Required practical - Specific heat capacity Revision Notes for AQA GCSE Physics Combined Science

Required practical - Specific heat capacity

What is this practical about?

This practical lets you work out the specific heat capacity of water using an electric heater. You'll also explore what happens when ice melts and changes state.

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Understanding Specific Heat Capacity

Specific heat capacity means the amount of energy needed to heat up 1 kg of a substance by 1°C. This is a fundamental property that varies between different materials.

Water has a particularly high specific heat capacity of about 4200 J/kg°C, which is why it's so good at storing thermal energy and why it takes a lot of energy to heat up water compared to other substances.

Aim of the experiment

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Experimental Objectives

You want to find out:

The specific heat capacity of water through direct measurement
How ice behaves when it melts (changes state) and the energy changes involved

What you need (apparatus)

The following equipment is essential for this investigation:

Water (the substance being tested)
Thermometer (for temperature measurements)
Electric heater (energy source)
Power supply (to control voltage and current)
Insulation material (to minimise heat loss)
Beaker (to contain the water)
Electronic balance (for precise mass measurements)

Method - step by step

Step 1: Set up your equipment like the diagram shows. Put the electric heater in the beaker with water, ensuring it's fully submerged.

Step 2: Use the electronic balance to measure the mass (m) of the water accurately. Record this value as it's crucial for your calculations.

Step 3: Record the voltage (V) from the power supply and the current (I) going through the heater. These determine the power input.

Step 4: Take temperature readings every 30 seconds until the water reaches your target temperature. Write down the starting and ending temperatures to find the temperature change (ΔΘ).

Important tips for accuracy

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Critical Points for Accurate Results

Use insulation around the beaker. This stops heat escaping to the surroundings and gives you better results by ensuring most energy goes into heating the water.
Read the thermometer at eye level. This avoids parallax errors from looking at the wrong angle, which can significantly affect your temperature readings.
Take readings regularly at equal time intervals. Make sure the thermometer stays in the middle of the liquid for consistent measurements.
Stir the water gently to ensure even temperature distribution throughout the sample.

Working out the results

Plot a graph of temperature against time to visualise the heating pattern. Then use this fundamental formula to calculate specific heat capacity:

$c = \frac{V \times I \times t}{m \times \Delta\Theta}$

Where:

$c$ = specific heat capacity (J/kg°C)
$V$ = voltage (V)
$I$ = current (A)
$t$ = time (s)
$m$ = mass of water (kg)
$\Delta\Theta$ = temperature change (°C)

The product $V \times I \times t$ gives you the total electrical energy transferred to the water.

What happens to ice?

When ice melts, it changes state from solid to liquid through a fascinating process. The interesting thing is that during melting, the temperature stays constant even though you're continuously adding energy.

All the energy goes into breaking the bonds between water molecules rather than increasing their kinetic energy. This energy is called the latent heat of fusion and represents the energy required to change the state without changing temperature.

Example calculation

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Worked Example: Calculating Specific Heat Capacity

If you heat 0.5 kg of liquid and the heater transfers 28,800 J of energy, and the temperature rises from 20°C to 50°C:

Step 1: Identify the known values

Mass (m) = 0.5 kg
Energy transferred = 28,800 J
Initial temperature = 20°C
Final temperature = 50°C

Step 2: Calculate temperature change

Temperature change (ΔΘ) = 50°C - 20°C = 30°C

Step 3: Apply the formula

Specific heat capacity = Energy ÷ (mass × temperature change)
c = 28,800 ÷ (0.5 × 30) = 28,800 ÷ 15 = 1,920 J/kg°C

Key takeaways

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Key Points to Remember:

Specific heat capacity is the energy needed to heat 1 kg of a substance by 1°C
Water's specific heat capacity is approximately 4,200 J/kg°C, making it an excellent thermal energy store
Use insulation to get accurate results by reducing heat loss to surroundings
Formula: $c = \frac{V \times I \times t}{m \times \Delta\Theta}$
During melting, temperature stays constant even when adding energy due to latent heat
Accurate measurements of mass, temperature, voltage, and current are essential for reliable results

Required practical - Specific heat capacity (AQA GCSE Physics Combined Science): Revision Notes