The Law 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
- This means the total amount of energy in a closed system remains constant
- The total energy transferred into a system must be equal to the total energy transferred out of the system
- Therefore, energy is never '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, and can then be described as wasted 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
- Some of that energy is dissipated by heating to the thermal store of the bat, the ball, and the surroundings
Energy transfers taking place when a bat hits 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 unusefully to the thermal store of the plastic kettle
- And some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated
Energy transfer taking place as a kettle boils water
Example 3: Trampoline
- Whilst jumping, the person has energy in their kinetic store
- When the person lands on the trampoline, most of that energy is transferred to the elastic potential store of the trampoline
- That energy is transferred usefully back to the kinetic store of the person as they bounce upwards
- Energy is transferred from the kinetic store of the person to the gravitational potential store of the person as they gain height
- Some of the energy is dissipated by heating to the thermal store of the surroundings (the person, the trampoline and the air)
- The useful energy transfers taking place are:
elastic potential energy ➝ kinetic energy ➝ gravitational potential energy
Energy transfers taking place when a person jumps on a trampoline
Worked example
The diagram shows a rollercoaster going down a track.
The rollercoaster takes the path A → B → C → D.
Which statement is true about the energy changes that occur for the rollercoaster down this track?
A. EK → ΔEP → ΔEP → EK
B. EK → ΔEP → EK → ΔEP
C. ΔEP → EK → EK → ΔEP
D. ΔEP → EK → ΔEP → EK
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 A:
-
- 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 B:
-
- At point C:
- Energy is transferred mechanically from the gravitational potential store to the kinetic store
- At point C:
-
- 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
- At point D:
-
- 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
- We say this energy is dissipated to the surroundings
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 exam questions. By familiarising yourself with the energy stores and transfer pathways, you will be able to relate these to the situation in the question. For example, a ball rolling down a hill is transferring energy from the ball's gravitational potential energy store to its kinetic energy store mechanically, whilst a spring transfers energy from its elastic potential energy store to its kinetic energy store mechanically.
