Sec 4 Physics Current of Electricity notes

Electric current is the rate of flow of charge: I = Q ÷ t, measured in amperes (A) where 1 A means 1 coulomb passing a point each second. Rearranged, Q = I × t gives the charge delivered. A 2.0 A charger running for 30 minutes ships Q = 2.0 × 1800 = 3600 C — about 2.25 × 10²² electrons, since each carries only 1.6 × 10⁻¹⁹ C.

Direction matters. Conventional current is drawn flowing from the positive terminal, through the external circuit, to the negative terminal of the source. The mobile charges in a metal wire are electrons, which drift the opposite way — from − toward +. Both descriptions describe the same physics; exam convention (and the ammeter’s plus-sign) follows the conventional direction.

An ammeter measures current and must be wired in series with the component, so the same current passes through both. Its ideal resistance is tiny so it barely disturbs the circuit.

The electromotive force (e.m.f.) of a source is the work done (energy given) per unit charge driving charge around the complete circuit: 1.5 V means every coulomb leaving the battery carries 1.5 J. When cells are connected in series and aiding, their e.m.f.s add: two fresh 1.5 V cells in series drive each coulomb with 3.0 J.

The potential difference (p.d.) across a component is the work done per unit charge in pushing charge through that component — the energy each coulomb spends there, delivered as heat, light or motion. The distinction: e.m.f. is a source property (energy supplied per coulomb); p.d. is a consumer property (energy converted per coulomb). A voltmeter measures p.d. and connects in parallel with the component, drawing almost no current itself.

Energy bookkeeping: a charge Q driven through a p.d. V converts W = Q × V of electrical energy. A 12 V battery pushing 250 C through a starter motor converts W = 12 × 250 = 3000 J of electrical energy inside the motor.

Resistance measures how hard a component fights the flow: R = V ÷ I, in ohms (Ω) — one ohm lets 1 A through when 1 V drives it. A 12 Ω lamp running at 6.0 V draws I = V ÷ R = 0.50 A. The same lamp driven harder at 12 V and drawing 0.80 A has R = 15 Ω — the resistance grew with temperature, a signature of filaments we meet below.

For a wire of given material, resistance follows two proportions: R ∝ length L (twice as long, twice the toll gates) and R ∝ 1 ÷ cross-sectional area A (twice the area, half the resistance). Diameter matters through the area: doubling the diameter gives 4× the area and ¼ the resistance. Combine both by ratios: double the length and double the diameter gives R × 2 ÷ 4 = half the resistance.

Temperature acts oppositely on metals and most insulators’ semiconductors: in a metal, hotter ions vibrate more and collide with drifting electrons more often, so resistance rises; in a thermistor (semiconductor) higher temperature releases far more charge carriers, so resistance falls — that NTC behaviour is exploited in temperature-sensing circuits.

Ohmic conductors (metals at constant temperature): current is directly proportional to voltage, so the I–V graph is a straight line through the origin with constant gradient 1 ÷ R. Double the voltage, double the current — resistance is a fixed number.

Filament lamp: at first the graph rises almost straight, then bends over (curve gets shallower) as the glowing filament heats up; its resistance climbs with temperature, so successively more voltage buys successively less extra current. The curve is symmetric for both current directions — a lamp does not care about polarity.

Diode: almost no current in reverse (huge resistance) and, in forward direction, negligible current until a small turn-on voltage is reached, then current rises steeply. The diode is a one-way valve for current — the graph is the valve’s spec sheet, and its asymmetric shape is unlike any other component in the syllabus.

An ammeter counts the current through a branch, so it is connected in series — the quantity must pass through the meter. A voltmeter compares energy per coulomb at two points, so it is connected in parallel, across the component. Wire them the wrong way round and the circuit misbehaves: a series voltmeter’s huge resistance strangles the current, while a parallel ammeter’s tiny resistance short-circuits the component — the classic lab short that blows a fuse.

Read meters like a physicist: note the range and the value of one small division before reading, estimate one figure between divisions on an analogue scale, and check the terminal polarity so the needle swings the right way. A digital meter trades parallax for convenience but still deserves a sanity check against the circuit’s expected size.