JAMB Physics Past Questions & Answers - Page 395

1,971.

A 120 V, 60 W lamp is to be operated on 220 V a.c. the supply mains. calculate the values of non-inductive resistance that would be required to ensure that the lamp is run on current value.

A.

100 Ω

B.

200 Ω

C.

300 Ω

D.

500 Ω

Correct answer is B

Power of the lamp = 60W = (V2)/R and the voltage rate = 120v.
R = (V2)/60 = (120 X 120)/60 = 240 Ω, the fuse wire that should ensure current does not exceed this value should be rated 200 Ω

1,972.

The instrument that measures both a.c and d.c is?

A.

a current balance

B.

a moving current ammeter

C.

a moving iron ammeter

D.

an inverter

Correct answer is C

No explanation has been provided for this answer.

1,973.

A 40 kW electric cable is used to transmit electricity through a resistor of resistance 2.0Ω at 800 V. The power loss as internal energy is?

A.

4.0 x 102W

B.

5.0 x 102W

C.

4.0 x 103W

D.

5.0 x 103W

Correct answer is D

In general, power = IV; 40kw = IV.
∴ 40,000 = 1 x 800
1 = (40.000)/800 = 50A. tune the current through resistor = 50A.
∴ power loss = 12R = 502 X 2 = 2500 X 2 = 5.0 X 103W

1,974.

A conductor has a diameter of 1.00 mm and length 2.00m. if the resistance of the material is 0.1Ω, its resistivity is?

A.

2.55 x 105 Ωm

B.

2.55 x 102 Ωm

C.

3.93 x 10-6 Ωm

D.

3.93 x 10-8 Ωm

Correct answer is D

R = (1)/a, P = (R.a)/1 = (0.1 x 3.14)/2 (10-3/2)3
∴ P = (0.1 x 3.14 x 10-6)/2 x 4
= 0.3925 x 10-7 OR -3.930 X 10-8 Ωm

1,975.

If two charged plates are maintained at a potential difference of 3 kv, the work done in taking a charge of 600 µC across the field is?

A.

0.8 J

B.

1.8 J

C.

9.0 J

D.

18.0 J

Correct answer is B

Work done = product of the charge Q, and the p.d V.
(i)/e work done = Q.V = 600 x 10-6 x 3.0 x 10-3 = 1.8 J.