Question 2
The diagram shows a
circuit containing three resistors in parallel.
The battery has e.m.f.
12 V and negligible internal resistance. The ammeter reading is 3.2 A.
What is the resistance
of X?
A 2.1 Ω B 4.6 Ω C 6.0 Ω D 15 Ω
Reference: Past Exam Paper – November 2008 Paper 1 Q35
Solution:
Answer:
D.
For a parallel
combination, the p.d. across each branch is equal and in this case, the p.d.
across each branch is equal to the e.m.f. of 12 V.
To know the resistance of
X, we need to know the p.d. across it (= 12V) and the current flowing through
it (unknown).
The current of 3.2A coming
from the terminal of the battery splits at the junction.
Current across the 10Ω resistor = V/R = 12/10 = 1.2 A
Current through X = 3.2 –
1.2 – 1.2 = 0.8 A
From Ohm’s law, Resistance
of X = V/I = 12 / 0.8 = 15 Ω
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