WAEC Physics Past Questions & Answers - Page 232

1,156.
1,157.

The density of water is 1g\(cm^3\) while that of ice is 0.9\(cm^3\), calculate the volume change when 90g of ice is completely melted

A.

0\(cm^3\)

B.

9\(cm^3\)

C.

10\(cm^3\)

D.

90\(cm^3\)

E.

100\(cm^3\)

Correct answer is C

V = \(\frac{m}{p} \, ice volume= 90 cm^3;water volume =100 cm^3\\
Volume change = (100-90) cm^3= 10 cm^3\)

1,158.

The density of a body is \(5\times10^3kgm{^-3}\) and it weighs 1N in air. Calculate the apparent weight of the body when totally immersed in water. (Density of water =\(10^3kgm{-3}, g= 10ms^{-2}\) )

A.

0.02N

B.

0.08N

C.

0.20N

D.

0.80N

E.

1N

Correct answer is D

Upthrust = weight of the water displaced =pvg= 1000 x 2 x \(10^{-5}\)=0.20N \\ Apparent weight = weight in air – upthrust = 1-0.2N=0.8N

1,159.

A light wave travels from air into a medium of refractive index 1.54. If the wavelength of the light in air is \(5.9\times10^{-7}\)m, calculate its wavelength in the medium

A.

\(2.5\times10^{-7}\)m

B.

\(3.8\times10^{-7}\)m

C.

\(4.6\times10^{-7}\)m

D.

\(5.2\times10^{-7}\)m

E.

\(6.1\times10^{-7}\)m

Correct answer is B

n =\(\frac{wavelength\, in\, air}{wavelength\, in \,medium}= \frac{5.9\times10^{-7}}{1.54}= 3.8\times10^{-7}\)m

1,160.

Calculate the time taken to heat 2kg of water from 50°C to 100°C in an electric kettle taking 5A, from a 210V supply. (Specific heat capacity of water = 4200\(kg^{-1}K{-1}\)

A.

2.5 minutes

B.

5.7 minutes

C.

6.7 minutes

D.

7.5 minutes

E.

9.5 minutes

Correct answer is C

t = \(\frac{mc\theta}{p}= \frac{2\times4200\times50}{210\times5} \\ = 400s = 6.7\)