Page 34 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current
Answer: B
Volume is constant,
A 1 l 1 = A 2 l 2
A 1 = l 2
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A 2 l 1
ρl
R = ⇒ R ∝ l
A A
⇒ R 1 ∝ l 1
A 1
R 2 ∝ l 2
A 2
\ R 2 = l 2 × A 1
R 1 A 2 l 1
= l 2 × A 1
l 1 A 2
= 1 2 2
l 2
l 1
R 2 = 1 2 + 0.001 2 2
20 2
R 2 = 20.02 Ω

Question 3 Semester

A hollow copper rod has an inner radius of 0.65 cm and an outer radius of
1.0 cm as shown in the diagram.
P 2
Q
O
0.65 cm
20 cm
1.0 cm
A current flows from P to Q. If the length of the rod is 20 cm and its resistivity
is 1.7 × 10 Ω m, what is the resistance of the copper rod?
–8
A 3.4 × 10 Ω C 1.8 × 10 Ω
–4
–9
B 1.9 × 10 Ω D 2.9 × 10 Ω
–5
–3

Answer: B
ρ l
R =
A
2 2
Cross-sectional area, A = πr 2 – πr 1
= π(1 × 10 ) – π(0.65 × 10 )
–2 2
–2 2
= 1.8143 × 10 m 2
–4
233

014 STPM Q&A Physics T2.indd 233 17/02/2022 10:12 AM