Internal energy: the energy stored by a system’s particles
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.
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.
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
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.
Question: A student heats a solid block until it melts. Explain how heating affects the internal energy of the block.
Model answer: Energy is transferred to the block by heating, which increases its internal energy because the particles gain energy. At first the temperature rises as the particles gain kinetic energy and vibrate faster. When the block melts, the energy supplied changes the potential energy of the particles as their arrangement changes, so the temperature stays constant during melting.
- 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.
- 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.
- 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?
