Electromagnetism & AC generation

Year 9 answers

Fluency

Point the right thumb in the direction of conventional current; the fingers curl in the direction of the magnetic field.
(a) Inside a solenoid: strong, nearly uniform field running along the axis. (b) Outside: weaker field that loops from one end to the other like a bar magnet.
E.g. MRI scanners, scrap-metal cranes, relays, doorbells, loudspeakers, electric motors.
A changing magnetic field through a coil induces a voltage (EMF) in that coil; if the circuit is closed, a current flows.
$50$ Hz.
Coal, natural gas, wind, hydro, nuclear (globally), biomass. Any three.

Reasoning

AC reverses direction (at 50 Hz here), DC does not. AC voltage is easy to change with transformers; DC needs electronic converters. AC is used for transmission and mains; DC is used in batteries and electronics.
AC: mains power, home outlets. DC: battery (torch, phone, car), solar panel.
AC can be stepped up to very high voltage with a transformer, reducing $I^2 R$ transmission losses. Electronics need stable low-voltage DC for logic circuits; wall adaptors rectify AC to DC.
An inverter converts DC from solar panels or batteries into AC at mains voltage and frequency, so it can feed household appliances or be exported to the grid.
The alternator produces AC efficiently via rotation; its output is rectified (by diodes) to DC for battery charging and car electronics.

Problem solving

Labelled diagram: magnet N/S poles either side; rectangular coil with axis horizontal; slip rings on the axle; two brushes in contact with slip rings; leads to external circuit. Arrow on one side of the coil shows induced current direction at the instant drawn.
(a) Coal: chemical (coal) -> heat (combustion) -> kinetic (steam turbine) -> electrical (generator). (b) PV: light energy -> electrical (directly, via semiconductor junction).
Voltage doubles (faster change of flux) and the frequency of the AC also doubles.
An electromagnet can be switched on to pick up iron and switched off to drop it; a permanent magnet could not release the load.

Reasoning

For a fixed power $P = VI$ , raising $V$ allows $I$ to be lower. Since $P_{\text{loss}} = I^2 R$ depends on the square of current, halving current cuts losses to one-quarter. That is why transmission uses 275-500 kV, then transformers step voltage down for distribution and use.
Induced voltage roughly doubles from faster rotation (greater rate of change of flux) and doubles again from twice the turns, so $\approx 4$ times greater. (Faraday’s law: induced EMF $\propto N \cdot d\Phi/dt$ .)
Combustion converts only some chemical energy to useful heat (boiler losses, flue-gas losses). Steam turbine: thermodynamic limit — a substantial share of heat must be dumped at the cold end (large). Generator: small resistive/mechanical losses. Transmission: $I^2 R$ losses and transformer losses, typically a few percent. Largest losses are in the heat-to-kinetic stage (Carnot limit) and waste heat at the condenser.
PV: energy from sunlight; capacity factor roughly 15-25% (depends on weather and latitude); low operating impact but land use and manufacturing footprint. Hydro: energy from gravitational potential in water; high capacity factor (often 40-60%) and dispatchable; big ecological impact from dams, displacement, and habitat change.