Energy transfers

Name 4 methods of energy transfer

1. mechanically

2. electrically

3. heating

4. radiation

mechanically

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

electrically

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)

Name the 8 energy stores

1. chemical

2. kinetic

3. gravitational potential

4. elastic

5. thermal

6. magnetic

7. electrostatic

8. nuclear

Kinetic

Moving objects have energy in their kinetic store

Gravitational

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

Elastic

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

Magnetic

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

Electrostatic

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

Chemical

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

Nuclear

Atomic nuclei release energy from their nuclear store during nuclear reactions

Thermal

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

State the principal of conservation of energy

energy cannot be create or destroyed, only transferred from one store to another.

equation linking efficiency, useful energy output, and total energy output

efficiency =

useful energy output/total energy output x100

Name the 3 methods of thermal energy transfer

1. Convection - occurs in substances that flow

2. Conduction - mainly in solids

3. Radiation - electromagnetic waves

Describe how thermal energy transfer takes place by conduction

• vibrating particles transfer energy from their kinetic energy store to the kinetic energy stores of neighboring particles.

• kinetic energy is gradually passed through solid, causing a rise in temperature.

describe how thermal energy transfer takes place by convection

occurs in liquids and gases

more energetic particles move from the hotter region to the cooler region → transferring energy as they do so.

explain the role of convection in an immersion heater

1. energy transferred from heater coils to the thermal energy store of the water (by conduction.)

2. particles near coil gain energy and move faster.

3. bigger distance between particles.

4. water expands and becomes less dense.

5. reduction in density = hotter water rises above colder water (which is more dense.)

6. hot water rises and displaces colder water.

7. colder water sinks towards heating coils.

8. process repeats

9. convection currents occur - circulating energy through the water.

explain the process of convection

1. liquid or gas heated at the bottom.

2. liquid gains thermal energy.

3. liquid expands (particles move further apart.)

4. liquid becomes less dense.

5. less dense liquid rises.

6. liquid reaches top and cools.

7. liquid contracts as it cools.

8. liquid particles become more dense.

9. more dense liquid sinks.

10. convection current is established.

11. cycle repeats.

describe an experiment to investigate convection currents using coloured crystals

1. fill beaker with cold water and place on Bunsen, tripod and gauze.

2. put potassium manganate crystals in centre of beaker.

3. gently heat water under the crystals.

4. temp increase = potassium manganate crystals dissolve.

5. bright purple solution formed.

6. purple solution carried through the water by convection.

7. purple solution traces out the path of convection currents in the water.

describe an experiment to investigate the conductivity of metals (using 4 metals)

-aluminum, copper, brass, iron…

1. the four metals are places centrally on the tripod.

2. water placed into the hollows at the end of each metal.

3. Bunsen burner placed in the centre (so all metals are heated equally.)

4. water on the best conductor will boil first.

describe an experiment to show conduction (using beads and wax)

1. attach beads at regular intervals along a metal bar, using wax.

2. hold metal bar in clamp stand.

3. at other end of metal bar (away from beads) heat with a Bunsen flame.

4. overtime, energy is transferred along the bar by conduction.

5. as temp increases, wax gradually melts and beads will fall.

6. this shows conduction.

explain why thicker walls reduce unwanted energy transfer

• brick and glass are good thermal conductors.

• the thicker the walls, the slower the rate of energy transfer

explain why insulation reduces unwanted energy transfer

-to reduce energy transfers away from a system by conduction, use materials with low thermal conductivity…

• insulation traps pockets of air.

• air can’t move → energy conducts very slowly through the pockets of air, in addition to the material in-between, also with a low thermal conductivity.

explain why double glazing reduces unwanted energy transfer

• layer of gas between two pains of glass insulates windows

describe how thermal energy transfer occurs by radiation

• thermal radiation consists of electromagnetic waves emitted by the sun.

• all objects are continuously emitting and absorbing infrared radiation.

(radiation) an object hotter than its surroundings…

emits more radiation than it absorbs

(radiation) an object cooler than its surroundings…

absorbs more radiation than it emits

Black

good absorber

good emitter

dull/dark

reasonable absorber

reasonable emitter

white

poor absorber

poor emitter

shiny

very poor absorber

very poor emitter

if the rate of absorption is greater than the rate of emission..

the temperature of the body increases

if the rate of absorption is equal to the rate of emission…

the temperature of the body remains constant

if the rate of absorption is less than the rate of emission

the temperature of the body decreases

describe an experiment to investigate the emission of thermal radiation

• a Leslie Cube is a hollow metal cube.

• the 4 vertical faces have different surfaces. (black paint, matt white paint, shiny metal, and dull metal)

1. fill Leslie cube with boiling water.

2. hold thermometer against each of the 4 faces. they all have the same temperature.

3. hold infrared detector at set distance.

4. record the amount of IR radiation detected.