Specific thermal capacity of aluminium more accurately

Class practical

Measuring the specific thermal capacity (also called the specific heat capacity) of aluminium, including the use of a cooling correction.

Apparatus and Materials

For each group of students

• Immersion heater, 12 V 100 W (older, 60 W types will do)
• Aluminium calorimeter with holes for heater and thermometer (see discussion below)
• Ammeter (0-5 amp)
• Stopwatch or stopclock
• Thermometer
• Voltmeter, 0-15 volt (see discussion below)
• Rheostat (10-15 ohms, rated at 5A or more)
• Top pan balance
• 12 volt supply, i.e. LV power supply with high current smoothing unit

Health & Safety and Technical Notes

Read our standard health & safety guidance

If you drop some paraffin-oil into the thermometer hole it will ensure good thermal contact between the block and the thermometer. It is not necessary to use oil with the immersion heater. In fact, as there is a danger of cracking any oil which is left on the heater when it is removed from the block, it is wiser not to use it.

Procedure

1. Find the mass of the aluminium block using the top pan balance. Connect the immersion heater to the 12-volt supply in series with an ammeter and a rheostat. The immersion heaters are 12 volt, 60 or 100 watt, so adjust the rheostat to give a current of about 4 amps. Switch the heater off.
2. Insert the immersion heater in the aluminium block and place the thermometer into its hole. Before switching on for the experimental run, wait for five minutes before taking the temperature of the block. Switch on the heater and start the clock.
3. The easiest way to measure the temperature rise is to leave the heater switched on until a rise of about 10°C is achieved. Switch off the heater and continue to monitor the temperature until it begins to fall. Note the maximum temperature reached by the block.
4. A more accurate method is to take temperature readings every half minute and to plot a graph of the results both whilst the heater is on and for approximately the same time after the heater is switched off. A cooling correction can be applied to the temperature rise measured, using a standard technique.

Teaching Notes

• The specific thermal capacity can be determined from the relationship: mass x specific thermal capacity x rise in temperature/ time = current x p.d.
• The ratio temperature rise / time can be obtained from the slope of a graph of temperature plotted against time.
• The aluminium blocks can be lagged by enclosing them in foamed polystyrene.
• You can attach the immersion heater to a joulemeter and so measure directly the energy transferred from the electrical supply to the material.
• The choice of power supply makes a lot of difference in this experiment. You will get an accurate result with AC supplies and AC meters because the AC meters have been constructed to read correct values.
• However, most school power supplies providing currents high enough to warm up immersion heaters are not smoothed at all and the DC terminals give voltages which are extremely bumpy. External smoothing units will help.
• If you use moving coil meters, students will need to make a correction for the electrical power calculation.
• A moving coil meter reads time-averaged values, which are (2/π) x peak value.
• So actual power = 1.2 x power calculated from meter readings.
• For further information, see the guidance note:

  Explaining rms voltage and current

This experiment was safety-tested in August 2007