Energy changes in systems Revision Notes for AQA GCSE Physics

Energy changes in systems

What is specific heat capacity?

Specific heat capacity is the amount of thermal energy needed to raise the temperature of 1 kg of a material by 1°C.

Different materials need different amounts of energy to heat up. For example, water needs much more energy to warm up than metal does. This is why water takes longer to heat up on the stove than a metal pan.

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Understanding specific heat capacity helps explain why different materials heat up at different rates. This concept is crucial for understanding thermal energy transfer in everyday situations.

Water has a high specific heat capacity of 4200 J/kg°C. This means you need 4200 J of energy to heat up 1 kg of water by just 1°C.

The energy equation

To calculate how much thermal energy is needed to change the temperature of a material, we use this equation:

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$\Delta E = mc\Delta\theta$

Where:

$\Delta E$ = change in thermal energy (in joules, J)
$m$ = mass of the material (in kg)
$c$ = specific heat capacity (in J/kg°C)
$\Delta\theta$ = change in temperature (in °C)

Common specific heat capacity values

You should remember these typical values:

Material	Specific heat capacity (J/kg°C)
Air	1005
Water	4200
Aluminium	900
Concrete	880

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Water has the highest value, which is why it's excellent for heating systems in homes and why coastal areas have more moderate temperatures than inland areas.

How to find specific heat capacity

Scientists can find the specific heat capacity of a material using electrical heating. They:

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Experimental Method:

Use a heater to supply a known amount of electrical energy to a metal block
Measure the temperature change with a thermometer
Use insulation to reduce energy loss to surroundings
Calculate the specific heat capacity using the rearranged equation: $c = \frac{\Delta E}{m \times \Delta\theta}$

Worked example

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Worked Example: Calculating Thermal Energy

Problem: A 0.8 kg metal block with specific heat capacity 900 J/kg°C is heated from 20°C to 80°C. How much thermal energy is needed?

Solution:

$m = 0.8$ kg
$c = 900$ J/kg°C
$\Delta\theta = 80°C - 20°C = 60°C$

Using $\Delta E = mc\Delta\theta$ : $\Delta E = 0.8 \times 900 \times 60 = \text{:success[43,200 J]}$

Remember!

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Key Points to Remember:

Specific heat capacity is the energy needed to heat 1 kg of material by 1°C
Water has the highest specific heat capacity at 4200 J/kg°C
Use $\Delta E = mc\Delta\theta$ to calculate energy changes when temperature changes
Different materials need different amounts of energy to heat up by the same amount
Always check your units - mass in kg, temperature change in °C, energy in J

Energy changes in systems (AQA GCSE Physics): Revision Notes