Internal energy (AQA GCSE Physics Combined Science): Revision Notes
Internal energy
What is internal energy?
Internal energy is all the energy that particles in a system have stored inside them. Think of it like the total energy "bank account" that all the particles in an object share together.
Every system - whether it's a gas, liquid, or solid - has internal energy because its particles are always moving and interacting with each other.
The key concept here is that all matter has internal energy - it's not something that only appears under special conditions. Even a seemingly "cold" ice cube has internal energy because its particles are still moving and positioned relative to each other.
The two types of internal energy
Internal energy is made up of two main parts that work together:
1. Kinetic energy of particles
- This comes from how fast the particles are moving
- When particles move faster, they have more kinetic energy
- This is linked to temperature - hotter objects have faster-moving particles
2. Potential energy of particles
- This comes from where the particles are positioned relative to each other
- It depends on the forces between particles (like attraction and repulsion)
- This energy changes when particles get closer together or further apart
The total internal energy can be written as:
How internal energy changes
You can change a system's internal energy by heating it or cooling it. When you add thermal energy to a system, one of two things happens:
Temperature changes (particles move faster)
When you heat something and its temperature goes up:
- The particles gain kinetic energy and move faster
- The potential energy stays roughly the same
- Example: Heating water in a kettle from 20°C to 80°C
Critical Formula for Temperature Changes:
When calculating energy needed for temperature changes, always use:
Where:
- = energy change
- = mass
- = specific heat capacity
- = temperature change
State changes (particles rearrange)
When you heat something and it changes state (like ice melting):
- The temperature stays constant during the change
- The kinetic energy of particles doesn't change
- But the potential energy increases as particles rearrange
- Example: Ice melting at 0°C or water boiling at 100°C
Critical Formula for State Changes:
When calculating energy needed for state changes, use:
Where:
- = energy change
- = mass
- = specific latent heat
Key examples
Worked Example: Heating a Gas
When you heat a gas in a sealed container:
- The particles move faster and collide more frequently
- This increases their kinetic energy
- The result is a temperature rise
- The potential energy remains relatively unchanged since the particles don't get significantly closer or further apart
Worked Example: Water Changing State
When ice melts at 0°C or water boils at 100°C:
- The temperature remains constant throughout the process
- Energy goes into breaking bonds between water molecules
- This changes the potential energy as particles rearrange into new positions
- No change in kinetic energy occurs since temperature is constant
Key Points to Remember:
- Internal energy is the total energy stored by all particles in a system
- It has two components: kinetic energy (from particle motion) and potential energy (from particle positions)
- Heating can either increase temperature (kinetic energy changes) or cause state changes (potential energy changes)
- During state changes, temperature stays constant even though energy is being added
- Use different equations for temperature changes versus state changes:
- Temperature change:
- State change: