P1* - Moving/Storing Heat, Conduction/Convection, Heat radiation, Saving Energy

Question 1

What happens to a substance when it is heated?

Answer 1

When a substance is heated, it’s particles gain kinetic energy (KE). This makes the particles in a gas or liquid move around faster. In a solid, the particles vibrate more rapidly. This may eventually cause solids to melt, liquids to boil.

Question 2

What scale is heat energy measured on?

Answer 2

Heat energy is measured on an absolute scale.

Question 3

What is an absolute scale?

Answer 3

It means that it can’t go lower than zero, this is because there is a limit to how slow particles can move.

Question 4

What is the unit of heat energy?

Answer 4

Joules.

Question 5

What is temperature a measurement of?

Answer 5

Temperature is a measure of the average kinetic energy of the particles in a substance, the hotter something is, the higher its temperature, and the higher the average KE of its particles.

Question 6

What is temperature measured in?

Answer 6

Temperature is usually just measured in degrees celsius.

Question 7

Why is temperature not measured on an absolute scale?

Answer 7

It is not measured on an absolute scale, this is because temperatures can go below zero.

Question 8

What direction does energy flow in?

Answer 8

Energy tends to flow from hot objects to cooler objects. E.g. warm radiators heat the cold air in your room.

Question 9

What correlation does temperature difference have with rate of cooling?

Answer 9

The greater the difference in temperature the faster the rate of cooling, e.g. a hot cup of coffee will cool down quicker in a cold room rather than in a warm room.

Question 10

What happens to materials that need to gain lots of energy, when they cool down?

Answer 10

Materials which need to gain lots of energy to warm up also release loads of energy when they cool down.

Question 11

What is specific heat capacity?

Answer 11

• Specific heat capacity is the measure of how much energy a substance can store.
• Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1 degrees celsius.

Question 12

Why is water good for central heating systems?

Answer 12

Specific heat capacity of water is high, once water is heated it stores a lot of energy, which makes it good for central heating systems.
Water is a liquid, so it is can easily be pumped around a building.

Question 13

What happens when you heat a liquid ( process of boiling )?

Answer 13

When you heat a liquid the heat energy makes the particles move faster. Eventually when enough of the particle have enough energy to overcome their attraction to each other, big bubbles of gas form in the liquid, this is boiling.

Question 14

How do you get convection currents?

Answer 14

• When you heat up a liquid, the particles move faster and the fluid ( liquid or gas ) expands, becoming less dense
• Warmer, less dense fluid rises above its colder denser surroundings.
• As warm fluid rises, colder fluid takes its place, as this process continues you end up with a circulation of fluid ( convection currents ). This is one way in which immersion heaters work.

Question 15

When does convection occur?

Answer 15

Convection occurs when the more energetic particles move from the hotter region to the cooler region, and take their heat energy with them.

Question 16

Why cannot convection happen in solids?

Answer 16

Convection cannot happen in solids because the particles cannot move, they just vibrate on the spot.

Question 17

How can you reduce convection?

Answer 17

To reduce convection you need to stop the fluid moving,:

Question 18

How does cavity wall insulation prevent convection?

Answer 18

Cavity wall foam insulation all work by trapping pockets of air. The air can’t move so the heat has to conduct very slowly through the pockets of air, as well as the material in-between.

Question 19

Besides convection, and conduction, how else is heat transferred?

Answer 19

Heat can be transferred by radiation.

Heat is radiated as infra-red waves.

Question 20

What are infra-red waves?

Answer 20

Infra-red waves are electromagnetic waves that travel in straight lines at the speed of light.

Question 21

How is radiation different from convection/conduction?

Answer 21

• It doesn’t need a medium to travel through, so it can occur in a vacuum like space, this is the only way radiation reaches us from the sun.
• It can only occur through transparent substances, like air, glass, and water.
• The amount of radiation emitted or absorbed by an object depends to large extent on its surface colour and texture, this isn’t true for conduction and convection.

Question 22

What are all hot objects always doing?

Answer 22

All objects are continually emitting and absorbing heat radiation.

Question 23

What happens to an object the hotter it gets, in regards to radiation?

Answer 23

the hotter an object gets, the more heat radiation it emits.

Question 24

Why do colder objects get hotter?

Answer 24

Cooler objects will absorb the heat radiation emitted by hotter things, so their temperature increases.
For example, you can feel heat radiation if you are indoors and the sun shines on you through a window.

Question 25

What type of surface is a very good absorber of radiation?

Answer 25

A Matt black surface is a very good absorber of radiation, they are also very good emitters of radiation
Radiators could be painted black to help emit heat radiation, and fridges white to help reflect radiation.

Question 26

What type of objects are poor emitters and absorbers of radiation?

Answer 26

Light coloured, smooth and shiny objects are very poor emitters and absorbers of radiation, they effectively reflect heat radiation
e.g. some people put shiny foil behind their radiators to reflect radiation back into the room rather than heat the wall up

Question 27

Why are panels for solar water heating painted black?

Answer 27

They are painted blak to absorb as much heat as possible.

Question 28

How do Grills and toasters heat food?

Answer 28

Grills and toasters heat food by infrared (heat) radiation:

• The heat radiated by a grill is absorbed by the surface particles of the food, increasing their kinetic energy.
• The heart energy is then conducted or convected to more central parts

Question 29

Why do people often line their grill with shiny foil?

Answer 29

This reflects heat radiation onto the bottom of the food being grilled so the food is cooked more evenly.

Question 30

How do Microwaves cook food?

Answer 30

Microwaves penetrate about 1 cm into the outer layer of food where they’re absorbed by water or fat molecules, increasing their kinetic energy. The energy is then conducted or convected to other parts.

Question 31

Why do you not cover food with foil in a microwave?

Answer 31

The microwaves would then be reflected away, they will not cook food and cause dangerous sparks inside the microwave.
It is however okay to cover them in plastics, or glass, as the microwaves will pass right through.

Question 32

What do microwaves use to cook food?

Answer 32

Microwave ovens use radiation to cook food:

They use microwaves, these are electromagnetic waves that have a different wavelength to infra-red.

Question 33

What are things that emit energy called?

Answer 33

Things that emit energy are called sources, e.g. radiators

Question 34

What are things that transfer and waste or lose energy called?

Answer 34

things that transfer and waste or lose energy are called sinks, e.g. windows and computers.

Question 35

How can you save energy?

Answer 35

• You can save energy by insulating your house, so the sinks “drain” less energy.
  e. g. Use curtains to reduce energy loss.
• You can make sources and sinks more efficient, so they waste less energy, e.g. use energy saving light bulbs instead of normal ones.

Question 36

What is payback time?

Answer 36

Payback time is the time it takes for the money you have saved on energy bills to equal the cost to buy the insulation.

Question 37

What are the various methods of insulating the home?

Answer 37

• Loft insulation
• Hot water tank jacket
• Cavity walls & insulation
• Double Glazing
• Thick Curtains
• Draught Proofing

Question 38

How does loft insulation reduce energy loss?

Answer 38

• Fibreglass “wool” laid on the loft floor and sealing reduces energy loss from the house by conduction and convection

Question 39

What form of energy loss does Hot Water Tank Jacket reduce?

Answer 39

• It reduces conduction

Question 40

How does Cavity Wall & Insulation reduce energy loss?

Answer 40

• Two layers of bricks with a gap between them reduce conduction, but you still get some energy loss by convection
• Squirting insulating foam into the gap traps pockets of air to minimise convection.
  /// However Heat is still lost through the walls through radiation, also if there are spaces where the air is not trapped, there will still be some convection too. \\

Question 41

What type of energy loss do thick curtains reduce?

Answer 41

Thick curtains reduce heat loss through convection and conduction through the windows.

Question 42

How does double glazing reduce heat loss/

Answer 42

Double glazing has two layers of glass with an air gap between reduce conduction.

Question 43

What is a thermogram?

Answer 43

A thermogram is a picture taken with a thermal imaging camera.

Question 44

What happens when you heat a solid?

Answer 44

When you heat a solid, heat energy makes the particles vibrate faster until eventually the forces between them are overcome and the particles start to move around and flow, this is melting.