Thermal Physics Questions

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The specific latent heat of vaporization of a substance is the quantity of energy required to
A.raise the temperature of a unit mass of a substance by one degree Celsius.
B.convert a unit mass of liquid to vapour at constant temperature and pressure.
C.convert a unit mass of solid to vapour at constant temperature and pressure.
D.convert a unit mass of liquid to vapour at a temperature of 100°C and a pressure of one atmosphere.

Thermal energy is transferred through the glass windows of a house mainly by
A.conduction.
B.radiation.
C.conduction and convection.
D.radiation and convection.

Thermal energy is transferred through the glass windows of a house mainly by
A.conduction.
B.radiation.
C.conduction and convection.
D.radiation and convection.

Some students were asked to design and carry out an experiment to determine the specific latent heat of vaporization of water. They set up the apparatus shown below.

The current was switched on and maintained constant using the variable resistor. The readings of the voltmeter and the ammeter were noted. When the water was boiling steadily, the reading of the top-pan balance was taken and, simultaneously, a stopwatch was started. The reading of the top-pan balance was taken again after 200 seconds and then after a further 200 seconds.

The change in reading of the top-pan balance during each 200 second interval was calculated and an average found. The power of the heater was calculated by multiplying together the readings of the voltmeter and the ammeter.

(a)Suggest how the students would know when the water was boiling steadily.
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(1)
(b)Explain why a reading of the mass lost in the first 200 seconds and then a reading of the mass lost in the next 200 second interval were taken, rather than one single reading of the mass lost in 400 seconds.

(2)
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The students repeated the experiment for different powers supplied to the heater. A graph of the power of the heater against the mass of water lost (the change in balance reading) in 200 seconds was plotted. The results are shown below. (Error bars showing the uncertainties in the measurements are not shown.)

(c)(i)On the graph above, draw the best-fit straight line for the data points.
(1)
(ii)Determine the gradient of the line you have drawn.
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