IB Physics SL

Revision Notes

2.3.6 Principle of 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 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

Types of EnergyEnergy types table, downloadable AS & A Level Physics revision notes

Transfers and stores of energy, downloadable AS & A Level Physics revision notes

Energy types can be separated into transfers or stores

Energy Dissipation

  • 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

Useful and wasted energy conversions for a television

  • The energy changes in an electrical heater:

    electrical energy ➝ thermal energy + sound energy + light energy

  • In a gas cooker, the energy transfers are similar but the initial source of energy is different:

chemical energy ➝ thermal energy + sound energy + light energy

  • In both these cases, thermal energy is useful, whereas sound and light are not

Useful and wasted energy conversions in an electric heater and gas cooker

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.     KE – GPE – GPE – KE

B.     KE – GPE – KE – GPE

C.     GPE – KE – KE – GPE

D.     GPE – KE – GPE – KE

     ANSWER: D

  • 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

Applications of Energy Conservation

  • 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

Conservation of Energy Trampoline, downloadable AS & A Level Physics revision notes

Energy transfers whilst jumping on a trampoline

  • There may also be work done against resistive forces such as friction
  • For example, if an object travels up a rough inclined surface, then

Loss in kinetic energy = Gain in gravitational potential energy + Work done against friction

Worked Example

A simple pendulum has a mass of 640 g and a length of 0.7 m. It is pulled out to an angle of 20° from the vertical.

The pendulum is released. Assuming negligible air resistance, calculate the maximum speed of the pendulum bob as it passes through the vertical position.

Energy Conservation Worked Example, downloadable AS & A Level Physics revision notes

Applications of Energy Conservation WE 1Applications of Energy Conservation WE 2

Spring Energy Conservation

  • When a vertical spring is extended and contracted, its energy is converted into other forms
  • Although the total energy of the spring will remain constant, it will have changing amounts of:
    • Elastic potential energy (EPE)
    • Kinetic energy (KE)
    • Gravitational potential energy (GPE)
  • When a vertical mass is hanging on a spring and it moves up and down, its energy will convert between the three in various amounts

Change in Spring Energy, downloadable AS & A Level Physics revision notes

  • At position A:
    • The spring has some EPE since it is slightly compressed
    • Its KE is 0 since it is stationary
    • Its GPE is at a maximum because the mass is at its highest point
  • At position B:
    • The spring has some EPE since it is slightly stretched
    • Its KE is at a maximum as it passes through the equilibrium position at its maximum speed
    • It has some GPE since the mass is still above the ground
  • At position C:
    • The spring has its maximum EPE because it is at its maximum extension
    • Its KE is 0 since it is stationary
    • Its GPE is at a minimum because it is at its lowest point above the Earth’s surface
  • For a horizontal mass on a spring system, there is no gravitational potential energy to consider. The spring only converts between kinetic and elastic potential energy
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