Physics 9702 · AS & A Level · Electric current

Electric current — practice question

A cell with electromotive force (e.m.f.) $0.48\,\text{V}$ is linked to a metal wire X, as shown in Fig. 6.1. The cell has internal resistance. The current in the cell is $0.80\,\text{A}$. Wire X has length $3.0\,\text{m}$, cross-sectional area $1.3 \times 10^{-7}\,\text{m}^2$ and resistance $0.40\,\Omega$.
(a)[2]

Calculate the charge that flows through the cell in a time of $7.5$ minutes.

(b)[3]

Calculate the percentage efficiency with which the cell supplies power to wire X.

(c(i))[1]

The volume of wire X contains $3.2 \times 10^{22}$ free (conduction) electrons. For wire X, calculate the number density $n$ of the free electrons.

(c(ii))[2]

The volume of wire X contains $3.2 \times 10^{22}$ free (conduction) electrons. For wire X, calculate the average drift speed of the free electrons.

(d)[3]

Wire Y matches wire X in cross-sectional area and is made from the same metal. Wire Y is longer than wire X. Wire X in the circuit is now replaced with wire Y. Assume that wire Y has the same temperature as wire X. State and explain whether the average drift speed of the free electrons in wire Y is greater than, the same as, or less than that in wire X.

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