AS Daily A Level Physics question
In a lab test, a bench power supply is set to 6.0 V and connected to a resistor using long, thin leads. The current is 0.50 A. A voltmeter across just the resistor reads 5.8 V. Which statement must be true about the energy transfer per coulomb in this circuit?
Answer
The correct answer is B.
Correct: B — Each coulomb gains 6.0 J from the source; 5.8 J is delivered to the resistor and 0.2 J is dissipated in the leads. A confuses source e.m.f. with the terminal p.d. across the load: the supply provides 6.0 J per coulomb, not 5.8 J. B is correct because e.m.f. is the energy per coulomb supplied; the 0.2 V drop in the leads means 0.2 J per coulomb is dissipated there. C invents energy gain in the supply; the 0.2 J per coulomb is not stored but lost as heating in the leads indicated by their voltage drop. D ignores the measured 0.2 V across the leads; a non-zero p.d. across them means energy per coulomb is dissipated in the leads, not all in the resistor.