Electrical Quantities: Drift, Ohm, Resistivity and the Real Cell

In the paper

Unit 4 carries 9 of the 50 physics marks, the largest single block in the paper; with seven chapters sharing them, this one averages a little over one (MEC publishes weights by unit, not by chapter). It is also the chapter the rest of the unit is written in. A Kirchhoff loop is Ohm's law applied five times in a row, a potentiometer is nothing but a potential gradient along a wire, and an LRC circuit is Ohm's law with impedance where resistance used to be. Nobody loses marks on impedance because impedance is hard; they lose them because V = IR was never solid.

The MEC scope lines are: Ohm's and Joule's laws; Resistance, emf, potential difference, energy and power. The two headings below are those points, in MEC's order.

Three ways it is asked:

• Recall — the SI unit of resistivity, what happens to the resistance of a semiconductor when it is warmed, the value of one unit of domestic electricity in joules.
• Understanding — why a filament lamp is non-ohmic, why the drift velocity is a fraction of a millimetre per second while a lamp lights the instant the switch closes, why the 60 W bulb is the brighter one when two bulbs are put in series.
• Application — a drift velocity from n, A and I; the resistance of a wire after it has been stretched; the terminal pd of a cell delivering a stated current; the cost of running a heater; the power shared between two bulbs in series.