Page 35 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current
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R = (1.7 × 10 )(20 × 10 )

1.8143 × 10 –4
= 1.87 × 10 Ω
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Question 4
The conductivity of a copper wire decrease as the temperature increases because
A the number of free electrons decreases.
B the lattice atoms vibrate with smaller amplitude.
C the drift velocity of the electrons decreases.
D the mean time between collision decreases.

Answer: D

Exam Tips
Exam Tips
Copper wire has a constant number of free electrons.
When the temperature increases, the lattice atoms vibrate with greater amplitude.
Hence the mean time between collision, τ decreases because the mean free path
of the free electrons decreases.
2
ne τ↓
↓σ =
m
Section B Structured Questions
Question 5
Semester
2 A metal rod consists of a copper sheath with an iron core as shown in the
diagram.
Sheath

Core

The diameter of the sheath is 4 mm and the diameter of the core is 3 mm. The
length of the rod is 50 cm. If the potential difference applied across the ends of
the rod is 12 mV, calculate the
(a) resistance of the copper shealth,
(b) resistance of the iron core,
(c) total resistance of the metal rod,
(d) current flow in the metal rod.
[Resistivity of copper = 1.7 × 10 Ω m, resistivity of iron = 1.0 × 10 Ω m]
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014 STPM Q&A Physics T2.indd 234 17/02/2022 10:12 AM