WAEC Physics Past Questions & Answers - Page 71

351.

A wire of length 100 cm has a resistance of 10\(\Omega\). If its cross-sectional area is 0.005cm2, determine its resistivity

A.

0.0005\(\Omega\)cm

B.

0.0015\(\Omega\)cm

C.

0.0016\(\Omega\)cm

D.

0.0700\(\Omega\)cm

Correct answer is A

e = \(\frac{RA}{L} = \frac{10 \times 0.005}{100} = 0.0005\)

352.

In the circuit diagram above, E is a battery of negligible internal resistance. If its emf is 9.0V. Calculate the current in the circuit 

A.

1.8A

B.

1.0A

C.

0.8A

D.

0.3A

Correct answer is B

\(\frac{1}{r} = \frac{1}{r_1} + \frac{1}{r_2}\)

= \(\frac{1}{5} + \frac{1}{20}\)

= \(\frac{4 + 1}{20} = \frac{5}{20}\)

r = \(\frac{20}{5}\)

= 4\(\Omega\)

R = R1 + R2

= 5 + 4

= 9\(\Omega\)

but V = IR

I = \(\frac{V}{R} = \frac{9}{9}\)

= 1A

353.

Two identical charges situated 20cm apart in vacuum repel each other with a force of 1.0N.Calculate the value of each other with a force of 1.0N. Calculate the value of each charge. [(14 \(\pi \varepsilon_o)^{-1}\) = 9 x 109Nm2C-2]

A.

21\(\mu C\)

B.

3.6\(\mu C\)

C.

4.0\(\mu C\)

D.

4.4\(\mu C\)

Correct answer is A

F = \(\frac{Q^2}{4 \pi \varepsilon_o r^2}\)

I = 9 x 109 x \(\frac{Q^2}{0.2^2}\)

Q = \(\sqrt{\frac{0.04}{9 \times 10^9}}\)

= 2.11 x 10-6

354.

A 15 \(\mu F\) capacitor is connected to a 240V, 50Hz ac. source. Calculate the reactance of the capacitance [\(\pi = 3.142\)]

A.

16\(\Omega\)

B.

68\(\Omega\)

C.

106\(\Omega\)

D.

212\(\Omega\)

Correct answer is D

\(X_c = \frac{1}{2 \pi fc}\)

= 2 x 3.142 x 50 x 15 x 10-6

= 212.179\(\Omega\)

355.

An electric motor converts

A.

electrical energy to mechanical energy

B.

mechanical energy to electrical energy

C.

mechanical energy to chemical energy

D.

electrical energy to thermal energy

Correct answer is A

No explanation has been provided for this answer.