Page 123 - Physics Form 5 KSSM_Neat

Solving Problems Involving Wire Resistance LET’S ANSWER CHAPTER 3
LET’S ANSWER
Example 1 Electricity

The resistivity of constantan is 49 × 10 W m. Calculate the resistance of a
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constantan wire with a length of 50.0 cm and a diameter of 0.6 mm.
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Solution ly/3jgPsJU

Step 1: Step 2: Step 3: Step 4:
Identify the Identify the Identify the formula Solve the problem
problem information given that can be used numerically

1 Constantan wire resistance, R 3 Cross-sectional area of wire, A = πr 2
rl
Resistance, R =
2 Resistivity, ρ = 49 × 10 W m A
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Length of wire, l = 50.0 cm
= 0.5 m 4 A = π(3 × 10 ) m 2
–4 2
Diameter of wire, d = 0.6 mm (49 × 10 )(0.5)
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0.6 × 10 m R = π(3 × 10 )
–3
–4 2
Radius of wire, r =
2 = 0.867 W
= 3 × 10 m
–4
Example 2

Azwan is an electrical wiring contractor in Taman Kota Puteri. He found that a coil of pure
copper wire with a length of 500 m and a radius of 0.5 mm has a resistance of 10.8 W. What is
the resistivity of copper?

Solution

Resistance of wire, R = 10.8 W Resistivity of copper, r = RA
l
Length of wire, l = 500 m Rπr 2
Radius of wire, r = 0.5 mm = l
−3
= 0.5 × 10 m (10.8)π(0.5 × 10 )
–3 2
=
500
= 1.696 × 10 W m
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Formative Practice 3.2

1. List the factors that affect the resistance of a wire.
2. Calculate the total resistance of a coil of copper wire with a length of 50.0 m
and a cross-sectional area of 2.5 mm , given that the resistivity of copper at a
2
temperature of 20°C is 1.72 × 10 W m.
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LS 3.2.6 113