JAMB Physics Past Questions & Answers - Page 387

1,931.

I. for current amplification.

II. for voltage stabilization.

III. for power amplification.

IV. as a switch.

which of the above are the uses of transistor?

A.

I,II,III and IV

B.

I,III, and IV only

C.

I, II and III only

D.

I,II and IV only

Correct answer is A

Transistor is used for amplification of current, power', voltage stabilization and as a switch in logic gates.

1,932.

When a pure semiconductor is heated, its resistance?

A.

increases

B.

decreases

C.

remains the same

D.

increase and then decreases

Correct answer is B

Heating a pure semiconductor increases the mobility of electrons and holes and hence the conductivity. thus, the resistance decreases on heating

1,933.

zener diode is used for?

A.

current amplification

B.

power amplification

C.

voltage regulation

D.

energy conversion

Correct answer is C

Zener diode is useful in stabilized power supply i.e to keep steady the voltage of a power supply.

1,934.

caesium has a work function of 3 x 10-19J. the maximum energy of liberated electrons when it is illuminated by light of frequency 6.7 x 1014

HZ is?

[h = 6.6 x 1010-34Js]

A.

1.42 x 10-19J

B.

3.00 x 10-19J

C.

4.42 x 1010-19J

D.

7.42 x 10-19J

Correct answer is A

Max kinetic energy = Energy of incident electron - work function. EK = hf - w
= 6.6 x 10-34 x 6.7 x 1014 - 3 x 10-19
= 4.422 x 10-19 - 3 x 10-19
= 1.422 x 10-19J

1,935.

A step-down transformer has a power output of 50 W and efficiency of 80%. if the mains supply voltage is 200 v, calculate the primary current of the transformer.

A.

0.31 A

B.

3.20 A

C.

3.40 A

D.

5.00 A

Correct answer is A

Efficiency = (power output)/power input x 100%
= 1s VS x 100% 1p
Vp
∴ (80)/100 = (50)/IP x 200
I P = (50 x 100)/80 x 200 = 0.3125 A