GCSE AQA Physics: Magnetism and Electromagnetism Revision Notes

Magnetism and Magnetic Fields

Magnets have poles where magnetic forces are strongest. When two magnets interact, like poles repel and unlike poles attract each other - this is a non-contact force that works across space.

A permanent magnet produces its own magnetic field, while an induced magnet only becomes magnetic when placed near another magnet. The induced magnetism always creates attraction and disappears when the inducing field is removed.

The magnetic field is the region around a magnet where forces act on other magnets or magnetic materials (iron, steel, cobalt, and nickel). This field is strongest at the poles and weakens with distance. Magnetic field lines run from north to south poles, showing the direction a free north pole would move if placed at any point in the field.

Pro Tip: When plotting a magnetic field, mark where a compass points, move to that marked position, and repeat until you return to the magnet. Connect these points with arrows pointing from north to south.

Earth itself has a magnetic field, which is why compass needles (small bar magnets) point north - they're aligning with Earth's magnetic field!

Electromagnetism

When current flows through a wire, it creates a magnetic field around it. The field's strength depends on both the current and your distance from the wire. This relationship between electricity and magnetism is what powers much of our technology!

A solenoid (a wire coiled into a helix) concentrates this effect, creating a strong, uniform magnetic field inside the coil. This field resembles a bar magnet's field in shape. Add an iron core to the solenoid, and you've got an electromagnet with an even stronger field.

You can determine the direction of the magnetic field using the right-hand thumb rule: point your thumb in the direction of the current, and your curled fingers show the field direction around the wire.

Remember this: Electromagnets are incredibly useful because, unlike permanent magnets, you can switch them on and off! That's why they're used in devices like electric bells, where the alternating attraction and release of an armature creates the ringing sound.

When a current-carrying conductor sits in a magnetic field, the interaction between the two fields creates a force - the motor effect. Fleming's Left Hand Rule helps you predict the direction of this force, forming the foundation of electric motors and many other devices.

The Motor Effect and Electric Motors

The force on a current-carrying conductor in a magnetic field increases when you boost the current, lengthen the conductor in the field, or increase the magnetic flux density. This relationship is expressed as:

Force = magnetic flux density × current × length $F = BIl$

Where force is in newtons (N), magnetic flux density in tesla (T), current in amperes (A), and length in metres $m$.

Fleming's Left Hand Rule helps you work out directions in motor effect situations:

• First finger: direction of the magnetic field (North to South)
• Middle finger: direction of the current
• Thumb: direction of the resulting force

Electric motors use this principle to convert electrical energy to movement. When current flows through a coil in a magnetic field, opposite sides of the coil experience forces in opposite directions. This creates a turning effect! The clever bit is the split-ring commutator, which reverses the current every half-turn, ensuring continuous rotation.

Physics in action: Every time you use an electric drill, toy car, or fan, you're seeing the motor effect at work! The electric energy is being transformed into the kinetic energy of rotation.

Speakers and the Generator Effect

Loudspeakers convert electrical signals into sound using the motor effect. An alternating current flows through a coil attached to a cone, sitting in a magnetic field. As the current direction changes, the coil moves back and forth, pushing the cone to create sound waves. Headphones work the same way but on a smaller scale with a thin diaphragm.

If you move a conductor through a magnetic field (or change the field around the conductor), you create a potential difference across it. This is the generator effect - essentially the opposite of the motor effect. If the conductor forms a complete circuit, you get an induced current.

The size of this induced voltage or current depends on:

• How fast you're moving
• How strong the magnetic field is
• The number of turns in the coil (if using one)

Fascinating fact: The induced current creates its own magnetic field that opposes whatever change caused it in the first place - this is why generators require constant force to keep turning!

The direction of the induced current reverses if you change the direction of movement or swap the magnetic poles. This principle is fundamental to electricity generation worldwide.

Electricity Generation

The generator effect powers our modern world through two key devices: alternators and dynamos.

In an alternator, a coil spins in a magnetic field, creating a changing magnetic field through the coil. This induces a potential difference that drives current through a circuit. The coil connects to slip rings and brushes that maintain contact with the external circuit. As the coil rotates, the current reverses direction every half-turn, producing alternating current (AC).

A dynamo works similarly, with a coil rotating in a magnetic field, but uses a split-ring commutator instead of slip rings. This clever mechanism swaps the connections every half-turn, ensuring the current always flows in one direction in the external circuit, producing direct current (DC).

Think about it: Almost all electricity powering your home comes from power stations using the generator effect - whether from wind turbines, hydroelectric dams, or steam turbines in nuclear or coal plants!

The generator effect demonstrates the beautiful symmetry in physics - just as electricity can create movement through the motor effect, movement can create electricity through the generator effect.

Microphones and Transformers

Microphones use the generator effect to convert sound waves into electrical signals. Sound causes a diaphragm to vibrate, moving an attached coil within a magnetic field. This movement induces a voltage across the coil that exactly matches the pattern of the sound waves. The resulting electrical signal can then be amplified or recorded.

Transformers are devices that can change the voltage of alternating current electricity. They consist of primary and secondary coils wound around an iron core. The alternating current in the primary coil creates an alternating magnetic field in the iron core, which then induces an alternating voltage in the secondary coil.

The ratio of the voltages depends on the ratio of turns in the coils:

• Step-up transformers (where secondary voltage > primary voltage) have more turns on the secondary coil
• Step-down transformers (where secondary voltage < primary voltage) have fewer turns on the secondary coil

Why this matters: Transformers are crucial to our power grid! Electricity is transmitted at high voltages to reduce energy loss, then transformed down to safer voltages for home use.

In an ideal transformer, electrical power output equals input. This relationship between electricity and magnetism underpins our entire electrical infrastructure.