Contents
1. Magnetic Fields
A magnetic field is a region where a magnetic force acts on a magnet or moving charge. Field lines run from north pole to south pole outside the magnet; the closer together the lines, the stronger the field.
- Like poles repel; unlike poles attract.
- Magnetic materials: iron, steel, nickel, cobalt.
- Soft iron is easily magnetised and demagnetised (used in electromagnet cores).
- Steel is harder to magnetise but retains magnetism (used in permanent magnets).
Field pattern for a straight wire carrying current
Concentric circles around the wire. Direction: use the right-hand grip rule — wrap the right hand around the wire with the thumb pointing in the direction of conventional current; fingers curl in the direction of the field.
Field pattern for a solenoid
Similar to a bar magnet — field lines emerge from one end (north pole) and enter the other (south pole). Inside the solenoid the field is uniform and parallel.
2. Electromagnets
An electromagnet is a coil of wire (solenoid) with a soft-iron core that becomes magnetic when current flows.
Ways to increase electromagnet strength
- Increase the current through the coil.
- Increase the number of turns in the coil.
- Use a soft-iron core (instead of air).
Uses of electromagnets
Electric bells, relays, circuit breakers, cranes for lifting scrap metal, MRI machines, speakers.
A relay uses a small current in a control circuit to switch on a large current in a separate output circuit. The electromagnet attracts a soft-iron armature that closes the contacts of the output circuit — useful when the control circuit must be kept isolated from the high-power circuit.
3. Motor Effect
A current-carrying conductor in a magnetic field experiences a force. This is the motor effect.
Point the left hand so that: First finger → magnetic Field direction; seCond finger → conventional Current direction; thuMb → direction of Motion (force). All three must be perpendicular to each other.
DC motor
A coil in a magnetic field rotates when current flows. The commutator (split-ring) reverses the current direction every half turn, ensuring the coil keeps rotating in the same direction. Carbon brushes maintain contact with the spinning commutator.
Increasing motor speed
- Increase the current.
- Increase the number of turns on the coil.
- Use a stronger magnet.
- Use a soft-iron core inside the coil.
Fleming's Left-Hand Rule is for the motor effect (force on a conductor). Fleming's Right-Hand Rule is for electromagnetic induction (generator effect). Using the wrong hand is a common 1-mark error.
4. Electromagnetic Induction
An e.m.f. (and current if the circuit is complete) is induced in a conductor when there is a change in the magnetic flux linking the conductor. This is Faraday's law.
Ways to increase induced e.m.f.
- Move the magnet faster (increase rate of change of flux).
- Use a stronger magnet.
- Increase the number of turns in the coil.
Lenz's Law
The induced current always flows in a direction that opposes the change causing it. This is why you feel resistance when pushing a magnet into a coil — the induced current creates a magnetic field that repels the incoming magnet.
AC generator
A coil rotating in a magnetic field produces an alternating e.m.f. Slip rings (not a commutator) maintain contact and allow the alternating output. Output frequency = rotation frequency.
5. Transformers
A transformer uses electromagnetic induction to change voltage. It only works with AC (alternating current) — a changing current in the primary coil creates a changing magnetic field in the iron core, inducing a changing e.m.f. in the secondary coil.
A transformer has 200 primary turns and 1000 secondary turns. The primary voltage is 230 V. Find the secondary voltage.
V_s = V_p × (N_s / N_p) = 230 × (1000/200) = 1150 V
This is a step-up transformer (secondary voltage > primary voltage).
Why transmit at high voltage?
At high voltage, current is lower (P = IV, P constant). Lower current means less power lost as heat in the transmission cables (P_loss = I²R). Step-up transformers raise voltage before transmission; step-down transformers reduce it for safe use in homes.
- Fleming's Left Hand Rule (motor effect): thuMb = Motion, First finger = Field (B), seCond finger = Current (I)
- Fleming's Right Hand Rule (generator/induction): thuMb = Motion, First finger = Field, seCond finger = induced Current
- Electromagnet strength: increase current, increase number of turns, add iron core.
- Transformer: only works with AC. Step-up (Ns > Np): voltage increases, current decreases.
- Step-down (Ns < Np): voltage decreases, current increases. Ideal: VpIp = VsIs (power conserved).
- Induced EMF increases with: faster movement, stronger magnet, more turns in coil.
6. Common Exam Traps
A transformer requires a changing magnetic field to induce an e.m.f. in the secondary coil. DC produces a constant field — no change, no induction. "A transformer works with DC" is always wrong.
If a transformer steps up voltage by a factor of 10, it steps down current by a factor of 10 (power is conserved). Students often forget this and calculate current incorrectly.
Holding a stationary magnet inside a coil induces nothing. You must move the magnet (or change the current) to change the flux linkage and induce an e.m.f.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.