Page 22 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current
Question 3

A neon lamp is connected to a battery. If a constant current of 1.25 A flows to
the lamp, how long will it take for a charge of 15 C to flow through the lamp?
A 8.3 s C 16 s
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B 12 s D 19 s

Answer: B
Q = It
t = Q
I
= 15
1.25
= 12 s

Section B Structured Questions
Question 4
(a) Free electrons are charge carries in metals. What is meant by free electrons?
(b) Explain why the random motion of free electrons does not produce a
current. Semester
(c) When a metal conductor is connected to a battery, a current flows in the
circuit. Explain the mechanism of conduction of electricity in metals.
2
Answer:
(a) Free electrons are electrons that are not used in bonding. They move
randomly throughout the metal. There is no net flow in any specific
direction.
(b) When there is no electric field applied to a conductor, the free electrons
move randomly. There is no net flow of electrons in any specific direction.
Hence the average velocity of the free electrons is zero. Thus there is no
net flow of charge and the current is zero.
(c) When a battery is connected across the ends of a metal conductor, an
electric field is set up. Electrons are negatively charged, so they drift in the
opposite direction to the field. The free electrons are now accelerated by
the field. They drift in a particular direction from the low potential end to
the high potential end. This produces the electric current.

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014 STPM Q&A Physics T2.indd 221 17/02/2022 10:12 AM