Principle of Conservation of Energy (DP IB Physics) : Revision Note

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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

  • In physics, a system is defined as:

    An object or group of objects

  • Defining the system in physics is a way of narrowing the parameters to focus only on what is relevant to the situation being observed

  • When a system is in equilibrium, nothing changes and so nothing happens

  • When there is a change in a system, things happen, and when things happen, energy is transferred

Types of Energy

Energy types table, downloadable AS & A Level Physics revision notes
  • Kinetic energy, gravitational potential energy, and elastic potential energy are collectively known as mechanical energy types

Energy Dissipation

  • No energy transfer is 100% efficient

  • When energy is transformed from one form to another, some of the energy is dissipated to the surroundings

  • Dissipated energy usually ends up as thermal energy transferred to the surroundings where it cannot be easily used for another purpose 

  • Therefore, dissipated energy is usually regarded as wasted energy

 

  • A kettle transforms electrical energy into thermal energy

  • The thermal energy in the heating element is transferred to thermal energy in the water

    • Some thermal energy is also transferred to the plastic casing

    • Some thermal energy is also dissipated to the surrounding air

  • The energy transfers that are useful for heating the water are considered useful energy transfers

  • The energy transfers that are not useful for heating the water are considered wasted energy transfers

Applications of Energy Conservation

  • In mechanical systems, the energy transferred is equivalent to the work done

    • A falling object (in a vacuum, where no energy is not dissipated into the surroundings) transfers its gravitational potential energy into kinetic energy 

    • Horizontal mass on a spring transfers its elastic potential energy into kinetic energy

    • A battery or cell transfers its chemical energy into electrical energy

    • A car transfers chemical energy from the fuel into kinetic energy of the car

    • A person bouncing on a trampoline is transferring energy from elastic potential to kinetic to gravitational potential 

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

Spring Energy Conservation

  • When a vertical spring oscillates, 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)

Change in Spring Energy, downloadable AS & A Level Physics revision notes
  • At position A:

    • The spring has some EPE because it is slightly compressed

    • Its KE is zero because 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 because 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 because the mass is still raised

  • At position C:

    • The spring has its maximum EPE because the spring is at its maximum extension

    • Its KE is zero because it is stationary

    • Its GPE is at a minimum because the mass is at its lowest point 

  • For a horizontal mass on a spring system, you do not need to consider the gravitational potential energy because this does not change

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

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Ashika

Author: Ashika

Expertise: Physics Content Creator

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

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