Electrical quantities

The mark-scheme form is p.d. ÷ current, $R = V/I$. Writing $R = I/V$, $R = V \times I$ or "resistance ÷ current" scores zero. A definition needs both parts: the ratio of the p.d. across a component to the current through it, AND the unit, the ohm ($\Omega$). "Opposition to current" alone earns nothing. Rearranged: $V = IR$ and $I = V/R$.

Only electrons transfer when two insulators are rubbed — protons stay locked in the nuclei. So an object becomes positively charged because electrons leave it, not because positive charge is added or protons arrive. Charge is conserved: if a polythene rod ends up negative it has gained electrons, and the cloth has lost the same number and is left equally positive.

An ammeter measures current, so it goes in series — in the same loop, with the current it measures flowing through it. A voltmeter measures p.d., so it goes in parallel, across the component, reading between two points. Swapping them is a fatal circuit error and one of the most repeated diagram mistakes.

In a metal the moving charges are the delocalised (free) electrons, drifting through a lattice of positive ions. Naming "ions", "protons" or "the current itself" as the charge carriers is wrong — ions carry charge in an electrolyte, not in a solid metal. Those free electrons make metals and graphite conductors; plastic, rubber, glass and dry air are insulators.

Resistance is proportional to length ($R \propto L$) but inversely proportional to cross-sectional area ($R \propto 1/A$). Doubling the length doubles the resistance; halving the area also doubles it. The common slip is treating a thicker wire as more resistive — a thicker wire has a larger area, so less resistance and a larger current for the same p.d.

Charge ($Q$, coulombs) is the amount of electricity; current ($I$, amperes) is how fast that charge flows past a point — different quantities, different units, linked by $I = Q/t$ (Extended). Writing "a current of 90 coulombs" or "a charge of 3 amperes" loses the mark even when the arithmetic is right, so check the unit matches the quantity named.

e.m.f. is the work done per unit charge by the source — energy supplied to the circuit. p.d. is the energy transferred per unit charge by a component — energy used. Both are energy per unit charge and both are measured in volts, which is exactly why they get muddled; a question asking you to distinguish them wants the source-versus-component contrast.