Current, potential difference and resistanceEdexcel A-Level Physics: Revision notes

Electric current is the rate of flow of charge:

I = Q / t

where Q is the charge in coulombs (C) and t is the time in seconds. The unit of current is the ampere (A); one ampere is one coulomb per second (C s⁻¹). In a metal the charge carriers are electrons, each with a charge of magnitude 1.60 × 10⁻¹⁹ C, so the number of electrons passing a point in time t is Q divided by the charge of one electron.

Worked example. A current of 2.0 A flows for 45 minutes. Q = It = 2.0 × 2700 = 5.4 × 10³ C.

The potential difference (p.d.) between two points is the energy transferred per unit charge as charge moves between them:

V = W / Q

where W is the energy transferred (work done) in joules. The unit is the volt (V); one volt is one joule per coulomb (J C⁻¹). Rearranged, W = QV, and since Q = It, W = VIt.

Worked example. The charger above, at 5.0 V, transfers W = QV = 5.4 × 10³ × 5.0 = 2.7 × 10⁴ J.

The resistance of a component is the ratio of the p.d. across it to the current through it:

R = V / I

The unit is the ohm (Ω); one ohm is one volt per ampere. For a given p.d., a larger resistance gives a smaller current. Resistance is found for a particular p.d. and current: for a component where R changes, calculate it separately for each pair of readings, and do not use the gradient of the graph unless the line is a straight line through the origin.

Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature is constant. A conductor that obeys it is ohmic. For an ohmic conductor, a graph of I against V is a straight line through the origin and R = V / I is constant.

Many components are non-ohmic. A filament lamp has a current-p.d. graph that curves, because the filament gets hotter as the current rises, so its resistance increases. In a metal the ions vibrate with greater amplitude at higher temperature, so electrons collide with them more often.

Connect the component in series with an ammeter, and a voltmeter in parallel across it. Vary the p.d. in steps using a variable power supply (or potential divider) and record the current at each value. Use small currents and allow time to cool so the temperature stays constant. Plot I against V, and calculate R = V / I for each reading.

A straight line through the origin means the component is ohmic. A curve with a changing R means it is not.