A-LevelPhysicsElectricity9702/23 · May/June 2020 · Q66 Marks
The current I in a metal wire is given by the expression I = Anve where v is the average drift speed of the free electrons in the wire and e is the elementary charge.
- (a)State what is meant by the symbols A and n. A: n:[2]
- (b)Use the above expression to determine the SI base units of e. Show your working.[2]
- (c)Two lamps P and Q are connected in series to a battery, as shown in Fig. 6.1. [Figure 6.1] The radius of the filament wire of lamp P is twice the radius of the filament wire of lamp Q. The filament wires are made of metals with the same value of n. Calculate the ratio average drift speed of free electrons in filament wire of P average drift speed of free electrons in filament wire of Q[2]
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Mark scheme
Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.
- [B1] A: cross-sectional area
- [B1] n: number density of free electrons
- [B1] units of I: A and units of A: m² and units of v: m s⁻¹
- [A1] units of e: A/(m² m⁻³ m s⁻¹) = A s
- [C1] ratio = A_Q / A_P
- [A1] = [πr²] / [π(2r)²] = 0.25
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