Page 27 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current
Answer:
(a) Drift velocity is the average velocity of the electrons when they move
and collide with the positive ions in the opposite direction to the electric
field.
(b)
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E
+ e –
Z A
Q P
v
Let A = cross-sectional area
n = density of free electrons / number of electrons per unit volume
v = drift velocity
In one second, the electrons move from P to Q by a distance of v.
The volume from P to Q = Av
\ The total number of free electrons flowing from P to Q in 1 second
= nAv
The total charge flow from P to Q = nAve
\ Current flow = rate of flow of charge
I = nAve
Exam Tips
Exam Tips
(c) The number of charge carriers per unit
volume remains unchanged. Formulae used:
The drifts velocity increases as the increase V = IR ρl
of potential difference will increase the R = A
Semester
2 current flow in the conductor. v = I
v = I , V = IR nAe
nAe
The drift velocity is directly proportional to the current.

(d) (i) N = m
N A M
N = mN A
M
N = mN A
V VM

= eN A
M
(8.93 × 10 )(6.02 × 10 )
3
23
=
63.5 × 10 –3
= 8.466 × 10 atoms per m 3
28
226

014 STPM Q&A Physics T2.indd 226 17/02/2022 10:12 AM