JAMB Physics Past Questions & Answers - Page 258

1,286.

When a resistance R is across a cell, the voltage across the terminals of the cell is reduced to two-third of its normal value. The internal resistance of the cell is

A.

\(\frac{2}{1}\)R

B.

\(\frac{1}{2}\)R

C.

\(\frac{3}{2}\)R

D.

5R

Correct answer is B

IR = \(\frac{2}{3}\) I(R + r)

r = \(\frac{1}{2}\)R

1,287.

A cell of internal resistance 2 ohms supplies current to a 6 ohms resistor. The efficiency of the cell is

A.

12.0%

B.

25.0%

C.

33.3%

D.

75.0%

Correct answer is D

Efficiency = \(\frac{IR}{I(R + r)}\) x 100

= \(\frac{6}{I(2 + 6)}\) x 100

= 75%

1,288.

A spectrum of sunlight is said to be impure when

A.

it is made of only three colours

B.

it has no red colour in it

C.

the different colours in it overlap

D.

it does not contain white light

Correct answer is C

No explanation has been provided for this answer.

1,289.

Which of the following statement is TRUE of ultraviolet, visible and infra-red rays?

A.

they are all electromagnetic waves with the same wavelength

B.

ultra-violet rays have shorter wavelengths than infra-red rays and produce flourescence

C.

infrared rays have shorter wavelength than visible light and produce the sensation of heat

D.

the wavelenghts increase in the sequence: visible, ultra-violet, infra-red

Correct answer is B

No explanation has been provided for this answer.

1,290.

When white light is dispersed by a spectrometer, the component having the shortest wavelength is

A.

orange

B.

green

C.

violet

D.

red

Correct answer is C

No explanation has been provided for this answer.