Internal energy is the total kinetic energy and potential energy of all the particles that make up a system. Free GCSE Physics notes and 13 FAQs.
Updated 20 Mar 2026 · Revise Right Team
Internal Energy: What's Inside? is part of Internal energy and energy transfers in GCSE AQA Higher Physics.
3 min
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13
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GCSE AQA Higher
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What this lesson covers
Internal energy is the total kinetic energy and potential energy of all the particles that make up a system. Free GCSE Physics notes and 13 FAQs.
Define internal energy as the total kinetic and potential energy of all particles in a system
Explain how heating changes the energy stored within a system by increasing particle energy
Describe the relationship between internal energy, temperature changes, and changes of state
Apply the concept of internal energy to explain everyday thermal processes
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Use this lesson to stay inside Internal energy and energy transfers while connecting the detail back to the wider Particle model of matter topic.
Particle model of matter
Internal energy and energy transfers
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Internal Energy: What's Inside? before testing yourself from memory.
1. What's Really Going On Inside Matter?
Right now, the atoms in your desk are jiggling about. The molecules in the air are zooming around at hundreds of metres per second. Even in a 'solid' ice cube, water molecules are vibrating constantly. At the microscopic level, the motion of particles and the forces between them contribute to internal energy.
All this hidden motion and the forces between particles store energy inside every object. This energy has a name: internal energy. You can't see it, but it's there - and it explains why things get hot, why ice melts, and why steam burns.
2. The Two Components: Kinetic and Potential
Internal energy has two parts, like a bank account with two different savings pots. Let's break this down using something familiar - a glass of water.
In your glass of water, every H₂O molecule is moving randomly - some fast, some slow, but all in constant motion. The faster they move, the more kinetic energy they have. When you heat the water, you're making these molecules move faster on average.
Water molecules also attract each other (that's why water forms droplets). When molecules are close together, they have less potential energy. When you pull them apart, you increase their potential energy - like stretching a spring.
3. How Heating Changes Internal Energy
Here's where it gets interesting. When you heat something, you're pumping energy into the system. But that energy doesn't always do what you might expect.
Sometimes heating makes particles move faster (increasing kinetic energy) - and you see the temperature rise. Other times, heating gives particles enough energy to break free from their positions (increasing potential energy) - and you see a change of state instead.
Worked example: What Happens When You Heat Ice
Trace the energy changes as ice at -10°C is heated until it becomes steam at 110°C
Energy → faster vibrations — Kinetic energy increases, temperature rises, ice stays solid
Energy → breaking bonds between molecules — Potential energy increases, temperature stays constant, ice becomes water
Energy → faster molecular motion — Kinetic energy increases, temperature rises, water stays liquid
Energy → molecules escaping liquid — Potential energy increases, temperature stays constant, water becomes steam
Energy → faster gas molecule motion — Kinetic energy increases, temperature rises, steam gets hotter
Answer: When ice is heated from -10°C to 0°C, the energy increases the kinetic energy of the ice, raising its temperature. At 0°C, the energy is absorbed to overcome intermolecular forces, increasing potential energy during melting while temperature remains constant. As the liquid water heats from 0°C to 100°C, kinetic energy increases and temperature rises. At 100°C, during boiling, energy again contributes to increasing potential energy by breaking intermolecular bonds, so the temperature remains constant. Finally, when steam is heated from 100°C to 110°C, the kinetic energy increases resulting in a temperature rise.
Key insight: During state changes (melting, boiling), all the energy goes into changing potential energy. Temperature stays constant even though you're still heating. This is why it takes ages to boil a kettle once it reaches 100°C.
4. States of Matter and Internal Energy
Different states of matter have different internal energies, even at the same temperature. Think about water at 0°C - it can be solid ice or liquid water. Same temperature, but very different internal energies.
State
Particle Arrangement
Kinetic Energy
Potential Energy
Total Internal Energy
Solid
Fixed positions, vibrating
Low
Low
Lowest
Liquid
Close but mobile
Medium
Medium
Medium
Gas
Far apart, fast moving
High
High
Highest
This explains why steam burns are so dangerous. Steam at 100°C has much more internal energy than water at 100°C. When steam condenses on your skin, it releases all that extra potential energy as heat.
5. Exam Success: Internal Energy Questions
Exam questions on internal energy often test whether you understand the difference between temperature and internal energy, and what happens during state changes. Here's how to tackle them.
Watch out for questions that give you heating curves or ask you to compare internal energies of different states. The key is always to think about what's happening to the particles - are they moving faster (kinetic) or changing their arrangement (potential)?
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Internal energy is the total kinetic energy and potential energy of all the particles that make up a system. Heating a system increases its internal energy, either by making particles move faster, which raises the temperature, or by changing their arrangement during a change of state.
What is the kinetic energy of particles?
The kinetic energy of particles is the energy they have due to their random motion, and faster moving particles have more kinetic energy. Heating water makes its molecules move faster on average, which raises its temperature.
What is the potential energy of particles?
The potential energy of particles is the energy stored in the forces between them, which depends on how far apart they are and how strongly they attract each other. Pulling particles apart increases their potential energy, like stretching a spring.
Why are steam burns worse than boiling water burns?
Steam burns are worse than boiling water burns because steam at 100°C has much more internal energy than water at 100°C. When steam condenses on skin, it releases all that extra potential energy as heat.
How does heating change internal energy?
Heating increases the internal energy of a system. Sometimes it makes particles move faster, increasing kinetic energy so the temperature rises; at other times it gives particles enough energy to break free from their positions, increasing potential energy and causing a change of state.
Why does temperature stay constant when ice melts?
Temperature stays constant when ice melts because the energy is absorbed to overcome intermolecular forces, increasing potential energy rather than kinetic energy. During state changes such as melting at 0°C and boiling at 100°C, all the energy goes into changing potential energy.
Which state of matter has the most internal energy?
A gas has the highest internal energy, with high kinetic and potential energy, a liquid has medium internal energy, and a solid has the lowest. Different states have different internal energies even at the same temperature, such as ice and liquid water at 0°C.
What happens to energy when ice is heated until it becomes steam?
When ice at −10°C is heated to steam at 110°C, kinetic energy and temperature rise while it is solid, liquid and gas. At 0°C while melting and at 100°C while boiling, the energy increases potential energy instead, so the temperature stays constant.
What is the difference between temperature and internal energy?
Temperature is linked to the kinetic energy of particles, while internal energy is the total kinetic and potential energy of all the particles. Water at 0°C can be solid ice or liquid water: the same temperature, but very different internal energies.
Do ice and water at 0°C have the same internal energy?
Ice and water at 0°C have different internal energies even though their temperature is the same. Liquid water has more potential energy, because energy was absorbed to overcome the forces between molecules during melting.
What happens to internal energy during boiling?
During boiling, internal energy increases because the heating energy increases the potential energy of the particles as they escape the liquid. The temperature stays constant at 100°C for water, which is why a kettle takes ages to boil once it reaches 100°C.
Why does the temperature of boiling water stay at 100°C?
Boiling water stays at 100°C because all the heating energy goes into increasing the potential energy of the molecules, breaking the bonds between them, rather than increasing their kinetic energy. Only once all the water has become steam does the temperature rise.
Why are particles in a solid always moving?
Particles in a solid are always moving, because even in an ice cube the water molecules are vibrating constantly in their fixed positions. This motion, together with the forces between particles, stores internal energy in every object.