Internal energy (AQA GCSE Physics): Revision Notes
Internal energy
What is internal energy?
Internal energy is all the energy stored inside a system by its tiny particles. Every object around you has internal energy because its particles are constantly moving and positioned in different ways.
Think of internal energy as having two main parts:
-
Kinetic energy - This comes from particles moving around. The faster they move, the more kinetic energy they have.
-
Potential energy - This comes from where the particles are positioned relative to each other.
We can write this as:
The internal energy of any system is determined by the microscopic motion and interactions of all its particles. Even seemingly stationary objects contain enormous amounts of internal energy due to constant particle motion at the atomic level.
How does internal energy change?
When you add thermal energy to a system (like heating it up), you increase the energy stored within it. This happens by giving more energy to the particles inside.
Here's what happens when you heat something:
- The kinetic energy of particles increases
- This makes the temperature of the system go up
- If the system changes state (like ice melting), the potential energy of particles changes too, but kinetic energy stays the same
Key Concept: When thermal energy is added to a system, it can either increase the kinetic energy of particles (raising temperature) OR change the potential energy of particles (causing a state change), but not both simultaneously.
Temperature changes vs state changes
When temperature changes
When you heat something and it doesn't change state (like warming up water in a kettle), the particles move faster and faster. The more kinetic energy they have, the higher the temperature becomes.
Equation to use:
Where:
- = Energy (J)
- = mass (kg)
- = specific heat capacity (J/kg°C)
- = temperature change (°C)
Worked Example: Heating a Gas
When heating a gas, particles move faster, increasing their kinetic energy and raising the temperature. For example, if you heat 2 kg of air with a specific heat capacity of 1000 J/kg°C by 10°C:
When state changes
When you heat something and it does change state (like ice melting into water), something different happens:
- The temperature stays constant (for example, ice melts at exactly 0°C)
- The kinetic energy of particles doesn't change
- The potential energy increases as particles rearrange themselves
- All the added energy goes into changing the arrangement, not the movement
Equation to use:
Where:
- = Energy (J)
- = mass (kg)
- = specific latent heat (J/kg)
Worked Example: Water Vapour Condensing
When water vapour condenses into liquid water at 100°C, the temperature doesn't change but the particles move closer together. If 0.5 kg of steam condenses (latent heat of vaporisation = 2,260,000 J/kg):
This energy is released as the particles rearrange from gas to liquid state.
Real-world examples
Understanding internal energy helps explain many everyday phenomena:
Worked Example: Heating a Gas
When thermal energy is added to gas particles, they move faster. This increases their kinetic energy and raises the temperature. You can calculate this energy change using the specific heat capacity equation:
Worked Example: Water Vapour Condensing
When steam turns into liquid water at 100°C, the temperature stays the same. The kinetic energy doesn't change, but the potential energy decreases as particles get closer together in the liquid state. The energy calculation uses:
Key Points to Remember:
- Internal energy has two parts: kinetic energy (from particle movement) and potential energy (from particle positions)
- Heating increases internal energy by giving particles more energy
- Temperature changes happen when particles move faster (kinetic energy increases)
- State changes happen at constant temperature (potential energy changes, kinetic energy stays same)
- Use different equations: for temperature changes vs for state changes
- Energy is always conserved - it just moves between different forms