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1.2 Conservation of Energy

GCSE Physics · AQA · Chapter 1: Conservation and Dissipation of Energy

A free GCSE Physics lesson for AQA (8463) from OBA School: the principle of conservation of energy, closed systems, pendulums, bungee jumps and roller coasters. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • State the principle of conservation of energy
  • Explain what a closed system is and why its total energy stays the same
  • Describe the changes in energy stores for a swinging pendulum, a bungee jump and a roller coaster
  • Use conservation of energy to calculate energy transferred to the surroundings

Key words

  • Conservation of energy: Energy cannot be created or destroyed. It can only be transferred from one store to another.
  • Closed system: An isolated system: no energy is transferred into it or out of it, so its total energy never changes.
  • Open system: A system that can exchange energy with its surroundings.
  • Dissipated: Spread out into the surroundings, usually warming them slightly.
  • Pendulum: A mass (the bob) on a string or rod that swings to and fro about a fixed point.

Energy cannot be made or destroyed

Scientists have measured energy changes in countless experiments. Every time, the total amount of energy after a change is the same as before. This is the principle of conservation of energy:

Energy cannot be created or destroyed. It can only be transferred usefully, stored or dissipated.

It applies to every energy change. When you stretch an elastic band, the elastic potential store of the band increases by exactly the amount of energy transferred from your muscles’ chemical store (minus any that warms your muscles).

Closed systems

A system is an object or a group of objects. A closed system is isolated from its surroundings: no energy is transferred into or out of it. Energy can move between stores inside a closed system, but the total energy of the system stays the same.

Real systems are usually not closed. Air resistance and friction transfer energy to the thermal store of the surroundings. We can still use conservation of energy by counting the surroundings too:

energy at the start = energy at the end + energy dissipated to the surroundings

Exam tip: If a question says “ignore air resistance” or “no energy is lost to the surroundings”, treat the object as a closed system: the decrease in one store equals the increase in another.

A swinging pendulum

  • A → B: the bob swings down and speeds up. Gravity transfers energy from its GPE store to its kinetic store.
  • B → C: the bob swings up and slows down. Energy goes from its kinetic store back to its GPE store.
  • If air resistance were zero (a pendulum in a vacuum), the bob would rise to the same height on each side and keep swinging for ever: a closed system.
  • In air, a little energy is transferred to the thermal store of the air on every swing, so each swing is slightly lower and the pendulum eventually stops.

Practical tip: Measuring how high the bob rises is tricky because it is only still for an instant. Put a ruler or a marked board right behind the bob, look straight across at it (eye level) to avoid parallax, or film it and pause the video. Repeat and take a mean.

A bungee jump

  • Rope slack: the jumper accelerates downwards. Energy goes from the GPE store to the kinetic store.
  • Rope tightening: the rope stretches and slows the jumper. The kinetic store decreases and the elastic potential store of the rope increases. The GPE store keeps decreasing as the jumper falls.
  • Lowest point: the jumper is momentarily at rest. Almost all the energy is in the rope’s elastic store.
  • Bouncing back up: the rope shortens; energy goes from its elastic store to the jumper’s kinetic store, then to the GPE store. The jumper does not get back to the platform because some energy has been dissipated as the rope stretched and relaxed and by air resistance.

A roller coaster

Going down from A to B, most of the energy from the GPE store goes to the kinetic store. Friction and air resistance transfer some to the thermal store of the surroundings, and the rattling makes sound. Hill C must be lower than A, because the train can never have more energy than it started with. At D the brakes transfer energy from the kinetic store to the thermal store of the brakes and the air.

Worked example: energy dissipated on the first drop

From A to B the GPE store of the train decreases by 320 kJ. At B its kinetic store is 296 kJ. It started at rest.
energy dissipated = decrease in GPE store − increase in kinetic store
= 320 kJ − 296 kJ = 24 kJ

Answer: 24 kJ transferred to the thermal store of the surroundings (and by sound)

Common mistakes

  • Saying energy is “lost” or “destroyed” by friction. It is transferred to the thermal store of the surroundings.
  • Saying a pendulum stops because it “runs out of energy”. Its energy is gradually dissipated by air resistance.
  • Forgetting the elastic store of the rope in a bungee jump.
  • Thinking a closed system has no energy changes at all. Energy moves between stores inside it, but the total stays the same.

Revision questions

Try these, then check your answers below each one.

Question 1. Which is the principle of conservation of energy? (Choose from: Energy cannot be created or destroyed., Energy is always wasted in any transfer., Energy is used up when a machine works., Energy can be created by machines but not destroyed..)

Answer: Energy cannot be created or destroyed.

Working: Energy cannot be created or destroyed – it can only be transferred usefully, stored or dissipated.

Question 2. True or false? In a closed system, the total energy decreases when energy is transferred between stores.

Answer: False

Working: False. No energy enters or leaves a closed system, so its total energy stays the same, even though energy moves between stores inside it.

Question 3. A roller coaster train starts at rest at the top of a drop. Its GPE store decreases by 150 kJ on the way down, and 12 kJ is dissipated to the surroundings. What is its kinetic store at the bottom, in kJ?

Answer: 138 kJ

Working: Kinetic store = 150 kJ − 12 kJ = 138 kJ.

Related lessons

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This lesson is section 1.2 (AGCPH0102) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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