- The Principle of Conservation of Energy states that:
- Energy cannot be created or destroyed, it can only change from one form to another
- This means the total amount of energy in a closed system remains constant, although how much of each form there is may change
- Common examples of energy transfers are:
- A falling object (in a vacuum): gravitational potential energy ➝ kinetic energy
- A battery: chemical energy ➝ electrical energy ➝ light energy (if connected to a bulb)
- Horizontal mass on a spring: elastic potential energy ➝ kinetic energy
- When energy is transferred from one form to another, not all the energy will end up in the desired form (or place)
- Dissipation is used to describe ways in which energy is wasted
- Any energy not transferred to useful energy stores is wasted because it is lost to the surroundings
- These are commonly in the form of thermal (heat), light or sound energy
- What counts as wasted energy depends on the system
- For example, in a television:
electrical energy ➝ light energy + sound energy + thermal energy
- Light and sound energy are useful energy transfers whereas thermal energy (from the heating up of wires) is wasted
- Another example, in a heater:
electrical energy ➝ thermal energy + sound energy
- The thermal energy is useful, whereas sound is not
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. KE – GPE – GPE – KE
B. KE – GPE – KE – GPE
C. GPE – KE – KE – GPE
D. GPE – KE – GPE – KE
- At point A:
- The rollercoaster is raised above the ground, therefore it has GPE
- As it travels down the track, GPE is converted to KE and the roller coaster speeds up
- At point B:
- KE is converted to GPE as the rollercoaster rises up the loop
- At point C:
- This GPE is converted back into KE as the rollercoaster travels back down the loop
- At point D:
- The flat terrain means the rollercoaster only has KE
- At point A:
You may not always be given the energy transfers happening in the system in exam questions.
By familiarising yourself with the transfers and stores of energy, you will be expected to relate these to the situation in question. For example, a ball rolling down a hill is transferring gravitational potential energy to kinetic energy whilst a spring converts elastic potential energy into kinetic energy.