Contents
1 · Magnetic Fields
A region of space in which a magnetic force can be experienced by a magnetic material or a moving charge. Represented by magnetic field lines that run from north pole to south pole outside a magnet.
Key properties of magnetic field lines:
- They never cross each other.
- Closer lines indicate a stronger field.
- They always form closed loops (inside the magnet they run S → N).
Field Around a Straight Wire
A straight current-carrying wire produces concentric circular field lines. Use the right-hand grip rule: point your right thumb in the direction of conventional current; your fingers curl in the direction of the magnetic field.
Field Inside a Solenoid
A solenoid (coil of wire) produces a uniform field inside that resembles a bar magnet. The end where current flows anticlockwise is a North pole; clockwise is a South pole. Adding a soft-iron core greatly increases the field strength — this is the basis of an electromagnet.
Field line diagrams are high-value drawing questions. Practise drawing the field between two attracting poles (lines curve from N to S) and two repelling poles (lines bow away from each other).
2 · Force on a Current-Carrying Conductor
When a current-carrying conductor is placed in an external magnetic field, it experiences a force. This is because the external field and the field created by the current interact, creating a region of stronger field on one side and weaker field on the other — the conductor is pushed toward the weaker side.
The force is greatest when the conductor is perpendicular to the field, and zero when it is parallel.
A wire of length 0.25 m carries a current of 4.0 A in a magnetic field of flux density 0.30 T, perpendicular to the field. Calculate the force on the wire.
F = BIL = 0.30 × 4.0 × 0.25 = 0.30 N
3 · Fleming's Left-Hand Rule (Motor Effect)
Hold the left hand with the thumb, index finger and middle finger mutually perpendicular. The First finger points in the direction of the magnetic Field; the seCond finger points in the direction of Conventional current; the thuMb points in the direction of the Motion (force).
Memory aid: FBI — Field (first finger), current (second finger / Bumbling finger), motion/force (thumb). Or: First finger = Field, seCond = Current, thuMb = Motion.
Students often mix up the left-hand rule (motors — input is electrical, output is mechanical) with the right-hand rule (generators — input is mechanical, output is electrical). Motor = Left hand. Generator = Right hand.
4 · The d.c. Motor
A d.c. motor converts electrical energy into kinetic (mechanical) energy. The key components are:
| Component | Function |
|---|---|
| Rectangular coil | Carries the current; experiences the force |
| Permanent magnet | Provides the external magnetic field |
| Split-ring commutator | Reverses current direction every half-turn so the coil always rotates in the same direction |
| Carbon brushes | Maintain electrical contact with the rotating commutator |
| Soft-iron cylinder (armature) | Concentrates the field, making it radial so force is always maximum |
The coil experiences maximum torque when the plane of the coil is parallel to the field. Torque is zero when the coil is perpendicular to the field (the "dead point" — which the momentum of the coil carries it through).
How to Increase Motor Speed
- Increase the current.
- Increase the magnetic field strength (stronger magnet or more turns on the field coil).
- Increase the number of turns on the armature coil.
5 · Electromagnetic Induction
The production of an electromotive force (e.m.f.) — and therefore a current in a closed circuit — when the magnetic flux through a conductor changes.
Faraday's Law: The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage.
Lenz's Law: The induced current flows in a direction such that its magnetic effect opposes the change that caused it. (This is conservation of energy — if induced current aided the change, you would get energy for free.)
Factors that Increase the Induced e.m.f.
- Moving the magnet faster (greater rate of change of flux).
- Using a stronger magnet.
- Increasing the number of turns on the coil.
When a question asks you to explain why an e.m.f. is induced, always reference the change in magnetic flux — not just "the magnet moves". The key phrase is: "the rate of change of magnetic flux linkage increases/decreases, inducing a greater/smaller e.m.f."
6 · Fleming's Right-Hand Rule (Generator Effect)
The right-hand rule gives the direction of the induced current in a conductor moving through a magnetic field. Hold the right hand with thumb, index and middle fingers mutually perpendicular:
- Thumb → direction of Motion of conductor
- First finger → direction of Field
- Second finger → direction of induced Current
An a.c. generator (alternator) uses slip rings (not a split-ring commutator) so that the output current alternates direction every half-turn, producing an alternating current.
7 · Transformers
A device that uses electromagnetic induction to change the voltage (and current) of an alternating supply. It consists of two coils (primary and secondary) wound on a shared soft-iron core.
| Type | Turns ratio (Np : Ns) | Effect on voltage | Effect on current |
|---|---|---|---|
| Step-up | Np < Ns | Voltage increases | Current decreases |
| Step-down | Np > Ns | Voltage decreases | Current increases |
A transformer has 200 turns on the primary and 50 turns on the secondary. The primary voltage is 240 V. Find the secondary voltage and the secondary current if the primary current is 0.5 A (assume 100% efficiency).
Vs = Vp × (Ns / Np) = 240 × (50/200) = 60 V
Is = (Vp × Ip) / Vs = (240 × 0.5) / 60 = 2.0 A
Transformers only work with alternating current. A d.c. supply produces a steady magnetic flux — no change in flux means no induced e.m.f. in the secondary. Many students forget this and lose easy marks.
8 · The National Grid
Electricity from power stations is transmitted across long distances via the national grid. High voltage, low current transmission is used because power loss in cables is given by P = I²R. By reducing current, resistive losses are greatly reduced.
- Power station generates at ~25 kV.
- Step-up transformer raises voltage to 400 kV for long-distance transmission.
- Step-down transformers reduce voltage in stages to 230 V for domestic use.
A common Paper 2 question asks you to explain why high voltage is used for transmission. Full marks require: (1) state P = I²R; (2) high voltage → low current (from P = VI, fixed P); (3) low current → less power lost as heat in cables; (4) therefore more efficient transmission.
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