Page 25 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current

Section B Structured Questions
Question 5
A current flows in a wire of length L and radius r with the free electrons travelling
–1
at a drift velocity of 6.25 × 10 m s for 1.0 minute.
–3
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(a) If the number of free electrons in 1.0 m of wire is 5.0 × 10 , calculate the
19
3
current flow in the circuit.
(b) Calculate the drift velocity for the same current in a wire of the same
1
materials but with radius r.
2
(c) The wire is then replaced with a wire of the same length that tapers
uniformly from end P to end Q as shown in the diagram.
P Q

The diameter at P is twice that at Q. Compare the drift velocity of the
conduction electrons at P and at Q.

Answer:

(a) N = 5.0 × 10 N = 5.0 × 10 19
19
A
Total charge = Ne
= (5 × 10 )(1.6 × 10 )
19
–19
= 8.0 C
Q
Current, I =
Semester
2 t
= 8 = 0.13 A
1 × 60
(b) v = I = I
nAe nπr e
2
v ∝ 1
r 2
= 2
v 2 r 1 2
v 1 1 r 2
v 2 = r 2
1
6.25 × 10 –3 1 2
r
2
= 4
v 2 = 4(6.25 × 10 )
–3
= 0.025 m s –1

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014 STPM Q&A Physics T2.indd 224 17/02/2022 10:12 AM