Magnetism and Electronics
Magnetism and Electronics covers magnetic forces, electromagnetic induction, transformers and the basic semiconductor and logic content of the syllabus.
Magnetic forces and flux
A straight wire carrying current I in a field B experiences F = BIL sin θ, maximum when the wire is perpendicular to the field; a moving charge experiences F = Bqv sin θ. Direction follows Fleming's left-hand rule. Magnetic flux Φ = BA cos θ; flux linkage = NΦ. Faraday's law gives induced EMF = −N dΦ/dt, and Lenz's law says the induced current opposes the change producing it.
Transformers
For an ideal transformer, Vs/Vp = Ns/Np and VpIp = VsIs, so Ip/Is = Ns/Np. Step-down transformers have fewer secondary turns.
Electronics
A silicon diode conducts when forward biased above about 0.7 V and blocks reverse current; one diode gives half-wave rectification, four in a bridge give full-wave. A capacitor in parallel with the load smooths the output. Logic gates: AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts; NAND is AND followed by NOT.
Worked example
A transformer has 1200 turns on the 240 V primary and 60 turns on the secondary. Vs = 240 × 60/1200 = 12 V. If the secondary supplies 2.0 A, an ideal transformer draws Ip = VsIs/Vp = (12 × 2.0)/240 = 0.10 A.
Traps
Using peak values where rms is required; treating a real transformer as ideal and ignoring efficiency; reversing the turns ratio; omitting the negative sign that expresses Lenz's law; assuming a diode conducts in both directions.