Overview
You should be able to:
- Describe magnetic materials, poles and magnetic field lines.
- Draw and interpret the field of a bar magnet and between two poles.
- Explain how a current-carrying wire or coil creates a magnetic field.
- Describe and use electromagnets in simple devices (relays, cranes, bells, etc.).
- Explain and use the motor effect and the factors affecting the force on a wire in a field.
- Describe electromagnetic induction and the basic idea of generators and transformers.
- Use the transformer equation Vₚ/Vₛ = Nₚ/Nₛ and VₚIₚ ≈ VₛIₛ for an ideal transformer.
Most of this topic is diagrams + “which way does it move?” – once the pictures make sense, the marks are very gettable.
1. Permanent Magnets & Magnetic Fields
Magnetic Materials
- Common magnetic materials: iron, steel, cobalt, nickel.
- Magnets have two ends called north (N) and south (S) poles.
- Like poles repel (N–N or S–S), unlike poles attract (N–S).
- Permanent magnets keep their magnetism; induced magnets become magnetic when near a magnet and lose magnetism when removed.
Magnetic Field Lines
- A magnetic field is the region where a magnetic material or moving charge feels a force.
- Field lines show the direction a compass (tiny north pole) would point.
- Field lines go from the north pole to the south pole of a magnet.
- Closer field lines = stronger field.
Bar Magnet Field
In an exam, draw field lines smooth, not crossing, denser near the poles, and label N and S clearly.
Quick Check
Q1. A steel paperclip is attracted to a magnet. Is the paperclip itself now a magnet?
Show answer
Yes, it becomes an induced magnet while in the field – it gains poles. When removed, it may lose most of its magnetism.
2. Electromagnets & Fields from Currents
Current in a Straight Wire
- A current-carrying wire produces a magnetic field made of concentric circles around the wire.
- The direction of the field can be found using the right-hand grip rule: thumb = current, fingers = field direction.
Electromagnets (Solenoids)
- A coil of wire (solenoid) with current acts like a bar magnet.
- Putting a soft iron core inside makes the field stronger.
- Electromagnets are useful because they can be switched on and off.
Uses: relays, electric bells, scrap-yard cranes, MRI scanners, door locks, etc.
Quick Check
Q2. Give one advantage of an electromagnet over a permanent magnet.
Show answer
It can be switched on and off (and its strength can be changed by altering current or turns on the coil).
3. The Motor Effect
Force on a Wire in a Magnetic Field
- If a wire carrying current is placed in a magnetic field, it experiences a force – this is the motor effect.
- The force is largest when the wire is at 90° to the field.
- Reversing the current or reversing the magnetic field reverses the direction of the force.
You should know the factors that increase the force:
- stronger magnetic field
- larger current
- longer length of wire in the field
F = B I L
Motor Effect Diagram
In school you might learn Fleming’s left-hand rule to remember the directions (Force, Field, current).
Simple D.C. Motor (Idea Only)
- A coil of wire in a magnetic field carries current.
- One side of the coil is forced up, the other side down → coil spins.
- A split-ring commutator reverses the current every half turn, so the coil keeps turning in the same direction.
Quick Check
Q3. A horizontal wire carries current from left to right between N and S poles (field from N → S). Will the force be up, down, into the page or out of the page?
Show answer
Using Fleming’s left-hand rule: field from N to S, current left → right, so the force is out of the page.
4. Electromagnetic Induction
Basic Idea
- If a wire cuts magnetic field lines, a voltage (emf) is induced across the ends.
- If the circuit is complete, this induced voltage can cause a current.
- This is called electromagnetic induction.
The induced voltage (and current) is larger if:
- the magnet is stronger,
- the wire/coil moves faster,
- the coil has more turns.
Reversing the direction of movement or the pole (N/S) reverses the direction of induced current.
Moving Magnet in a Coil
Quick Check
Q4. Name two ways to increase the size of the induced voltage when moving a magnet in a coil.
Show answer
Move the magnet faster, use a stronger magnet, or use a coil with more turns (any two).
5. Transformers
Structure & Types
- A transformer has two coils of wire (primary and secondary) wound on an iron core.
- It only works with a.c. in the primary coil.
- Changing current in the primary creates a changing magnetic field in the core, which induces a voltage in the secondary.
- Step-up transformer: more turns on secondary → higher voltage, lower current.
- Step-down transformer: fewer turns on secondary → lower voltage, higher current.
Subscripts: p = primary, s = secondary. Always keep p’s with p’s and s’s with s’s.
Transformer Diagram
In power networks, step-up transformers reduce current (to reduce heating losses) and step-down transformers make voltages safe for homes.
Quick Check
Q5. A transformer has 200 turns on the primary and 800 turns on the secondary. The primary voltage is 50 V. What is the secondary voltage?
Show answer
Vₚ/Vₛ = Nₚ/Nₛ → 50 / Vₛ = 200 / 800 → 50 / Vₛ = 1/4 → Vₛ = 200 V (step-up).
What Next?
To make this topic safe for the exam:
- Practise drawing: bar magnet fields, field between two poles, solenoids, transformer diagrams.
- Do past-paper questions on the motor effect, direction of force and simple D.C. motors.
- Practise transformer calculations with Vₚ/Vₛ = Nₚ/Nₛ and VₚIₚ = VₛIₛ.
- Explain aloud how an electromagnet works and where it’s used in everyday devices.