Sec 4 Combined Science (Physics) Charge & Current notes

Electric charge comes in exactly two kinds: positive and negative. The rule between them is simple and absolute: unlike charges attract, like charges repel. Charge is measured in coulombs (C). Electrons carry the negative charge that does the moving in metals; a coulomb is a very large number of electrons' worth — the coulomb is the practical counting unit for circuits.

A battery or power supply acts like a charge pump: it gives charges the energy needed to keep circulating. Everything in this chapter — current, voltage, resistance — is a way of describing how that pumped charge behaves.

Current is the rate of flow of charge — how much charge passes a point each second — measured in amperes (A) with an ammeter connected in series. One ampere means one coulomb passing per second.

Two arrows exist on every circuit diagram. Conventional current — the arrow tradition uses — flows from the + terminal round the circuit to −. The electrons themselves drift the opposite way, from − towards +, because they are negatively charged. Exams accept either, provided you say which you are describing: "electron flow" or "conventional current".

Both e.m.f. (electromotive force) and potential difference (p.d.) are measured in volts (V) with a voltmeter connected in parallel. The e.m.f. of a source is the energy it gives to each unit of charge — the push. The p.d. across a component is the work done per unit charge as the charge passes through it — the energy each coulomb delivers there: 1 volt = 1 joule per coulomb. A 6 V lamp takes 6 J from every coulomb that passes through it.

Round a complete circuit the arithmetic must balance: the energy per coulomb handed out by the source equals the sum of the energies per coulomb delivered across the components. That bookkeeping idea returns in the next chapter (D.C. circuits).

Components oppose the flow: that opposition is resistance R, measured in ohms (Ω) and defined by R = V ÷ I — the p.d. across the component divided by the current through it. Rearrangements: V = IR and I = V ÷ R. A 6 V supply driving 0.5 A through a lamp means the lamp's resistance is 12 Ω at that current.

For a wire of given material at a given temperature: R ∝ length — double the length, double the resistance, because charge fights through twice the obstacles. And R ∝ 1 ÷ cross-sectional area — double the thickness (area), halve the resistance, because a wider road passes more traffic. Hence the memorable shape: long and thin = high resistance; short and thick = low resistance.