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8/10/2019 S.4 Physics -- Measuring the Specific Latent Heat
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Why should crushed ice be used in the experiment?
To increase the surface area of ice and
keep good contact with the heater.Crushed ice should be used because this enhances the
thermal contact between the ice and the heater. Therefore,
less heat is wasted to heat up the air between the ice
cubes.
Would you expect the value of the specific latent heat of fusion
of ice obtained in the experiment to be higher than, equal to,
or lower than the accepted value? Explain briefly.
The value obtained is higher than the accepted value. This is
because some energy supplied by the heater is lost to the
surroundings. The actual energy absorbed by the ice should
be lesser.
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Describe, with the aid of a diagram, how a control set can be
set up to improve the accuracy of the experiment.
An immersion heater is set up without connecting to the
power supply. The actual mass of ice melted by the energy
supplied by the heater is the mass difference between the
masses of water in the two beakers.
Why should the melting ice be used?
Melting ice is used to make sure that the temperature of theice is at 0 C. Hence, no energy is absorbed as the heat
capacity to raise the temperature of the ice to 0 C.
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Why is the control necessary?
This is to account for the ice melted at room temperature.
After switching off the heater, the cups are not removed until
the drip rates become steady. Why?
This is to allow time for the energy from the heater to be
conducted to the ice to melt it (water and ice are poor
conductors of heat).
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The immersion heater should not be switched on unless it is
fully immersed in water. Why?
Otherwise the heater may overheat and gets damaged.
What are the possible sources of error in the experiment?
Sources of Error
Calculated result
higher / lower thanaccepted value?
Steam condensing on the top of the heater
drips back into the beaker.
Higher
Some water spills out from the beaker. LowerEnergy is lost to the surroundings. Higher
Energy from the non-immersed part of the
heater is lost to the surrounding and not
transferred to the water.
Higher
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A student suggests adding a lid on the beaker so as to
improve the accuracy of the experiment.
If the beaker is covered with a lid, the steam will not be able
to escape from the beaker. As a result, water vapour will
condense on the lid and return back to the beaker. This will
lead to a serious experimental error.
Is the value of specific latent heat of vaporization obtained in
the experiment usually higher or lower than the accepted
value? Explain briefly.
Due to the heat loss to the surroundings, the energy recorded
by the kilowatt-hour meter is higher than the energy absorbed
by the water. The value obtained is usually higher than the
accepted value.
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Comment on the statement:
The calculated value of the specific latent heat of vaporization
of water should be lower than the accepted value because
some boiling spills out from the cup.
Although water may spill out from the beaker, the main source
of error is due to the heat lost to the surroundings.As the
energy supplied is always greater than the energy absorbed,
the calculated value would be greater than the accepted value.
Suggest a method to improve the accuracy of the experiment.
Use a polystyrene cup instead of a beaker because
polystyrene is a poor conductor of heat.
Wrapping the beaker with cotton wool.
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