Live practice

The earth wire connects the metal casing to ground through a low-resistance path. If the live wire touches the casing, fault current flows safely to earth and makes the fuse/MCB disconnect supply. The casing stays near zero potential, reducing electric-shock risk.

Electric power is the rate of energy transfer: \[\begin{aligned}P &= \frac{W}{t}\\&= VI\\&= I^2R\\&= \frac{V^2}{R}\end{aligned}\], depending on the circuit quantities.

(a) Concave: a source at its focus gives a strong parallel beam. (b) Convex: gives an upright diminished image and wide field. (c) Concave: concentrates parallel sunlight at its focus to generate high temperature.

Reproduction replaces individuals that die and transfers the basic body plan to the next generation. Controlled variation means offspring are similar enough to occupy the species' niche but not identical; some can survive environmental change. Birth and death together regulate population size over time.

Plants synthesize hormones that diffuse or are transported through tissues. Auxin, gibberellin and cytokinin coordinate elongation, germination and division; abscisic acid inhibits growth and closes stomata during water stress; ethylene promotes ripening. Unequal hormone distributions also produce directional growth responses.

Kidneys filter nitrogenous wastes and regulate water and ions by producing urine. They are organs of excretion.

Most carbon compounds have high calorific values and release large amounts of heat on combustion. Many ignite at manageable temperatures and, with sufficient oxygen, burn steadily to form mainly carbon dioxide and water. They are also readily available and can be transported or stored as solids, liquids or gases.

Metals above hydrogen in the reactivity series displace it from dilute non-oxidising acids; zinc and iron do so. Metals below hydrogen do not; copper and silver are examples.

The concentrations are unchanged, so the volume of acid required is directly proportional to the volume of base.

Principle. Useful electrical power per unit area equals incident irradiance times efficiency.