Page 153 - text book form physics kssm 2020

Chapter 4
Heat
Example 2
What is the amount of heat supplied by a water heater to change 0.75 kg of water at 30°C to
steam at 100°C? State the assumptions you make in your calculations.
–1
−1
3
[Specifi c heat capacity of water, c water = 4.20 × 10 J kg °C ,
−1
6
specifi c latent heat of vaporisation of water, l = 2.26 × 10 J kg ]
v
Solution:
0.75 kg Q = mcΔθ 0.75 kg Q = ml 0.75 kg
2
1
water at 30°C water at 100°C steam at 100°C
Make assumptions:
(i) All heat supplied by the heater is absorbed by the water.
(ii) No loss of heat to the surrounding during heating and change of phase.
Th ere are two stages of change:
(i) increase in water temperature from 30°C to its boiling point of 100°C
(ii) change of phase from water to steam without change in temperature.
Amount of heat supplied, Q = Q + Q
2
1
= mcΔθ + ml
3
6
= [0.75 × 4.2 × 10 × (100 – 30)] + (0.75 × 2.26 × 10 )
6
= 1.92 × 10 J

Formative Practice 4.3
4.3

1. Figure 4.23 shows an electric steamer. Explain how the fi sh
is heated.

Fish
2. What is the amount of heat released when 0.8 kg of water at
25°C cools until it becomes ice at –6°C? State the assumptions
you make in your calculations.
Contain
–1
–1
3
[Specifi c heat capacity of water, c water = 4.2 × 10 J kg °C , water
3
–1
–1
specifi c heat capacity of ice, c = 2.0 × 10 J kg °C and
ice
5
–1
specifi c latent heat of fusion of ice, l = 3.34 × 10 J kg ]
f
Figure 4.23

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4.3.5
4.3.5 147