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WAEC Physics 2015 Paper

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Question : 40

Total: 50

A wire of cross-sectional area 0.60mm² has a restivity of 7.2 x 10^-7

\Omega

. Calculate the length of wire that will have a resistance of 3

\Omega

.

0.25m
2.50m
3.60m
36.00

Solution:

Concept:

Resistance of a wire depends on its resistivity, length, and cross-sectional area.

Explanation:

Use the formula

R = \frac{\rho L}{A}

.

Here,

R = 3\,\Omega

,

\rho = 7.2 \times 10^{-7}\,\Omega\text{m}

, and

A = 0.60\,\text{mm}^2

.

Convert area to

\text{m}^2

:

A = 0.60 \times 10^{-6}\,\text{m}^2

.

Rearrange the formula to find length:

L = \frac{RA}{\rho}

.

Substitute the values:

L = \frac{3 \times 0.60 \times 10^{-6}}{7.2 \times 10^{-7}}

.

Calculate:

L = \frac{1.8 \times 10^{-6}}{7.2 \times 10^{-7}} = 2.5\,\text{m}

.

This matches option B.

Answer:

B.

2.50\,\text{m}

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