PAG: Investigating Specific Heat Capacity (OCR GCSE Physics A (Gateway))

PAG: Investigating Specific Heat Capacity

• Two example practical investigations are outlined below, with an equipment list for both

Equipment List

• Resolution of measuring equipment:

  • Thermometer = 0.1 °C

  • Voltmeter = 0.1 V

  • Ammeter = 0.1 A

  • Stopwatch = 1 s

  • Digital balance = 1 g

Experiment 1: Determining the Specific Heat Capacity of Water

Aim of the Experiment

• The aim of this experiment is to determine the specific heat capacity of water by measuring the energy required to increase the temperature of a known amount by 1°C

Method

Apparatus for heating water and measuring energy supplied

1. Place the beaker on the digital balance and press 'tare'

2. Add approximately 250 ml of water and record the mass of the water

3. Place the immersion heater and thermometer in the water

4. Connect up the circuit as shown in the diagram, with the ammeter in series with the power supply and immersion heater, and the voltmeter in parallel with immersion heater

5. Record the initial temperature of the water at time 0 s

6. Turn on the power supply, set at approximately 10 V, and start the stopwatch

7. Record the voltage and current

8. Continue to record the temperature, voltage and current every 60 seconds for 10 minutes

• An example of a results table might look like this:

Analysis of Results

• Calculate the energy supplied each minute using the formula:

  Electrical energy (J) = voltage (V) × current (A) × time (s)

• Calculate the temperature change by subtracting the temperature at time 0 s from the temperature recorded each minute

• Calculate the average mass of the water by adding the mass at the start and the mass at the end and then dividing the total by two

• Plot a graph of the energy supplied (y-axis) against the temperature change multiplied by the average mass (x-axis)

• The gradient of this graph will be the specific heat capacity of the water

• Take the gradient in the straight-line region of the graph:

  • Divide the change on the y-axis between two points on the straight line region, with the change on the x-axis between the same two points

The gradient of the graph is equal to the specific heat capacity of the water, assuming a perfectly efficient immersion heater

Experiment 2: Obtaining a Temperature-Time Graph for Melting Ice

Aim of the Experiment

• The aim of this experiment is to plot a graph of the temperature of ice, against time, as it is heated to water

Method

Apparatus used to heat ice and measure its temperature as it melts

1. Place some ice in a beaker so it is about half-full

2. Place a thermometer in the beaker

3. Place the beaker on a tripod and gauze and slowly start to heat it using a bunsen burner

4. As the beaker is heated, take regular temperature measurements (e.g. at one minute intervals)

5. Continue this whilst the substance changes state (from solid to liquid)

6. The results can then be plotted on a graph

A heating curve will show a flat section whilst the ice is melting

Evaluating the Experiments

Systematic Errors:

• Ensure the digital balance is set to zero before taking measurements of mass

• It may be necessary to determine the efficiency of the immersion heater before experiment 1:

  • The specific heat capacity of water has a known value of 4200 J/kg/°C

  • If the efficiency of the heater is less than 1 then the values obtained for specific heat capacity will be larger than expected

• Remember to only take gradients on the straight-line region

  • Before this point the energy supplied is being used to heat the immersion heater itself

Random Errors:

• Stir the ice water constantly whilst heating in experiment 2

• When the current or voltage values appear to be changing between two values next to one another then be consistent in choosing the higher value

Safety Considerations

• The immersion heater will get very hot

  • Make sure not to touch it, and have a heatproof mat ready to place it on

• Make sure that the immersion heater is connected to a direct current supply

• The beaker may become unstable with an immersion heater and thermometer resting in it

  • If you feel this is the case then use a clamp stand to hold both

• Wear goggle while heating water

• Make sure to stand up during the whole experiment, to react quickly to any spills