Energy transfers

The 8 main energy stores

Chemical

Kinetic

Gravitational

Elastic

Thermal

Magnetic

Electrostatic

Nuclear

4 ways energy can be transferred

Mechanically

Electrically

By heating

By radiation

How is energy transferred mechanically?

When a force is applied to move an object through a distance

(e.g. when gravity causes an object to accelerate & gives it kinetic energy)

How is energy transferred electrically?

By an electric charge flowing through an object

(e.g. when a current passes through a lamp it emits light & heat)

How is energy transferred by heating?

When energy moves from the thermal store of a hotter object to the thermal store of a colder object

(e.g. when a fire is used to heat up an object)

How is energy transferred by radiation?

When energy is transferred by waves

(e.g. when vibrations causes waves to travel through the air as sound or when an object emits electromagnetic radiation)

The principle of energy conservation is…

Energy cannot be made or destroyed, but it can be transferred from one store to another

Energy input = energy output

What is the formula linking efficiency, useful energy output & total energy output?

Efficiency = (useful energy output ÷ total energy output) × 100%

Wasted energy is…

The energy that is not transferred to a useful output

(e.g. when boiling a kettle, the sound energy that is released is considered as waste energy because it is not being used to heat the water)

Sankey diagrams are…

Used to represent the transfer of input energy into useful energy & wasted energy

A skateboarder starts at rest at the top of a hill & then pushes herself down, coming to a rest at the bottom

What energy changes take place?

The skateboarder starts with GPE and does work to push herself down the hill

As she is descending, GPE is converted into KE and friction

Eventually, the friction causes all of the energy to be converted into thermal energy in the surroundings, meaning that the skateboarder no longer has any KE & therefore comes to a rest

What energy changes are involved when a moving car stops?

Kinetic energy is transferred to thermal energy (through friction) as well as sound energy

What energy changes are involved when a ball at rest then rolls down a hill?

As the ball rolls downwards, GPE is converted into KE

The speed of the ball will increase as the vertical height decreases

3 ways thermal energy can be transferred

Conduction

Convection

Radiation

Thermal conduction is…

The transfer of thermal energy in solids & liquids by the vibration of particles

How does thermal conduction work?

When a solid or liquid is heated, the particles vibrate more which causes them to collide with each other

When the particles collide, thermal energy is transferred from the hotter parts to the cooler parts

Metals are good thermal conductors because…

They have delocalised electrons which can easily move throughout the metal

Therefore the rate of energy transfer is fast

Non-metals are thermal insulators because…

They do not have any free moving particles, decreasing the rate of energy transfer

Therefore they are used to reduce unwanted energy transfer (e.g. in homes)

Thermal convection is…

The transfer of thermal energy through a fluid by the upward movement of warmer & less dense regions of fluid

How does thermal convection work?

When a fluid is heated it expands which decreases its density because the particles move further apart

This causes the regions of higher density to sink & the regions of lower density to rise

Thermal convection doesn’t occur in solids because…

The particles are in fixed positions

Thermal radiation is…

The transfer of thermal energy by infrared electromagnetic waves

Good absorbers & emitters of thermal radiation are…

Black bodies with dull texture

Good reflectors of thermal radiation are…

Whites bodies with shiny texture

They can be used to reduce unwanted energy transfer

Thermal radiation can be increased with…

Higher temperatures & greater surface area