Page 28 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current
(ii) In 1.0 m of copper, 8.466 × 10 atoms of copper contribute
3
28
8.34 × 10 free electrons.
28
8.466 × 10 atoms ⎯→ 8.34 × 10 free electrons
28
28
8.34 × 10 28
\ 1 atom ⎯→
8.466 × 10 28
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= 0.985 electrons
\ Each copper atom contributes 0.985 free electron.

14.3 Current Density

Section A Multiple-choice Questions

Question 1
A current of 3.0 A flows in an aluminium wire of cross-sectional area
2.5 × 10 m . What is the current density in the wire?
2
–6
A 1.2 × 10 A m C 7.5 × 10 A m –2
–2
6
6
–2
6
6
B 1.5 × 10 A m D 8.3 × 10 A m –2
Answer: A
Current density, J = I = 3 Semester
A 2.5 × 10 –6
6
= 1.2 × 10 A m
–2
2
Question 2
There are 6.23 × 10 free electrons per cm in a wire. The average drift velocity
3
24
of the electrons is 1.1 × 10 m s . Calculate the current density in the wire.
–1
–3
A 1.0 × 10 A m C 2.1 × 10 A m –2
–2
9
3
B 6.9 × 10 A m –2 D 1.1 × 10 A m –2
9
6
Answer: D
J = nve
6.23 × 10
= 1 1 × 10 m 24 3 2 (1.1 × 10 m s ) (1.6 × 10 C)
–3
–19
–1
–6
Exam Tips
= 1.1 × 10 C s m –2 Exam Tips
–1
9
I
= 1.1 × 10 A m –2 J = = nAve = nve
9
A A
n = density of electrons
v = drift velocity
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014 STPM Q&A Physics T2.indd 227 17/02/2022 10:12 AM