Physics - energy stores and transfers

A system

An object or group of objects, where energy is stored and transferred.

What are objects that are not part of the system classed as?

The surroundings.

When is the system in equilibrium?

When no change occurs.

What happens when a change occurs?

Energy is transferred to or from the system.

Energy stores:

• Kinetic

• Gravitational

• Elastic

• Electrostatic

• Chemical

• Nuclear

• Thermal

Kinetic energy store

Moving objects have energy in their kinetic store

Gravitational energy store

Objects gain energy in their gravitational potential store when they are lifted through a gravitational field

Elastic energy store

Energy stored when squished, bent, stretched.

Magnetic energy store

Magnetic materials interacting with each other.

Electrostatic energy store

Objects with charge interacting with one another

chemical energy store

Chemical reactions transfer energy into or away from a substance’s chemical energy store.

Nuclear energy store

Atomic nuclei releases energy from their nuclear store during nuclear reactions.

Thermal energy store

• All objects have energy in this store

• Hotter objects have more energy in their thermal energy store

units for energy

Joules (J)

Open system

Allows the exchange of energy / matter to or from

Principle of conservation of energy

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

Closed system

Can exchange energy but not matter to or from its surroundings

→ total amount of energy is constant

→ mass is preserved within the system’s boundary

Isolated system

Does not allow the transfer of matter or energy to or from its surroundings

Energy transfers

Energy is transferred between stores via transfer pathways

Transfer pathways

• Mechanically

• Electrically

• By heating

• By radiation

Mechanical working (transfer pathway)

When a force acts on an object (pulling, pushing, stretching)

Electrical working (Transfer pathway)

A charge moving through a potential difference

Heating by particles (energy transfer)

Energy is transferred from a hotter object to a colder one (conduction)

Heating by radiation (energy transfer)

Energy is transferred by electromagnetic waves

Example: energy transfer - hot coffee heating up cold hands

• The system is the coffee, mug and hand

• Energy is transferred by heating from the thermal store of the coffee to the thermal store of the mug

• Energy is transferred by heating from the thermal store of the mug to the thermal store of the hand

Example: energy transfer - a battery powering a torch

• The system is the battery and the torch

• The energy begins in the chemical store of the cells of the battery

• Energy is transferred to the thermal store of the bulb in the torch

• Energy is transferred by the flow of charge around the circuit, therefore the transfer pathway is electrical

• Energy is transferred electrically from the chemical store of the battery to the thermal store of the bulb

Example: energy transfer - a ball falling

• The system is the ball

• To fall the ball must have been raised, therefore the energy began in its gravitational potential store

• By falling the ball is moving, energy is transferred to its kinetic store

• For an object to fall, a force must be acting on it, the transfer pathway is mechanical

• Energy is transferred mechanically from the gravitational potential store of the ball to the kinetic store of the ball

How to describe an energy transfer

1) Determine the store that energy is being transferred away from

2) Determine the store that the energy is transferred to

3) Determine the transfer pathway