Page 29 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current
Question 3

A charge of 60 Coulomb flows in a wire for 45 minutes. If the diameter of the
wire is 1.0 mm, what is the current density in the wire?
A 2.2 × 10 A m C 7.6 × 10 A m –2
–2
7
4
4
–2
B 2.8 × 10 A m D 5.6 × 10 A m –2
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8
Answer: B
Exam Tips
Q 60 C Exam Tips
I = =
t 45 × 60 s
= 2.222 × 10 C s –1 Unit of current is Ampere (A) or
–2
Coulomb per second (C s ).
–1
Area = πd 2

4
–3 2
= π(1 × 10 )
4
= 7.8540 × 10 m 2
–7
Current density, J = I
A
2.222 × 10 –2
=
7.854 × 10 –7
= 2.83 × 10 A m –2
4
Semester
2 Question 4
A current flows in a copper wire. The current density in the wire is 1.4 × 10
6
28
A m . If the number of free electrons in 1 m of wire is 8.4 × 10 , what is the
3
–2
drift velocity of the electrons?
–4
A 1.04 × 10 m s
–1
B 3.10 × 10 m s –1
–4
C 1.67 × 10 m s
–3
–1
–3
D 1.90 × 10 m s –1
Answer: A
J = nve
1.4 × 10 = (8.4 × 10 ) v (1.6 × 10 )
6
28
–19
v = 1.04 × 10 m s –1
–4
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014 STPM Q&A Physics T2.indd 228 17/02/2022 10:12 AM