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

The principle of conservation of energy means that for a closed system, the total amount of energy is constant
The total amount of energy transferred into the system must be equal to the total amount of energy transferred away from the system

Therefore, energy cannot be ‘lost’, but it can be transferred to the surroundings
- Energy can be dissipated (spread out) to the surroundings by heating and radiation
- Dissipated energy transfers are often not useful, in which case they can be described as wasted energy

Examples of the principle of conservation of energy

Example 1: a bat hitting a ball

The moving bat has energy in its kinetic store
Some of that energy is transferred usefully to the kinetic store of the ball
Some of that energy is transferred from the kinetic store of the bat to the thermal store of the ball mechanically due to the impact of the bat on the ball
- This energy transfer is not useful; the energy is wasted
Some of that energy is dissipated by heating to the thermal store of the bat, the ball, and the surroundings
- This energy transfer is not useful; the energy is wasted
The total amount of energy transferred into the system is equal to the total amount of energy transferred away from the system

Conservation of energy: a bat hitting a ball

bat-and-ball-energy-flow-diagram-new

The principle of conservation of energy applied to a bat hitting a ball

Example 2: Boiling Water in a Kettle

When an electric kettle boils water, energy is transferred electrically from the mains supply to the thermal store of the heating element inside the kettle
As the heating element gets hotter, energy is transferred by heating to the thermal store of the water

Some of the energy is transferred to the thermal store of the plastic kettle
- This energy transfer is not useful; the energy is wasted
And some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated
- This energy transfer is not useful; the energy is wasted
The total amount of energy transferred into the system is equal to the total amount of energy transferred away from the system

Conservation of energy: a kettle boiling water

boiling-kettle-store-igcse-and-gcse-physics-revision-notes

The principle of conservation of energy applied to a kettle boiling water

Energy flow diagrams

Energy stores and transfers can be represented using a flow diagram
- This shows both the stores and the transfers taking place within a system

1-7-4-conservation-of-energy-flow-diagram

Energy flow diagram showing energy stores and transfers in a nuclear power plant.

Note the colour difference of the labels (stores) and the arrows (transfer pathways)

In an energy flow diagram, energy is always conserved

total energy in = total energy out

Worked example

The diagram shows a rollercoaster going down a track.

The rollercoaster takes the path A → B → C → D.

WE - Energy transfers question image, downloadable AS & A Level Physics revision notes

Which statement is true about the energy changes that occur for the rollercoaster down this track?

A. E_K → ΔE_P → ΔE_P → E_K

B. E_K → ΔE_P → E_K → ΔE_P

C. ΔE_P → E_K → E_K → ΔE_P

D. ΔE_P → E_K → ΔE_P → E_K

Answer: D

At point A:
- The rollercoaster is raised above the ground, therefore it has energy in its gravitational potential store
- As it travels down the track, energy is transferred mechanically to its kinetic store

At point B:
- Energy is transferred mechanically from the kinetic store to the gravitational potential store
- As the kinetic energy store empties, the gravitational potential energy store fills
At point C:
- Energy is transferred mechanically from the gravitational potential store to the kinetic store

At point D:
- The flat terrain means there is no change in the amount of energy in its gravitational potential store, the rollercoaster only has energy in its kinetic store
- The kinetic energy store is full
- In reality, some energy will also be transferred to the thermal energy store of the tracks due to friction, and to the thermal energy store of the surroundings due to sound
- We say this energy is dissipated to the surroundings
  - The total amount of energy in the system will be constant
  - Total energy in = total energy out

Exam Tip

It is helpful to think of energy stores as beakers and the total energy in the system as water. The water can be poured from one beaker into another, back and forth, as energy is transferred between stores.

You may not always be given the energy transfers happening in the system in your IGCSE Co-ordinated Sciences exam questions.

By familiarising yourself with the energy stores and transfer pathways, you should be able to relate these to the situation presented in the question. For example, a ball rolling down a hill transfers energy from the ball's gravitational potential energy store to its kinetic energy store mechanically, while a spring transfers energy from its elastic potential energy store to its kinetic energy store mechanically.

Conservation of Energy (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Revision Note