Page 26 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current

(c) v = I = I
nAe πd 2
n 1 4 2 e
\ v ∝ 1
d 2
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v P ∝ 1 .......................a
d P 2
v Q ∝ 1 .......................b

d Q 2
a ÷ b ,
v P = 1 d Q 2 2
v Q d P
d
= 1 2d 2 2

= 1
4
\ v Q = 4v P
The drift velocity at Q is four times that at P.

Section C Essay Question
Question 6 Semester
(a) What do you understand by drift velocity of the charge carries in a metal
wire? 2
(b) When a battery is connected to the ends of a metal conductor of cross-
sectional area A, the charge carriers in the conductor move with drift
velocity v. Thus a current I flows in the conductor. Derive an expression for
the current in terms of drift velocity and the cross-sectional area.
(c) The current in the metal conductor increases when the potential difference
between its ends is increased. Explain what will happen to the number of
charge carriers per unit volume in the conductor and the drift velocity of
the charge carriers.
(d) Copper has a density of 8.93 × 10 kg m and its relative atomic mass is
–3
3
63.5. The density of free electrons in the copper is 8.34 × 10 .
28
(i) Find the number of atoms in 1.0 m of copper.
3
(ii) Find the effective number of free electrons contributed by each copper
atom.

225

014 STPM Q&A Physics T2.indd 225 17/02/2022 10:12 AM