Required Practicals: Thermal Physics

AQA A-Level Physics includes several required practicals related to thermal physics. This lesson covers the practical skills, techniques, and analysis needed for the three key experiments: measuring specific heat capacity, verifying Boyle's law, and estimating absolute zero. Strong practical skills are tested in Paper 3 and also appear in context questions on Papers 1 and 2.

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Required Practical: Specific Heat Capacity by Electrical Method

Aim

To determine the specific heat capacity of a solid material (typically aluminium or copper) using an electrical heater.

Apparatus

• Metal block with two holes (one for the heater, one for the thermometer)
• Electric immersion heater
• Thermometer (or temperature sensor with data logger)
• Ammeter and voltmeter (or joulemeter)
• Power supply
• Stopwatch
• Electronic balance
• Insulation (e.g. cotton wool or bubble wrap)
• Small amount of oil (for the heater hole, to improve thermal contact)

Method

1. Measure and record the mass of the metal block (m) using the electronic balance.
2. Place the block on an insulating mat. Wrap the block in insulation to reduce energy losses.
3. Insert the heater into one hole and add a small drop of oil to improve thermal contact.
4. Insert the thermometer into the other hole (also with a drop of oil).
5. Record the initial temperature (θ₁).
6. Switch on the heater. Record the ammeter reading (I) and voltmeter reading (V).
7. Start the stopwatch.
8. Record the temperature at regular intervals (e.g. every 30 seconds) for a set time (e.g. 10 minutes).
9. Switch off the heater.
10. Continue monitoring the temperature — it may continue to rise briefly as heat distributes through the block. Record the maximum temperature reached (θ₂).

Calculation

Energy supplied: E = VIt (or read from joulemeter)

Temperature change: Δθ = θ₂ − θ₁

Specific heat capacity: c = E / (mΔθ) = VIt / (mΔθ)

Worked Example

Question: An aluminium block of mass 1.00 kg is heated for 300 s with a heater running at V = 12.0 V and I = 4.00 A. The temperature rises from 21.0 °C to 37.2 °C. Calculate the specific heat capacity.

Solution:

E = VIt = 12.0 × 4.00 × 300 = 14 400 J

Δθ = 37.2 − 21.0 = 16.2 K

c = E / (mΔθ) = 14 400 / (1.00 × 16.2) = 889 J kg⁻¹ K⁻¹

The accepted value for aluminium is 900 J kg⁻¹ K⁻¹, so this result is within 1.2% — indicating a well-conducted experiment with good insulation.

Sources of Error and Improvements

Source of Error	Type	Effect on c	Improvement
Energy lost to surroundings	Systematic	Measured c is too high (temperature rise is smaller than expected, so c = E/(mΔθ) gives a larger value)	Insulate the block thoroughly
Poor thermal contact	Systematic	Temperature reading lags behind true block temperature	Use oil in thermometer and heater holes
Non-uniform temperature distribution	Random	Temperature reading may not represent the whole block	Allow time for heat to spread; use a data logger to monitor continuously
Reading thermometer before equilibrium	Random	Final temperature may be inaccurate	Continue monitoring after switching off; record the maximum temperature
Assuming all electrical energy goes into heating	Systematic	Overestimates energy used for heating	Difficult to eliminate completely; good insulation minimises the error

Graphical Analysis

If you plot temperature against time, you can use the initial gradient of the line (before significant heat losses occur) to calculate c:

Gradient = Δθ/Δt = P/(mc) (where P = VI is the heater power)

Therefore: c = P / (m × gradient)

This method can give a more accurate result than using the total time and total temperature change, because it uses data from early in the experiment when heat losses are smallest.

Exam Tip: In exam questions about this practical, always discuss heat losses as the main source of systematic error. State that the measured value of c will be higher than the true value because some energy was lost to the surroundings, making the temperature rise smaller than expected.

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Required Practical: Boyle's Law Verification

Aim

To verify that pressure is inversely proportional to volume for a fixed mass of gas at constant temperature.

Apparatus

• Boyle's law apparatus (sealed tube containing a column of gas above oil, connected to a pressure gauge and hand pump)
• Alternatively: a large gas syringe with a pressure sensor
• Ruler or scale for reading the gas column length
• Pressure gauge (Bourdon gauge or digital sensor)
• Thermometer (to check constant temperature)

Method

1. Note the initial volume (or length of gas column) and pressure.
2. Use the hand pump to increase the pressure in small steps.
3. After each increase, wait for the temperature to return to the starting value (compression heats the gas temporarily).
4. Record the pressure (p) and volume (V) at each step.
5. Repeat for at least 6–8 data points over a wide range of pressures.
6. Also take readings while decreasing the pressure (to check for systematic errors such as friction).

Analysis

Method 1: Plot p against 1/V

If Boyle's law holds, p ∝ 1/V, so a plot of p against 1/V should give a straight line through the origin.

Calculate 1/V for each data point and plot the graph.

A straight line through the origin confirms Boyle's law.

Method 2: Plot pV against p

If pV = constant, a plot of pV against p should give a horizontal straight line.

Method 3: Plot log(p) against log(V)

If p ∝ V^n, then log(p) = −n log(V) + constant.

For Boyle's law, n = −1, so the gradient should be −1.

Worked Example — Data Analysis

A student records the following data:

p (×10⁵ Pa)	V (cm³)	1/V (cm⁻³)	pV (×10⁵ Pa cm³)
1.00	50.0	0.0200	50.0
1.25	40.2	0.0249	50.3
1.50	33.5	0.0299	50.3
2.00	25.0	0.0400	50.0
2.50	20.1	0.0498	50.3
3.00	16.8	0.0595	50.4

The pV values are approximately constant (50.0–50.4 × 10⁵ Pa cm³), confirming Boyle's law within experimental uncertainty.

Sources of Error