Sec 4 Physics Electromagnetic Induction notes

Connect a coil to a sensitive centre-zero meter and poke a bar magnet into it: the needle kicks while the magnet moves, then returns to zero the instant the magnet stops. Withdraw the magnet and the needle kicks the other way. This is electromagnetic induction: an e.m.f. appears in a circuit whenever the magnetic flux linkage through it changes — by moving the magnet, moving the coil, sliding a wire across field lines, or switching a neighbouring current on and off.

The size rule is Faraday’s law: the induced e.m.f. is proportional to the rate of change of flux linkage. In practice: move the magnet faster, use a stronger magnet, or use a coil with more turns — each multiplies the e.m.f. The critical exam sentence: it is the change that matters. A magnet sitting motionless inside a coil induces nothing at all, however strong it is.

Direction comes from Lenz’s law: the induced current flows so as to oppose the change producing it. Push a north pole towards a coil: the near face becomes a north pole, repelling your magnet — you must do work against it, and that work is exactly the electrical energy delivered (this is energy conservation, and quoting it earns marks). Pull the magnet out and the polarity flips, now pulling the magnet back in. A wire sliding across field lines behaves the same: drag it one way and the induced current feels a force dragging back.

For a straight conductor moving across a field, the right-hand rule gives the induced current direction — note the hand swap from Chapter 10: Motion = thuMb, Field = First finger, Current = seCond finger, all on the right hand. Memory hook: motors are left-handed (you supply current, get motion); generators are right-handed (you supply motion, get current).

Spin a coil in a magnetic field and you force the flux linkage through it to change continuously: an alternating e.m.f. is induced. Each side of the coil cuts the field lines on the way up and on the way down, so the output is sinusoidal — smooth rises and falls, switching direction every half revolution. The slip rings (two full, unbroken rings with brushes) keep the connection to the external circuit while the coil spins, delivering the alternating output — the mirror image of the d.c. motor’s split-ring commutator, which exists to stop the reversal.

Output controls, each straight from Faraday: rotate faster → higher peak e.m.f. and higher frequency; stronger field or more turns → higher peak e.m.f. only. A sketch drawn in words: starting with the coil horizontal (maximum cutting rate) gives maximum e.m.f.; at the vertical (sides sliding along the field, cutting nothing) the e.m.f. is zero. Real power-station alternators are exactly this design, scaled up: coils spun by steam turbines.

A transformer is two coils wound on a laminated soft-iron core. Alternating current in the primary makes a field that is continually changing; the core carries that changing field through the secondary, inducing an alternating e.m.f. in it. Induction with no motion and no wires touching — the changing current does the work. D.c. in the primary induces nothing in the secondary (except for a blip at switch-on and switch-off): steady current, steady field, no change, no e.m.f.

The turns ratio sets the voltage ratio: Vs ÷ Vp = Ns ÷ Np. More secondary turns → step-up; fewer → step-down. For an ideal transformer, IpVp = IsVs: power in equals power out, so stepping the voltage down steps the current up, and vice versa. Energy losses, with their fixes: winding resistance (thick copper wire), eddy currents heating the core (a laminated core — thin insulated sheets — breaks the circulating currents up), and field leakage (a continuous closed core). A real transformer still loses a little energy as heat and sound.

Cable loss is P_loss = I²R — current squared, so current is what kills. Send 24 kW down a 2.0 Ω cable at 240 V: the current is 24 000 ÷ 240 = 100 A, and the loss is 100² × 2.0 = 20 kW — over 80% of the power, wasted as hot cable. Send the same power at 24 kV: current 1.0 A, loss 2.0 W. Same cable, same power delivered, ten-thousand-fold less loss — that square is the whole argument.

The grid does exactly this: a step-up transformer raises the generator’s output to transmission voltage (hundreds of kilovolts), the current shrinks, and the cables waste almost nothing; near the customer, step-down transformers bring the voltage back to safe levels in stages. Explain any transmission question with the same three beats: P = VI fixes I for a given P; loss = I²R grows with I; transformers make high V cheap — and, crucially, transformers only work on a.c., which is the real reason the grid runs on alternating current.