Chapter 21: Thermal physics
2 This diagram shows a simple method to estimate the specific latent heat of vaporisation of water, L:
insulation (shown partly removed)
wattmeter
water heater balance g
A beaker is filled with hot water and placed on a balance. It is then heated with an electrical heater. The thermometer indicates the temperature of the water. a Explain what is meant by the specific latent heat of vaporisation of a liquid.
[2]
b The heater has a power rating of 25 W. Calculate the energy it supplies to the water in 500 s.
[1]
The water boils and the mass recorded on the balance gradually decreases. This table shows two measurements of the mass separated by a time interval of 500 s: Time / s
Mass / g
0
131.36
500
127.05
143
c Use this data to estimate a value for the specific latent heat of vaporisation of water L.
[2]
d Most of the energy supplied by the heater results in vaporisation of the water. However, some energy is lost to the surroundings and does not contribute to the vaporisation of the water. Suggest one way in which energy is lost to the surroundings.
[1]
e Explain whether energy losses to the surroundings mean that your estimated value for L in part c above is an under-estimate or an over-estimate.
[2]