Objectives:

  1. To understand how to practically determine the specific heat capacity of a substance
  2. To be able to use the equation \Delta E = mc \Delta \theta and understand what each term represents and the units of each term

Introduction 

A practical for specific heat capacity involves measuring the temperature changes of different materials when they are heated . An investigation involves linking the decrease of one store of one energy store to the increase in thermal energy store. As you would expect, the energy transferrer (work done) will cause a temperature to rise.

As you will have learned on the specific heat capacity page, the temperature rise of a material depends on its specific heat capacity. Materials with a low specific heat capacity (a low capacity to store thermal energy) will have a greater temperature increase than those with a high specific heat capacity.

Apparatus required

  • Aluminium block with two holes, one for a thermometer and one for a heater
  • 50 W, 12 V heater
  • 12 V power supply
  • Ammeter
  • Voltmeter
  • 5 leads
  • Stopwatch
  • Balance
  • Thermometer
  • Water
  • Beaker (250 cm3)

Safety precautions

  • Be careful with water around electrical appliances
  • The heating element will get very hot, especially if not inside a metal block. Take care not to burn yourself
  • Damaged equipment should not be used (e.g. bare wires etc.)
  • If you scald yourself with the heater or water then, cool under running cold water immediately for 10 minutes.

Method

  1. Measure the mass of the aluminium block using the balance, if recorded in grams, this should be converted into kilograms.
  2. Place the heater and thermometer into the aluminium block
  3. Connect the heating element up to the rest of the circuit as shown in the circuit diagram below, including an ammeter and voltmeter:
  4. Measure the starting temperature of the metal block (you may need to wait for the thermometer to stop changing first).
  5. Turn the power pack on and up to about 5V, this can be higher for certain heaters (but it will say the maximum on it)
  6. Record the ammeter and voltmeter readings every 60 seconds in a table like that shown further down this page. These values may vary during the experiment, but they shouldn’t do significantly. Whilst recording the ammeter and voltmeter reading, also record the new temperature of the block at each 60s interval.
  7. After about 10 minutes turn off the power supply.
  8. Keep the thermometer in the metal block for a while longer. Record the maximum temperature of the block. The heater will still have some energy after you have turned off the power supply so you want to record any additional temperature rise from this energy.
  9. Calculate the power of the heater using P = IV (Power equals current times voltage) and record the values.
  10. Using the power, calculate the work done using W = Pt (work done equal power times time) for each 60s interval.
  11. Plot your results on a graph, with temperature change (always from time t = 0 \ s ) on the y-axis and energy transferred on the x-axis.

Examples of results tables you should consider using:

Things to consider before experimenting

  • The heating element should fit very snuggly into the metal block, but there may be a small layer of air between the heating element  and the metal block. Add a drop of water before you put the heating element in to improve transfer of energy between the heating element and the metal block.
  • Remember to measure the mass of the metal block. These blocks are usually 1kg, but to make sure your calculations are accurate, you should take an accurate mass measurement.
  • To determine the power of your heater, attach an ammeter in series to the heater and a voltmeter in parallel, record these readings and you can calculate the power using the equation: P = IV (Power = Current x voltage)
  • Make sure you heat the metal block for at least 10 minutes; otherwise you will not be able to draw a graph with a good range of results.
  • Don’t forget to use your graph to find the gradient of the line. You will need this and the mass of the block to work out the specific het capacity  of the metal.

Analysing the results

After drawing you line of best and taking your gradient the specific heat capacity can be found by using the following equation:

\text{specific heat capacity} = \frac{1}{mass \times gradient}

Exemplar graph and results:

****waiting for a good graph to be drawn from a student ****

How to improve results

  • Usually, the value for specific heat capacity found is higher than it should be, this is because more energy is put into the system than that used to heat up the substance. Some energy goes into wasted energy, such as heat loss to the surroundings. To improve the results, an insulation material should be used around the block.
  • If you are trying to determine the specific heat capacity of a liquid, then the liquid should be stirred before each measurement to ensure all the water is the same temperature. Additionally, a lid should be used, since heat rises this is one way thermal energy can be lost to the surroundings.

Further reading: