. An electrical current (signal) flows through a coil.
2. The current interacts with a permanent magnet (the motor effect).
3. This creates a force that moves the coil and the attached cone.
4. The cone vibrates the air, creating sound waves.

Analogy:
Think of a microphone as an ear (listening and turning sound into signals) and a loudspeaker as a mouth (taking signals and shouting them out as sound).

Key Takeaway: Microphones use induction to create signals; loudspeakers use the motor effect to create sound.

Don't worry if this seems tricky at first! Magnetism is one of the more abstract parts of Physics. Just remember:
Electricity + Magnetism = Movement (Motor)
Movement + Magnetism = Electricity (Generator)