IB DP Physics: SL复习笔记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 Energy



Energy types can be separated into transfers or stores

  • The differences between energy stores and energy transfers can be confusing, so it is important to learn their differences and the different types of energy involved in each

Energy Stores

  • Energy stores keep or store energy within one part of a system
  • Kinetic energy store: the amount of energy in a kinetic energy store depends on the speed of the object
    • A runner with a high kinetic energy store in her muscles can run more quickly than a runner with a smaller kinetic energy store
    • A car with a high kinetic energy store within the engine can drive more quickly than a car with a smaller kinetic energy store
      • A racing car has a higher kinetic energy store than a tractor
  • Gravitational potential energy store: the amount of energy stored in an object depending on its height
    • The higher an object, the more gravitational potential energy it has
      • When a diver stands on a board 5 m high he has more gravitational potential energy than when standing on a board 3 m high
  • Magnetic energy store: magnets store magnetic energy until a magnetic material is present in its field
    • A piece of iron will be moved when it enters a magnetic field
    • The magnetic energy store is transferred to the kinetic energy of the iron
  • Chemical energy store: energy is stored as chemical energy and a chemical reaction takes place to release and then transfer it
    • Chemical energy is stored in our body for use when we think and move
    • Chemical energy is stored in a battery to be transferred to electrical energy
  • Thermal energy store: all objects store thermal energy
    • An object that is hotter stores more thermal energy than an object that is colder
  • Nuclear energy store: in a nuclear reactor, energy is stored as Uranium-235 until it is bombarded by neutrons and a huge amount of thermal energy is released

Energy Transfers

  • Energy transfers: give or transfer energy to different parts of a system
  • They act as a pathway around an energy system
    • Electrical transfer: when charge flows to produce an electric current
      • The current transfers the energy
    • Mechanical transfer: this occurs when a force is applied to move an object
      • This could be pushing a book across a desk
      • It could also be sound waves passing through a material causing the particles to move
    • Heating transfer: The internal energy of an object is determined by the temperature of the object
      • Energy is transferred from hotter to cooler areas
    • Waves transfer: When sound travels through a material it causes the particles to vibrate
      • Energy is transferred from an object that is moving/vibrating to generate the sound waves through other objects by the movement of particles
      • Light energy is transferred from the sun so we can see

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.


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

  • In mechanical systems when energy is transferred between stores it is equivalent to the work done:
    • A falling object (in a vacuum, where no energy is not dissipated into the surroundings): gravitational potential energy ➝ kinetic energy
    • Horizontal mass on a spring: elastic potential energy ➝ kinetic energy
  • We can also say energy is transferred between stores and transfers:
    • A battery connected to a bulb: chemical energy ➝ electrical energy ➝ light energy (if connected to a bulb)
    • A car: chemical energy ➝ mechanical energy ➝ kinetic energy


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


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


  • 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