Topic 8 - Energy - Forces Doing Work

Energy Store - Kinetic

Moving objects have energy in their kinetic store

Energy Store - Gravitational

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

Energy Store - Elastic

Objects have energy in their elastic potential store if they are stretched, squashed or bent

Energy Store - Magnetic

Magnetic materials interacting with each other have energy in their magnetic store

Energy Store - Electrostatic

Objects with charge (like electrons and protons) interacting with one another have energy in their electrostatic store

Energy Store - Chemical

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

Energy Store - Nuclear

Atomic nuclei release energy from their nuclear store during nuclear reactions

Energy Store - Thermal

All objects have energy in their thermal store, the hotter the object, the more energy it has in this store

Energy transfer pathways(4)

• Mechanical

• Electrical

• Heating

• Radiation

Mechanical working

When a force acts on an object (e.g. pulling, pushing, stretching, squashing)

Electrical working

A charge moving through a potential difference (e.g. current)

Heating (by particles)

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

(Heating by) radiation

Energy transferred by electromagnetic waves (e.g. visible light)

How do energy flow diagrams work?

Different labels represent the energy store at that point.

The arrows represent the energy transfer from one type of store to another.

How do Sankey diagrams work?

Arrows represent the types of energy transferred.

The ones that are straight are useful energy.

The ones that curve to the bottom are wasted energy.

The thickness of all the arrows correspond to a numerical value of the energy type in question.

The thickness of each arrow should be in proportion to each other.

system

an object or a group of objects

What happens when a system is in equilibrium?

Nothing changes, so overall nothing happens

3 states of a thermodynamic system

Closed

Open

Isolated

Open thermodynamic system

allows the exchange of energy and matter to or from its surroundings

Closed thermodynamic system

can exchange energy but not matter to or from its surroundings

Isolated thermodynamic system

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

principle of energy conservation

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

useful energy transfers when a bat hits a ball

transfer from kinetic store of bat to kinetic store of ball

wasted energy transfers when a bat hits a ball

kinetic store of bat to:

thermal store of bat

thermal store of ball

thermal store of surroundings

useful energy transfers when boiling water in a kettle

energy transferred from the mains supply to the thermal store of the heating element in the kettle as as that gets hotter, energy is transferred to the thermal store of the water

wasted energy transfers when boiling water in a kettle

Some of the energy is transferred to the thermal store of the plastic kettle (wasted energy transfer)

And some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated (wasted energy transfer)

How is energy transferred by heating?

Increases the kinetic store of the particles which increases the energy in the thermal store of the object

When is energy transferred by force

When mechanical work is done - when a force acts over a distance

How is energy transferred when a current flows?

Current is the flow of charge that occurs when a p.d. is applied to the circuit that is provided by the power supply or a cell

Energy is transferred electrically from the power supply to the circuit components(electrical work is done by the power supply)

Energy from the cell’s chemical store is electrically transferred to the lamp's thermal store as the filament heats up. From the lamp’s thermal store, energy can be transferred by heating and radiation to the thermal store of the surroundings

Energy is also transferred by heating to the thermal store of the wires due to resistance

How is work done?

When an object is moved over a distance by a force acting in the direction of its displacement

work done equation

E = F x d

E = energy in joules

F = force in newtons

d = distance in metres

gravitational potential energy

the energy an object has due to its height in a gravitational field

How do changes in height affect the GPE of an object?

If the object is lifted up then energy is transferred to its GP store

If the object falls then energy is transferred away from its GP store

GPE equation

ΔGPE = mgΔh 

ΔGPE = change in gravitational potential energy, in joules (J)

m = mass, in kilograms (kg)

g = gravitational field strength in newtons per kilogram (N/kg)

Δh = change in vertical height in metres (m)

kinetic energy

the amount of energy an object has as a result of its mass and speed

KE equation

KE = 0.5mv²

KE = kinetic energy in joules (J)

m = mass of the object in kilograms (kg)

v = speed of the object in metres per second (m/s)

What is a perfect energy transfer and when can it assumed to be so?

a transfer such that there is no wasted energy

it can be assumed to be perfect if the wasted energy transfer is negligible

Is a perfect energy transfer actually possible?

no g no

What is an example of a ‘perfect energy transfer‘

a pendulum constantly switching between KE and GPE

What is wasted energy

Energy that cannot be ‘gathered’ for any specific use

How does friction act as a means of wasted energy transfers?

Heats up working parts, causing energy to transfer from the kinetic store to the thermal store of the surroundings

How can energy transfer waste by friction be reduced?

Lubrication of the parts that rub together

dissipated

spread out

three factors of quality of insulation

thermal conductivity

density

thickness

How does thermal conductivity affect the insulation of a material?

The lower it is, the less energy is transferred

Lower the better

How does density affect the insulation of a material?

The more dense an insulator the more conduction can occur as the particles would be closer together allowing for energy transfer between them to be more easier

How does thickness affect the insulation of a material?

The thicker it is the better it will insulate

How are house lofts often insulated

using fibreglass/glass fibre

The insulation is often made from fibreglass (or glass fibre)

This is a reinforced plastic material composed of woven material with glass fibres laid across and held together

The air trapped between the fibres makes it a good insulator

How are gaps between external walls insulated?

Cavity wall insulation

This is often done by drilling a hole through the external wall to reach the cavity and filling it with a special type of foam which is made from blown mineral fibre filled with gas

This lowers the conduction of heat through the walls from the inside to the outside

power

the rate of energy transfer

power equation

P = E/t

P = power, measured in watts (W)

E = energy transferred or work done, measured in joules (J)

t = time, measured in seconds (s)

efficiency

the rate of useful energy output to its total energy output

efficiency equation

efficiency = useful power output/total power input

or

efficiency = useful energy transferred by device/total energy supplied to the device