Page 30 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current

Section B Structured Questions
Question 5
A current of 1.1 A flows in a copper wire of diameter 1.2 mm. The density of
copper is 8.93 × 10 kg m and the relative atomic mass of copper is 63.5.
3
–3
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Assuming that each copper atom contributes one free electron, calculate
3
(a) the number of free electrons in 1.0 m of wire,
(b) the current density in the wire,
(c) the drift velocity of the electrons in the wire.
Answer:
(a) Density = mass Exam Tips
volume Exam Tips
N × molar mass Number of
ρ = N A atoms = Mass
V Avogdro Molar mass
N = ρN A number
V M N = m
M
23
3
= (8.93 × 10 )(6.02 × 10 ) N A
63.5 × 10 –3
= 8.466 × 10 28
\ Number of the electrons per m = 8.466 × 10 28 Semester
3
(b) Cross-sectional area, A = πd 2
4
–3 2
= π(1.2 × 10 ) 2
4
= 1.131 × 10 m 2
–6
Current density, J = I
A
= 1.1
1.131 × 10 –6
= 9.726 × 10 A m –2
5
(c) J = nve
v = J
ne
= 9.726 × 10 5
(8.466 × 10 )(1.6 × 10 )
28
–19
= 7.18 × 10 m s –1
–5

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014 STPM Q&A Physics T2.indd 229 17/02/2022 10:12 AM