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

Phy: P1-P3 OCR > P1* - Moving/Storing Heat, Conduction/Convection, Heat radiation, Saving Energy > Flashcards

Flashcards in P1* - Moving/Storing Heat, Conduction/Convection, Heat radiation, Saving Energy Deck (44)
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1
Q

What happens to a substance when it is heated?

A

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.

2
Q

What scale is heat energy measured on?

A

Heat energy is measured on an absolute scale.

3
Q

What is an absolute scale?

A

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

4
Q

What is the unit of heat energy?

A

Joules.

5
Q

What is temperature a measurement of?

A

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.

6
Q

What is temperature measured in?

A

Temperature is usually just measured in degrees celsius.

7
Q

Why is temperature not measured on an absolute scale?

A

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

8
Q

What direction does energy flow in?

A

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

9
Q

What correlation does temperature difference have with rate of cooling?

A

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.

10
Q

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

A

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

11
Q

What is specific heat capacity?

A
  • 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.
12
Q

Why is water good for central heating systems?

A

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.

13
Q

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

A

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.

14
Q

How do you get convection currents?

A
  • 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.
15
Q

When does convection occur?

A

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

16
Q

Why cannot convection happen in solids?

A

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

17
Q

How can you reduce convection?

A

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

18
Q

How does cavity wall insulation prevent convection?

A

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.

19
Q

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

A

Heat can be transferred by radiation.

Heat is radiated as infra-red waves.

20
Q

What are infra-red waves?

A

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

21
Q

How is radiation different from convection/conduction?

A
  • 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.
22
Q

What are all hot objects always doing?

A

All objects are continually emitting and absorbing heat radiation.

23
Q

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

A

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

24
Q

Why do colder objects get hotter?

A

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.

25
Q

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

A

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.

26
Q

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

A

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

27
Q

Why are panels for solar water heating painted black?

A

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

28
Q

How do Grills and toasters heat food?

A

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
29
Q

Why do people often line their grill with shiny foil?

A

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

30
Q

How do Microwaves cook food?

A

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.

31
Q

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

A

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.

32
Q

What do microwaves use to cook food?

A

Microwave ovens use radiation to cook food:

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

33
Q

What are things that emit energy called?

A

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

34
Q

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

A

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

35
Q

How can you save energy?

A
  • 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.
36
Q

What is payback time?

A

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

37
Q

What are the various methods of insulating the home?

A
  • Loft insulation
  • Hot water tank jacket
  • Cavity walls & insulation
  • Double Glazing
  • Thick Curtains
  • Draught Proofing
38
Q

How does loft insulation reduce energy loss?

A
  • Fibreglass “wool” laid on the loft floor and sealing reduces energy loss from the house by conduction and convection
39
Q

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

A
  • It reduces conduction
40
Q

How does Cavity Wall & Insulation reduce energy loss?

A
  • 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. \\
41
Q

What type of energy loss do thick curtains reduce?

A

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

42
Q

How does double glazing reduce heat loss/

A

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

43
Q

What is a thermogram?

A

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

44
Q

What happens when you heat a solid?

A

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.

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