Magnetic field around a current-carrying wire: circles around the wire. Right-hand grip rule: thumb along the current, curled fingers show the field direction.
Solenoid right-hand rule: curl the fingers along the current; the thumb points to the north pole. Two long parallel wires with current in the same direction attract each other.
Force on a current-carrying conductor in a magnetic field: F = B I L (perpendicular). Fleming's left-hand rule: thumb = force (motion), forefinger = field, middle finger = current, all mutually perpendicular.
Electric motor: converts electrical energy to mechanical energy. Generator: converts mechanical energy to electrical energy.
Electromagnetic induction: an emf is induced in a conductor when the magnetic flux through it changes.
Lenz's law: the direction of the induced current is always such that it opposes the cause producing it.
Mutual induction: changing current in one coil induces an emf in a nearby coil.
Transformer: device to increase or decrease an alternating voltage. Step-up: more turns on the secondary. Step-down: fewer turns on the secondary.
Formulas
Vs/Vp = Ns/Np. Ideal transformer: Vp Ip = Vs Is (power in = power out). Transformation ratio = Ns/Np.
Must-know points
Factors affecting the induced emf: strength of magnet, speed of relative motion, number of turns of the coil, area of the coil.
Lenz's law and conservation of energy: when the north pole of a magnet is pushed towards a coil, the coil end becomes north and repels the magnet, so work must be done against this repulsion; that work becomes electrical energy. If induced current helped the motion, energy would be created from nothing, which is impossible.
DC motor (5 marks): parts: armature coil, permanent magnets, split-ring commutator, carbon brushes, battery. Working: current in the coil in the field feels forces (Fleming's left hand rule) on opposite sides, producing a torque; the commutator reverses the current each half turn so the coil keeps rotating the same way.
AC generator: coil rotating in a magnetic field, slip rings and brushes, gives alternating emf. Transformer is ineffective with d.c. because d.c. gives no changing flux.
Transformer working: a.c. in the primary makes a changing magnetic flux in the laminated soft iron core; it links the secondary and induces an emf by mutual induction. Energy changes: electrical to magnetic to electrical. Losses: heat in coils, eddy currents, flux leakage.
High voltage for long-distance transmission: for the same power, higher voltage means lower current, so less I²R heat loss in the cables.
Cathode rays (2025 paper): streams of fast electrons from a heated cathode in a vacuum tube. They travel in straight lines, are deflected by electric field towards the positive plate, are deflected by a magnetic field (perpendicular to field and motion), and cause fluorescence.
CRO components: electron gun (heated filament, grid, anodes), X and Y deflecting plates, fluorescent screen, evacuated glass tube. Waveform is controlled by applying a suitable electric field to the plates.
Burglar alarm (Section C, 2026 Set M): a door or window switch and relay in a circuit; when opened the circuit changes, the relay closes the alarm circuit and the bell sounds. Draw a circuit with battery, switch, relay, bell.
Past-paper questions with model answers
State Lenz's law; how it determines the direction of induced current; why the induced current opposes the change; how it obeys conservation of energy (asked in every set).
Factors affecting the magnitude of induced emf.
Describe d.c. and a.c.; a.c. preferred for domestic use (three reasons).
Step-up and step-down transformers; step-down 240 V to 12 V with 2000 primary turns: Ns = Np × Vs/Vp = 2000 × 12/240 = 100 turns.
Mutual induction in a transformer with diagram; energy transformation in a transformer (3+2).
Electric motor with split-ring and brushes, neat sketch (4+1); construction and working of a DC motor (3+2).
How do cathode rays behave in electric and magnetic fields; components of CRO with diagram (1+4).