GCSE Physics revision

Internal energy

Internal energy and energy transfers

AQA 4.3.2.1

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Energy is stored inside a system by the particles (atoms and molecules) that make up the system.

MOST 🎯🎯

Apply the specified scientific knowledge of internal energy to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of internal energy using the specified scientific ideas.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What is Kinetic Energy?

Kinetic energy is the energy an object possesses due to its motion, meaning any moving object — from a tiny particle to a jumbo jet — has kinetic energy.
The amount of kinetic energy an object has depends on two factors: its speed and its mass.

How Speed Affects Kinetic Energy

The faster an object moves, the greater its kinetic energy, because more energy must have been transferred to it to reach that speed.
Kinetic energy is stored in the kinetic energy store of an object, and increases as speed increases.

How Mass Affects Kinetic Energy

The greater the mass of an object, the more kinetic energy it has, provided its speed remains the same.
A plane travelling at the same speed as a tiny particle will have far more kinetic energy simply because its mass is so much larger.

The Kinetic Energy Equation

Kinetic energy is calculated using the equation Ek = 12 mv 2 , where Ek is kinetic energy in joules (J), m is mass in kilograms (kg), and v is speed in metres per second (m/s).
It is important to remember that only the velocity (v ) is squared in the equation, not the entire right-hand side.

Unit Conversions Before Calculating

Mass must always be in kilograms before substituting into the equation, so tonnes must be multiplied by 1,000 and grams must be divided by 1,000.
For example, 20 tonnes converts to 20,000 kg, and 0.1 g converts to 0.0001 kg (i.e.
0.1 ÷ 1000 = 0.0001 kg).

Worked Example: Plane vs. Particle

For a plane of mass 20,000 kg travelling at 5 m/s:
Ek = 0.5 × 20000 × 52 = 250,000 J (250 kJ).
For a particle of mass 0.0001 kg travelling at 4,000 m/s:
Ek = 0.5 × 0.0001 × 40002 = 800 J (0.8 kJ).
This example shows that even though the particle travels much faster, its kinetic energy is far less than the plane's because its mass is so much smaller.

6. Worked Example: Plane vs. ParticleAQA

Notes

For a plane of mass 20,000 kg travelling at 5 m/s:
Ek = 0.5 × 20000 × 52 = 250,000 J (250 kJ).
For a particle of mass 0.0001 kg travelling at 4,000 m/s:
Ek = 0.5 × 0.0001 × 40002 = 800 J (0.8 kJ).
This example shows that even though the particle travels much faster, its kinetic energy is far less than the plane's because its mass is so much smaller.

States of Matter: A Quick Recap

Solids have particles arranged in a regular, ordered structure that are close together and vibrate at fixed positions.
Liquids have more disorder than solids, with particles able to flow freely due to having greater energy.
Gases have the most energy and the most disorder, with particles moving rapidly and spread far apart.

7. States of Matter: A Quick RecapAQA

Notes

Solids have particles arranged in a regular, ordered structure that are close together and vibrate at fixed positions.
Liquids have more disorder than solids, with particles able to flow freely due to having greater energy.
Gases have the most energy and the most disorder, with particles moving rapidly and spread far apart.

State Changes: Heating Up

Melting is the process by which a solid changes into a liquid when heated.
Evaporation and boiling are the processes by which a liquid changes into a gas when heated.
Sublimation is a less common state change where a solid turns directly into a gas, skipping the liquid stage — dry ice is a classic example.

State Changes: Cooling Down

Condensing is the process by which a gas cools down and changes into a liquid, as particles slow down and move closer together.
Freezing is the process by which a liquid cools down and changes into a solid.
Mass is always conserved during physical (state) changes, and the changes are reversible.

Evaporation vs. Boiling

Evaporation is a slow process where particles escape from the surface of a liquid, often seen as steam or vapour rising from a liquid.
Boiling is a faster process where bubbles form throughout the liquid as particles gain enough energy to escape rapidly.
Both evaporation and boiling result in a liquid changing to a gas, but they differ in speed and where in the liquid the change occurs.
What Is Internal Energy?
Internal energy is the total energy stored within the particles of a system, including both their kinetic and potential energies.
The equation for internal energy is: Internal energy = Total kinetic energy + Total potential energy.
Kinetic energy is the energy stored in moving objects, given by KE = rac{1}{2}mv², where m is mass and v is velocity.
Potential energy in this context is the energy stored in the bonds between particles — energy is taken in when bonds break and released when bonds form.

10. Evaporation vs. BoilingAQA

Notes

Evaporation is a slow process where particles escape from the surface of a liquid, often seen as steam or vapour rising from a liquid.
Boiling is a faster process where bubbles form throughout the liquid as particles gain enough energy to escape rapidly.
Both evaporation and boiling result in a liquid changing to a gas, but they differ in speed and where in the liquid the change occurs.
What Is Internal Energy?
Internal energy is the total energy stored within the particles of a system, including both their kinetic and potential energies.
The equation for internal energy is: Internal energy = Total kinetic energy + Total potential energy.
Kinetic energy is the energy stored in moving objects, given by KE = rac{1}{2}mv², where m is mass and v is velocity.
Potential energy in this context is the energy stored in the bonds between particles — energy is taken in when bonds break and released when bonds form.

How Heating Affects Internal Energy

When thermal energy is supplied to a substance, the particles gain energy and their internal energy increases.
An increase in internal energy can either raise the temperature of the substance or cause a change of state.
During a state change, the energy supplied breaks or forms bonds between particles rather than raising the temperature.

11. How Heating Affects Internal EnergyAQA

Notes

When thermal energy is supplied to a substance, the particles gain energy and their internal energy increases.
An increase in internal energy can either raise the temperature of the substance or cause a change of state.
During a state change, the energy supplied breaks or forms bonds between particles rather than raising the temperature.

Heating and Cooling Curves

A heating curve shows temperature (°C) on the y-axis and time on the x-axis, illustrating how a substance changes as it is heated.
The sloped (gradient) sections of a heating curve show where temperature is rising and the state is not changing.
The flat (horizontal) sections of a heating or cooling curve show where a state change is occurring and temperature remains constant.
The flat sections occur because energy is being used to break bonds (on heating) or is released as bonds form (on cooling), rather than changing temperature.

12. Heating and Cooling CurvesAQA

Notes

A heating curve shows temperature (°C) on the y-axis and time on the x-axis, illustrating how a substance changes as it is heated.
The sloped (gradient) sections of a heating curve show where temperature is rising and the state is not changing.
The flat (horizontal) sections of a heating or cooling curve show where a state change is occurring and temperature remains constant.
The flat sections occur because energy is being used to break bonds (on heating) or is released as bonds form (on cooling), rather than changing temperature.

Reading Heating & Cooling Curves

The melting point of a substance can be identified on a heating curve as the temperature at which the first horizontal plateau occurs.
The boiling point of a substance can be identified as the temperature at which the second horizontal plateau occurs on a heating curve.
In an exam, you may be asked to plot heating or cooling curve data, or to identify the melting point, boiling point, or condensing point of a substance.

13. Reading Heating & Cooling CurvesAQA

Notes

The melting point of a substance can be identified on a heating curve as the temperature at which the first horizontal plateau occurs.
The boiling point of a substance can be identified as the temperature at which the second horizontal plateau occurs on a heating curve.
In an exam, you may be asked to plot heating or cooling curve data, or to identify the melting point, boiling point, or condensing point of a substance.

Key Equations to Know

The kinetic energy equation KE = rac{1}{2}mv² is provided on the equation sheet in the GCSE Physics exam, so you do not need to memorise it.
Internal energy can be calculated by adding together the total kinetic energy and total potential energy of all the particles in a system.
In exam questions, data may be provided so that you can calculate the total internal energy of a given system.

14. Key Equations to KnowAQA

Notes

The kinetic energy equation KE = rac{1}{2}mv² is provided on the equation sheet in the GCSE Physics exam, so you do not need to memorise it.
Internal energy can be calculated by adding together the total kinetic energy and total potential energy of all the particles in a system.
In exam questions, data may be provided so that you can calculate the total internal energy of a given system.