Revision summary

Charge and current

Charge, Q or q

Unit: coulomb, C; 1 C = 1 A s (one ampere-second). For steady current, Q = It. Convert time to seconds and milliamperes to amperes: 1 mA = 0.001 A.

Current, I

Rate of flow of charge; unit: ampere, A = C/s. I = Q/t gives the average over the stated interval. An ammeter connects in series with the measured path.

Direction in an external metal circuit

Conventional current goes from the discharging cell's positive terminal towards its negative terminal. Electron drift is opposite. A component transfers energy without consuming the charge.

Energy per charge

E.m.f., E

Work done per unit charge by the source in driving charge around the complete circuit; unit: volt, V = J/C.

Potential difference, V

Work done per unit charge in driving charge through a component; also measured in volts. 1 mV = 0.001 V. Work done = p.d. x charge. A voltmeter connects across the component's two terminals.

Ideal sources in series

Choose a reference direction and add signed e.m.f.s. Cross negative to positive: add. Cross positive to negative: subtract. Three 1.5 V cells all aiding give 4.5 V; reversing one gives 1.5 V.

Resistance and wire comparisons

Resistance, R

R = V/I; V = IR; I = V/R. Unit: ohm, Ω = V/A. Use the voltage across and current through the same component at the same operating point.

Wire dimensions

Same material and temperature: R ∝ L and R ∝ 1/A. For a circular cross-section, A ∝ d². Twice the length and twice the diameter give half the resistance.

Measurement

Use suitable meter ranges and d.c. polarities. Keep wire temperature steady in length comparisons and measure the active length between contacts. Contact resistance, heating and diameter variation need specific attention.

Temperature and I-V shapes

• Metallic conductor: increasing temperature increases resistance in the stated range. At constant temperature, its ohmic I-V graph is a straight line through the origin.
• Filament lamp: resistance rises as the filament heats. With I vertical and V horizontal, the curve becomes less steep as voltage magnitude increases.
• Diode: appreciable current flows forward under suitable conditions; reverse current is negligible within the illustrated range.
• Read a point: R = V/I. For a straight I-V line through the origin, gradient = 1/R; a curved graph's tangent gradient is not the same ratio.