Page 32 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current

Section C Essay Question
Question 7
A constant current of 2.0 A flows through a metal wire. Its diameter at end P is
smaller than the diameter at end Q as shown in the diagram.
(a) V = IRPelangi Sdn Bhd. All Rights Reserved.

I
P  Q

(a) Calculate the potential difference across PQ if the resistance of the wire is
0.5 Ω.
(b) Compare the
(i) mean drift speeds of the electrons flowing through the cross section
at P and Q,
(ii) current density at P and Q,
(iii) on the axes below, sketch a graph of current density against the distance
along the wire from P.
Current
density

P Q Semester
(c) The current is increased by increasing the potential difference across the
wire. State the effects on the concentration and the drift velocity of the 2
charge carriers. Exam Tips

Answer:
Exam Tips
Penerbitan at P is greater than that at Q. The v = nAe
I
= 2(0.5) = 1.0 V

(b) (i) The mean drift speeds of electrons
1
v ↑∝
A↓
drift velocity increases as the cross-
sectional area decreases.
A P , A Q
\ v P . v Q
(ii) The current density at P is greater

I
than at Q. The current density
increases as the cross-sectional area
J ∝
decreases. J = A I
A
A P , A Q
J P . J Q
231

014 STPM Q&A Physics T2.indd 231 17/02/2022 10:12 AM