Magnetism and Electromagnetism
What's covered
Key facts
Electromagnetic Induction
A potential difference is induced whenever a conductor cuts through magnetic field lines.
Electromagnets
An electric bell uses an electromagnet to repeatedly pull a striker against a bell.
Magnetic Fields
Magnetic field lines run from the north pole to the south pole outside a bar magnet.
The Generator Effect
A simple AC generator works because a coil rotates in a magnetic field, inducing an alternating PD.
The Motor Effect
A DC motor uses a split-ring commutator to reverse the current through the coil every half turn, keeping the coil rotating in one direction.
The National Grid
The National Grid transmits at very high voltage (up to 400 kV) because higher voltage means lower current, reducing I²R heat loss in cables.
Transformers
In an ideal (100% efficient) transformer, primary power equals secondary power.
Electromagnetic Induction
Electromagnetic induction is a potential difference induced in a conductor by changing magnetic flux.
Electromagnets
Electromagnets are used in relays, electric bells, loudspeakers, scrapyard cranes, MRI scanners, and motors.
Magnetic Fields
A compass detects magnetic field direction.
Sample questions
A taste of the 72 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What is electromagnetic induction?
- ✓A PD induced in a conductor by changing magnetic flux
- •Resistance that builds in a coil when the current through it increases
- •The creation of a magnetic field around a straight current-carrying wire
- •The force on a current-carrying conductor placed in a magnetic field
Give three applications of electromagnets in everyday devices.
- •Diesel engines, petrol generators, and steam turbines
- ✓Relay switches, electric bells, loudspeakers
- •Solar panels, wind turbines, and hydroelectric generators
- •Thermostats, spirit levels, and barometers
What do magnetic field lines show?
- ✓Direction a free north pole would move; closer lines = stronger field
- •Lines of equal gravitational potential running parallel to the magnet surface
- •The path of electrons through the magnet; closer lines mean slower electrons
- •Where the magnetic force is zero in the field; gaps indicate maximum strength
How does a simple AC generator produce electricity?
- ✓A coil rotates in a magnetic field, inducing an alternating PD
- •A current is passed through a coil in a magnetic field, causing it to rotate
- •A permanent magnet spins inside a stationary coil to create a steady DC
- •Electromagnets repel a rotating coil, building up electrostatic charge
What happens to the force on a wire if the current direction is reversed?
- •The force becomes perpendicular to the wire
- ✓The force direction reverses
- •The force disappears completely
- •The force doubles in size
Why does the National Grid transmit at very high voltage (up to 400 kV)?
- •Cables can be made thinner at high voltage, lowering installation costs
- •High voltage speeds up electrons in the cable, reducing transmission time
- ✓Higher voltage means lower current, reducing I²R heat loss in cables
- •Higher voltage prevents moisture corroding cables, reducing maintenance costs
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