IB Physics SL B.5.9 Current and Charge Flow

Practise calculating current as charge flow per time and extending it to carrier density, charge, cross-sectional area and drift speed for metals, liquids and moving charged…

Exam points

calculate charge or carrier number from Q = It, using the elementary charge for electrons
apply I = nqAvd with carrier density, charge, area and drift speed in consistent SI units
add conventional-current contributions from positive and negative carriers moving in opposite directions

[Maximum number: 1]

In an experiment a student constructs the circuit shown in the diagram. The ammeter and the voltmeter are assumed to be ideal.

Figure for Question B.5.9—Current and charge flow question 1 — IB Physics SL

A connecting wire in the circuit has a radius of 1.2 mm and the current in it is 3.5 A . The number of electrons per unit volume of the wire is $2.4 \times 10^{28} \mathrm{~m}^{-3}$ . Show that the drift speed of the electrons in the wire is $2.0 \times 10^{-4} \mathrm{~ms}^{-1}$ .

$3.5=2.4 \times 10^{28} \times \pi\left(1.2 \times 10^{-3}\right)^{2} \times 1.6 \times 10^{-19} \times v\left\|\Rightarrow v=2.0 \times 10^{-4} \mathrm{~ms}^{-1}\right\| v$

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B.5.9—Current and charge flow question 1