JAMB Physics Past Questions & Answers - Page 228

1,136.
1,137.

An electric kettle, connected to a 240V mains, produces 6.0 x 105J of heat energy to boil a quantity of water in 5 minutes. Find the resistance of the kettle.

A.

14.4\(\Omega\)

B.

28.8\(\Omega\)

C.

144\(\Omega\)

D.

288\(\Omega\)

E.

2880\(\Omega\)

Correct answer is B

Heat energy = Electrical energy

6 x 105 = \(\frac{V^2}{R}\)t

R = \(\frac{240 \times 240 \times 5 \times 60}{6 \times 105}\)

= 28.8\(\Omega\)

1,138.

A wire P has half the diameter and half the length of a wire Q of similar material. The ratio of the resistance of P to that of Q is

A.

8 : 1

B.

4 : 1

C.

2 : 1

D.

1 : 1

E.

1 : 4

Correct answer is C

Resistance \(\alpha\) \(\frac{length}{area}\)

\(\frac{R_Q}{R_P}\) = \(\frac{L_QA_P}{L_PA_Q}\)

LQ = 2LP

DQ = 2DP

AP = \(\pi\)\(\frac{(2D_P)^2}{2}\)

= \(\pi\)DP

AQ = \(\pi\)\(\frac{(D_P)^2}{2}\)

= \(\frac{1}{4}\)\(\pi\)DP

therefore, \(\frac{R_Q}{R_P}\) = \(\frac{2L_P \times \pi D_P}{L_P \times \frac{1}{4} \pi D_P}\)

= 2 : 1

1,139.

Which of the following will convert a milliammeter to a voltmeter?

A.

low series resistance

B.

low parallel resistance

C.

high series resistance

D.

high parallel resistance

E.

parallel resistance equal to milliammeter resistance

Correct answer is C

No explanation has been provided for this answer.

1,140.

A cell gives a current of 0.15A through a resistance of 8\(\Omega\) and 0.3A. When the resistance is changed to 3\(\Omega\) the internal resistance of the cell is

A.

0.05\(\Omega\)

B.

1.00\(\Omega\)

C.

1.50\(\Omega\)

D.

2.00\(\Omega\)

E.

2.50\(\Omega\)

Correct answer is D

The E.M.F of a circuit is given by 

\(E=I(R+r)\)

Hence, E = 0.15(8+r)  = 0.3(3+r)

1.2 + 0.15r = 0.9 + 0.3r

1.2 - 0.9 = 0.3r - 0.15r

0.3 = 0.15r

r = \(\frac{0.3}{0.15}\)

r = 2\(\Omega\)