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1 Energy
Practical
Energy transfer by heating process
An immersion heater is often used to heat the water in hot water tanks. Sometimes there are two
immersion heaters, one at the bottom of the tank and one near the top. This investigation will help
you to understand why this is a good idea.
Warning – the immersion heater will become hot so to take care in handling.
Method
1 Fill a boiling tube with water to about 2 cm from the top.
2 Clamp two thermometers carefully in the boiling tube. Clamp one
close to the bottom and one with the bulb only 1 cm below the
surface of the water (see figure).
3 Lower a small immersion heater into the water so that it is near but
not touching the lower thermometer.
4 Connect the immersion heater to a power pack in a circuit together
with a variable resistor and an ammeter.
5 Record the temperatures of the lower and upper thermometers in
a suitable table.
6 Turn on the heater and, using the resistor, adjust the current to 1.0 A or less. Record the two
temperatures every minute for the next 5 minutes.
7 Turn off the heater and replace the water with fresh cold water.
8 Reposition the immersion heater so that it is near the top thermometer and repeat the
experiment.
Questions
1 Describe the temperature changes of the water when the immersion heater is:
a) at the bottom
b) at the top of the water.
2 Use your knowledge of convection in liquids to explain why, when the water is heated at the
bottom:
a) the water at the top of the tube gets hotter
b) the water at the bottom gets warmer only slowly.
3 Use your knowledge of convection in liquids to explain why, when the water is heated at the top,
the water at the bottom does not get hot.
4 Which position of the immersion heater produces the highest temperature?
5 Which position of the immersion heater gives the most hot water?
6 If you had a two-heater hot water tank, which immersion heater would you switch on and why:
a) if you wanted a bath
b) if you wanted to do the washing up quickly.
© Hodder & Stoughton Limited 2016