Conservation & Dissipation of Energy (Oxford AQA IGCSE Physics)

Conservation of Energy

What is the conservation of energy?

• The law of conservation of energy states that

Energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed

• This means the total amount of energy in a closed system remains constant 

• The total energy transferred into an open system must be equal to the total energy transferred out of the system

Example 1: a bat hitting a ball

A bat striking a ball

• The moving bat has energy in its kinetic store

  • Some of that energy is usefully transferred mechanically 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 flow diagram for a bat striking a ball

Example 2: boiling water in a kettle

Energy transfers as a kettle boils water

• 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 to the thermal store of the plastic kettle

  • Some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated

Example 3: trampoline

A person bouncing on a 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 as the fabric is stretched

• 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

Dissipated Energy

• When energy is transferred only part of it may be usefully transferred

  • Unintended or wasted energy transfers are inevitable

  • There is no such thing as a perfect energy transfer

• Most wasted energy transfers result in the heating of objects and their surroundings

  • We say this energy is dissipated (spread out) to the thermal store of the surroundings

An electric kettle boiling water

• Useful energy transfer of a kettle

  • Energy is transferred by heating from the thermal store of the heating element in the kettle to the thermal store of the water

• Wasted energy transfers of a kettle

  • Energy is transferred from the thermal store of the water to the thermal store of the kettle casing

  • Energy is transferred from the thermal store of the kettle casing to the thermal store of the surroundings and the temperature of the air in the room will increase slightly

  • Energy is transferred from the thermal store of the water to the thermal store of the surroundings as water evaporates

• Energy transfers are deemed useful if they have a purpose or contribute to a process

  • The purpose of a kettle is to heat water so the energy transfer to the thermal store of the water is useful

• Energy transfers are deemed wasted if the energy can no longer be used for that purpose

  • The energy transferred to the air surrounding the kettle does not heat the water, therefore this energy is wasted

Energy & Friction

• Work done against air resistance, frictional forces, and resistance in wires all result in heating

• Energy is transferred to the thermal store of the surroundings increasing the temperature of the air particles and surrounding objects 

• Once energy is in the thermal store of the surroundings, it can not be 'gathered' for any specific use

• Therefore, it is referred to as wasted energy