Energy Stores & Transfers Notes

Edexcel IGCSE Physics: Revision notes

Key facts

  • Energy is held in stores and moved by transfer pathways: mechanically, electrically, by heating or by radiation.
  • Conservation of energy: energy cannot be created or destroyed, only transferred.
  • Efficiency =useful energy outputtotal energy input×100%= \dfrac{\text{useful energy output}}{\text{total energy input}} \times 100\%.
  • Thermal energy moves by conduction, convection and radiation.
  • Insulation, lubrication and streamlining reduce unwanted transfers and raise efficiency.

Stores and pathways

Energy is held in stores and moved between them by pathways: mechanically, electrically, by heating or by radiation.

To describe an energy change, name the energy store it starts in, the transfer pathway and the store it ends in.

Mechanical transfer is a force doing work; electrical is a current; radiation covers light and sound.

Stores of movement and position

  • Chemical
  • Kinetic
  • Gravitational
  • Elastic

Other stores

  • Thermal
  • Magnetic
  • Electrostatic
  • Nuclear

Transfer pathways

  • Mechanically
  • Electrically
  • By heating
  • By radiation
  1. 1

    Starting store

    gravitational

  2. 2

    Transfer pathway

    mechanically (gravity doing work)

  3. 3

    Ending store

    kinetic, as the ball speeds up

A falling ball

A battery lights a lamp. What is the pathway?

Conservation of energy

Energy cannot be created or destroyed, only transferred from one store to another.

The principle of conservation of energy says the total energy of a closed system is the same before and after any change.

Whatever leaves one store appears in others. Often some is transferred to the surroundings as wasted thermal energy.

020406080100InputUseful outputWasted outputEnergyEnergy (J)
Illustrative lamp: 100 J in always equals 100 J out, shared between the useful light store and wasted thermal energy.
  • Energy inenergy out (useful + wasted)

A lamp transfers 100 J of electrical energy, with 10 J as light. How much is wasted?

Efficiency

Efficiency is the fraction of the input energy that becomes useful output. Sankey diagrams show it with arrow widths.

Some of the energy supplied to any device is wasted, usually as heat. Efficiency compares the useful output with the total input.

In a Sankey diagram, the width of each arrow is proportional to the amount of energy.

050100150200Total inputUseful outputWastedEnergy (J)
A motor: 200 J in, 150 J useful, 50 J wasted.
  • Efficiencyuseful outputtotal input×100%\dfrac{\text{useful output}}{\text{total input}} \times 100\%

Worked example

A motor is supplied with 200 J and usefully transfers 150 J as kinetic energy. Find the efficiency.

Can a device be more than 100% efficient?

Thermal energy transfer

Thermal energy moves by conduction in solids, convection in fluids and radiation, which needs no medium.

Conduction: vibrating particles pass energy to neighbours. Convection: warmer, less dense fluid rises and cooler fluid sinks. Radiation: infrared waves, which can cross a vacuum.

Dark, matt surfaces are good emitters and absorbers of radiation; light, shiny surfaces are poor ones. Hotter objects emit more.

Conduction

Convection

Convection

Conduction passes energy between neighbouring particles, mostly in solids; in liquids and gases, warm less dense regions rise in convection currents.

Conduction

Where:
Solids mostly
How:
Particles vibrate and pass energy on

Convection

Where:
Liquids and gases
How:
Density differences make currents

Radiation

Where:
Infrared waves
How:
Works through a vacuum

How does energy reach us from the Sun?

Reducing unwanted transfers

Insulation, lubrication and streamlining reduce unwanted transfers, so more input energy reaches the useful output store.

Wasted energy is usually thermal energy lost to the surroundings. Reducing it increases a device's efficiency.

020406080No lubricationLubricatedMachineEnergy per 100 J input (J)
  • Useful
  • Wasted
Illustrative machine: lubrication reduces wasted energy from friction, so more of every 100 J input is useful.

Insulation

Example:
Loft and cavity wall insulation
Effect:
Cuts heat loss by conduction and convection

Lubrication

Example:
Oil between moving parts
Effect:
Cuts friction and heating

Streamlining

Example:
Smooth shape
Effect:
Cuts air resistance

Which reduces friction in a bicycle chain?

Try an exam question

A motor is supplied with 200 J of electrical energy and transfers 150 J as kinetic energy. (a) Calculate the efficiency of the motor. (b) State what happens to the other 50 J and how it could be reduced.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.