Magnetism and electromagnetism
Magnets and magnetic fields, electromagnets, the motor effect, electromagnetic induction, generators and transformers.
📌 Key points
- Like poles repel and unlike poles attract. Iron, steel, nickel and cobalt are magnetic materials. Soft iron is easily magnetised and demagnetised (used in electromagnets and transformer cores); steel keeps its magnetism (used for permanent magnets).
- Magnetic field lines go from the north pole to the south pole outside a magnet; the closer the lines, the stronger the field.
- A current in a straight wire makes circular field lines around it. A solenoid makes a field like a bar magnet; it is stronger with a larger current, more turns or an iron core.
- Motor effect: a current-carrying wire in a magnetic field feels a force. The force reverses if the current or the field is reversed.
- Fleming’s left-hand rule: first finger = field, second finger = current, thumb = motion (force).
- A d.c. motor turns faster with more turns on the coil, a larger current or a stronger field. A split-ring commutator reverses the current every half turn so it keeps turning.
- Electromagnetic induction: an e.m.f. is induced when a conductor cuts magnetic field lines, or when the field through a coil changes. It is larger with faster movement, a stronger magnet or more turns.
- The induced e.m.f. (and current) is in a direction that opposes the change producing it. An a.c. generator uses slip rings.
- Transformer: VₚVₛ = NₚNₛ. It only works with a.c. For an ideal transformer, IₚVₚ = IₛVₛ.
- Electricity is transmitted at high voltage so the current is small, which reduces the power lost as heat in the cables (P = I2R).
📖 Key terms
- Magnetic field
- A region in which a magnetic pole experiences a force.
- Soft iron
- A material that is easily magnetised and loses its magnetism easily.
- Solenoid
- A long coil of wire that produces a magnetic field when a current flows.
- Motor effect
- The force on a current-carrying conductor in a magnetic field.
- Commutator
- A split ring that reverses the current in a motor coil every half turn.
- Electromagnetic induction
- Producing an e.m.f. in a conductor by a changing magnetic field.
- Step-up transformer
- A transformer with more turns on the secondary coil, so the output voltage is higher.
✏️ Worked example
An ideal transformer has 200 turns on the primary coil and 4000 turns on the secondary. The input is 12 V a.c. Find the output voltage. If the output current is 0.10 A, find the input current.
- VₛVₚ = NₛNₚ, so Vₛ = 12 × 4000200 = 240 V.
- Output power = IₛVₛ = 0.10 × 240 = 24 W.
- For an ideal transformer, input power = output power = 24 W.
- Answer: Vₛ = 240 V and Iₚ = 2412 = 2.0 A.
Transformers only work with a.c.: a changing current in the primary coil produces a changing magnetic field in the core, which induces an e.m.f. in the secondary.
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