Changes in Energy — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

Section 1 of 8Four pathways

Energy transfer pathways

Heating by particles

• Heating by particles Particles collide and share kinetic energy, from the hotter to the colder object, by conduction or convection.

Heating by radiation

• Heating by radiation Energy travels as electromagnetic waves, e.g. infrared from a fire or light from the Sun.

Mechanical work

• Mechanical work A force moves an object through a distance, transferring energy between stores.

Electrical work

• Electrical work A current flows because of a potential difference, transferring energy from the power source to components.

Section 2 of 8Heating

Effect

• Effect Heating raises the kinetic energy of the particles in a system, which increases its thermal store.

Result

• Result The temperature rises, or the substance changes state, e.g. solid to liquid.

Example: pan on a hob

• Example: pan on a hob Electrical work moves energy from the mains to the thermal store of the hob; the hob then heats the pan,
  • increasing the thermal store of the pan.

Example: sunbathing

• Example: sunbathing Radiation from the Sun is absorbed by the skin,
  • so the thermal store of the body increases without any contact between the Sun and the person.

Section 3 of 8Work done by forces

1

A person pushes a box across a floor, applying a force over a distance.

2

Energy is transferred mechanically from the person's chemical store to the kinetic store of the box.

3

Friction between the box and floor transfers energy by heating to the thermal stores of both.

4

Sound waves carry a little energy to the air, increasing its thermal store.

Section 4 of 8Choosing the system

System	Energy transfer described
Person and box	Mechanical transfer from the person to the box's kinetic store
Box and floor	Heating from the box's kinetic store to the thermal store of the floor (friction)
Cell and lamp	Chemical store of the cell to the thermal store of the filament by electrical work; then radiation as light

Section 5 of 8Worked examples of transfers

Situation	Store that decreases	Pathway	Store that increases
A ball is dropped and falls	Gravitational potential	Mechanical work by the weight	Kinetic
A kettle boils water	Electrical supply (mains)	Electrical work, then heating	Thermal store of the water
A catapult fires a stone	Elastic potential	Mechanical work	Kinetic
A toaster browns bread	Electrical supply	Electrical work, then radiation	Thermal store of the bread
A car brakes to a stop	Kinetic	Friction doing work (heating)	Thermal store of brakes and surroundings

Section 6 of 8Closed and open systems

Closed system

• Closed system No energy enters or leaves the system, so the total energy stays the same even though it moves between stores inside it.

Open system

• Open system Energy can cross the boundary, e.g. a hot drink in a cup losing energy by heating to the air in the room.

Why it matters

• Why it matters Choosing a system with its surroundings included makes it easier to show that energy is conserved: the total is unchanged.

Section 7 of 8Key terms

Term	Meaning
System	The object or group of objects being considered in an energy transfer.
Mechanical work	Energy transferred when a force acts through a distance.
Electrical work	Energy transferred when a current flows through a circuit.
Potential difference	The energy transferred per unit charge, provided by a cell or power supply.
Radiation	Energy transferred by electromagnetic waves, without needing particles.
Conservation of energy	Energy cannot be created or destroyed, only transferred between stores or dissipated.

Section 8 of 8Exam technique

1

Treating 'heat' as a thing that is stored. Energy is stored in the thermal store and transferred by heating.

2

Forgetting that a current only flows when there is a potential difference.

3

Not defining the system, so transfers to the surroundings are missed or double-counted.

4

Saying radiation needs a medium. Electromagnetic waves can cross a vacuum.

5

Describing only the store that gains energy and forgetting to name the store that loses it.