AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Conservation of energy: Principle of conservation of energy.
The principle of conservation of energy states that energy cannot be created or destroyed; it can only be transferred from one store to another or transformed from one form to another.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The principle of conservation of energy states that energy cannot be created or destroyed; it can only be transferred from one store to another or transformed from one form to another. The total energy of a closed system remains constant. In a closed system, no energy enters or leaves, so the sum of all energy stores is constant. For example, a falling ball transfers gravitational potential energy to kinetic energy and internal energy; if air resistance is negligible, mgh = ½mv². When friction or drag acts, some energy is transferred to the internal energy store of the surroundings, often described as dissipated or wasted, but the total energy is still conserved. Energy conservation is a powerful tool for solving mechanics problems without knowing the details of the forces involved.
What earns the marks
- States that energy cannot be created or destroyed, only transferred between stores or transformed.
- Defines a closed system as one in which no energy enters or leaves, so the total energy remains constant.
- Applies conservation of energy to a mechanical process, for example equating gravitational potential energy lost to kinetic energy gained when friction is negligible.
- Explains that when friction or drag acts, some energy is transferred to the internal energy store of the surroundings, often called dissipated or wasted energy.
- Uses conservation of energy to calculate an unknown quantity such as speed, height or energy transferred, showing that the total before equals the total after.
Common mistakes and how to put them right
Claiming that energy is used up or lost.
Put it right by saying energy is dissipated or transferred to the surroundings.
Forgetting to include all energy stores in the system, such as elastic potential energy or internal energy, when writing an energy balance equation.
Assuming that mechanical energy is always conserved.
Put it right: In the presence of friction or drag, mechanical energy decreases as internal energy increases, although total energy is constant.
Mixing up the terms energy store and energy transfer.
Put it right: For example, saying kinetic energy is transferred to gravitational potential energy store rather than saying energy is transferred from the kinetic store to the gravitational potential store.
Exam tips
- Write a clear 'before = after' energy equation, listing each store with its correct expression, before substituting numbers.
- State explicitly that you are assuming the system is closed and that friction or air resistance is negligible when using conservation of mechanical energy.
- If energy is dissipated, calculate the energy transferred to internal energy as the difference between initial and final mechanical energy.
- Use standard symbols and units consistently, for example mgh for gravitational potential energy and ½mv² for kinetic energy.
Try the lesson
The start of the Conservation of energy: Principle of conservation of energy. lesson, made from checked teaching notes for this specification point.
Have a go first
A ball is released from rest and falls freely. Ignoring air resistance, which statement describes the energy transfer as the ball falls?
Listen
The principle of conservation of energy states that energy cannot be created or destroyed; it can only be transferred from one store to another or transformed from one form to another. The total energy of a closed system remains constant. In a closed system, no energy enters or leaves, so the sum of all energy stores is constant. For example, a falling ball transfers gravitational potential energy to kinetic energy and internal energy; if air resistance is negligible, mgh = ½mv². When friction or drag acts, some energy is transferred to the internal energy store of the surroundings, often described as dissipated or wasted, but the total energy is still conserved. Energy conservation is a powerful tool for solving mechanics problems without knowing the details of the forces involved. A strong answer earns marks for these points. States that energy cannot be created or destroyed, only transferred between stores or transformed. Defines a closed system as one in which no energy enters or leaves, so the total energy remains constant. Applies conservation of energy to a mechanical process, for example equating gravitational potential energy lost to kinetic energy gained when friction is negligible. Explains that when friction or drag acts, some energy is transferred to the internal energy store of the surroundings, often called dissipated or wasted energy. A common slip: Claiming that energy is used up or lost; energy is conserved but may be transferred to less useful stores such as internal energy. Instead, saying energy is dissipated or transferred to the surroundings. Exam tip: Write a clear 'before = after' energy equation, listing each store with its correct expression, before substituting numbers.
Key words
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Principle of conservation of energy
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Closed system
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Dissipated or wasted energy
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Energy conservation in mechanics
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See every definition
- Principle of conservation of energy
- Energy cannot be created or destroyed; it can only be transferred from one store to another or transformed from one form to another.
- Closed system
- One in which no energy enters or leaves, so the sum of all energy stores is constant.
- Dissipated or wasted energy
- Energy transferred to the internal energy store of the surroundings when friction or drag acts, though the total energy is still conserved.
- Energy conservation in mechanics
- A powerful tool for solving mechanics problems without knowing the details of the forces involved.
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