Internal energy: the energy stored by a system’s particles
Definition
Internal energy
The internal energy of a system is the total kinetic energy and potential energy of all the particles that make up the system, measured in joules, J\text{J}J.
Definition
System
A system is the object or group of objects being studied, such as a metal block, a beaker of water or the gas inside a container.
- Energy is stored inside a system by its particles, such as its atoms and molecules, and this store has two parts.
- Kinetic energy is the energy the particles have because they are moving.
- Potential energy is the energy the particles have because of their positions relative to one another.
- Internal energy is the total of both parts for all the particles in the system, not the kinetic energy of a single particle or the movement of the whole object.
Key Idea
Internal energy is an energy store, and like every energy store it is measured in joules, J\text{J}J.
How heating changes internal energy
Definition
Temperature
Temperature is a measure of the average kinetic energy of the particles in a system.
- When a system is heated, energy is transferred to it, and its internal energy increases because the particles gain energy.
- The transferred energy has one of two effects: it raises the temperature of the system, or it produces a change of state.
- If the temperature rises, the particles move or vibrate faster, so their average kinetic energy increases.
- If the substance changes state, the energy changes the potential energy of the particles as their arrangement changes, and the temperature stays the same during the change.
Common Mistake
- Do not say that an object “contains heat”, as heating is a way of transferring energy, not an energy store; the store held by the particles is the internal energy.
- Do not assume heating always raises the temperature, as the energy may instead produce a change of state.
Exam technique
- For an explanation question, use a clear chain of reasoning: energy transferred by heating →\rightarrow→ particles gain energy →\rightarrow→ internal energy increases →\rightarrow→ the temperature rises or a change of state occurs.
- If the temperature rises, refer to increased kinetic energy.
- If the state changes, refer to a change in the particles’ potential energy.
Self review
- What is internal energy?
- What are the two forms of energy the particles in a system have?
- What happens to the internal energy of a system when it is heated?
- What particle change causes a temperature rise?
Besides raising the temperature, what else can heating do?
