JAMB Physics Past Questions & Answers - Page 423

2,111.

A generator manufacturing company was contracted to produce an a.c dynamo but inadvertently produced a d.c dynamo. To correct this error, the

A.

armature coil should be made of silver

B.

commutator should be replaced with slip ring

C.

commutator should be replaced with split rings

D.

armature coil should be made of aluminium

Correct answer is B

No explanation has been provided for this answer.

2,112.
2,113.

The major difference between a pure semiconductor and a pure metal is that

A.

metals are harder than semiconductor

B.

while resistance of metals decrease with temperature , the reverse is the case for semiconductor

C.

the resistance of metals increases with temperature, while for semiconductors, it is the reverse

D.

metals have forbidden gaps while semiconductors have not

Correct answer is C

Experiment shows that increase in temperature increases the resistance of a pure metal but decreases the resistance of pure semiconductor

2,114.

In a semi conductor junction diode, as the depletion or barrier layer is forward-biased, the layer

A.

widens

B.

narrows

C.

remains constant

D.

widens then narrows

Correct answer is C

No explanation has been provided for this answer.

2,115.

When an alternating current given by I = 10sin (120π)t passes through a 12Ω resistor, the power dissipated in the resistor is

A.

1200W

B.

600W

C.

120W

D.

30W

Correct answer is A

An a.c current is usually expressed as I = Io sin wt; and comparing this with the given current I = 10sin (120π)t, => Io = 10.
and power in an a.c circuit is given as:
P = I2R
= (102 x 12)
= 1200W