State what is meant by Ohm’s law.
A battery with electromotive force (e.m.f.) 6.2 V and negligible internal resistance is joined in a circuit to a uniform resistance wire, a voltmeter, a fixed resistor and a switch, as shown in Fig. 7.1. The resistance wire has resistance 18 Ω, length 0.94 m and cross-sectional area 7.2 × 10^{-8} m^2. The slider S is placed half-way along the length of the wire.
Calculate the resistivity \rho of the material from which the resistance wire is made.
The switch is open. State the reading shown by the voltmeter.
The switch is now closed. State whether there is an increase, decrease or no change to: • the current in the battery • the voltmeter reading.
The switch stays closed. The slider S is moved along the resistance wire so that the voltmeter reading is 3.1 V. On Fig. 7.1, draw a cross (×) on the resistance wire to indicate one possible new slider position.
In (b), the circuit is changed by replacing the battery with one of a different e.m.f. The switch is open. A student notes the following data for the resistance wire: current in the wire: 0.93 A mean drift speed of charge carriers: 1.3 × 10^{-3} m s^{-1} number density of charge carriers: 9.0 × 10^{28} m^{-3}.
Determine the charge q of a charge carrier in the wire suggested by these data.
Using the value of q, explain why the data recorded by the student cannot be correct.