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Physics > Heat Transfer > Flashcards

Heat Transfer Flashcards

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1
Q

Why are metals good conductors of heat?

A

Because the outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms.

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2
Q

Why are insulators poor conductors?

A

They do not have free electrons.

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3
Q

Explain:

Two thin blankets are usually warmer than one thick one.

A

A layer of air is trapped between them ( and air is a good insulator)

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4
Q

Why do flames go upwards?

A

Because with gravity and an atmosphere that supports the flame will rise because hot air is lighter than cool air. The heated air is less dense so therefore it’ll rise.

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5
Q

How do a chimney and fire help ventilate a room?

A

As the fire in the fireplace consumes the oxygen from the surrounding air it is also heating this air. This heated air then rises and as it rises cooler air is drawn in from the surrounding room. As long as the air in the fireplace is warmer than the air in the room, this process will still continue.

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6
Q

Convection currents can form when liquids and gases are heated. The cold liquid falls and the hot fluid rises. Why does a convection current occur?

A
  1. The fluid is heated
  2. In the heated fluid the particles spread out.
  3. The heated fluid is less dense and so it rises. As it travels across the space it is in, it loses heat energy and begins to cool.
  4. The cooler fluid is more dense and falls. It then travels along the ground back to the heat source.
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7
Q

What is conduction?

A

Transfer of energy from particle to particle in matter.

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8
Q

What is convection?

A

The process energy transfer in fluids.

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9
Q

Explain how convection works in a room with a radiator

A
  1. The radiator heats the air particles increasing their kinetic energy.
  2. The hot air expands and becomes less dense
  3. The less dense air rise like a hot air balloon
  4. Cooler air is drawn in to take its place
  5. The heated air is cooling losing its kinetic energy becoming more dense again and falling back to the floor.
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10
Q

What is radiation?

A

Radiation is energy that comes from a source and travels through space and may be able to penetrate various materials.

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11
Q

Fill in:

Dark, matt surfaces ________and ________ more infra red radiation than light shiny surfaces.

A

Absorb and emit

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12
Q

Fill in:

Light, shiny surfaces _________ more infra red radiation than dark, matt surfaces.

A

Reflect

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13
Q

How does the temperature of an object affect the rate at which it emits infrared radiation?

A

The higher the temperature of an object the greater the rate at which it emits infra red radiation.

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14
Q

What is a vacuum?

A

A region that doesn’t contain anything at all, even gas particles.

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15
Q

Why are houses in hot countries often painted white?

A

Because white is the worst absorber of infra red radiation.

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16
Q

Why are the pipes on the back of a fridge usually painted black?

A

Because black surfaces are the best absorbers and emitters of infra red radiation.

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17
Q

Why are saucepans often made of metal with wooden handles?

A

The metal base of a saucepan is a good conductor so heated is conducted quickly to the food in the pan. The wooden handle is a poor conductor so it prevents someone picking up the saucepan getting burnt.

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18
Q

Why are materials that trap air good insulators?

A

Air is a poor conductor so materials that trap air are good insulators.

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19
Q

Why does convection happen in solids?

A

Because the particles in a solid are held together and unable to flow.

20
Q

Why does a fluid become less dense when it is heated?

A

Because it expands so there is the same mass of material in a larger volume.

21
Q

Why does painting an object dull black maximise the rate of energy transfer?

A

Because dull black surface are the best emitters of evaporation.

22
Q

Why does does trapping air in small pockets minimise the rate of energy transfer?

A

It minimises the rate of energy transfer because convection currents cannot be set up.

23
Q

What does the stopper prevent in a vacuum flask?

A

Prevents heat loss by convection and evaporation

24
Q

What does the extra layer of a cup prevent?

A

It traps an additional layer of air preventing convection

25
Q

What does the vacuum prevent in the flask?

A

Prevents heat loss by conduction and convection

26
Q

What do the silver surfaces prevent or do in the flask?

A

They stop radiation

27
Q

What do the corks prevent?

A

Cork is a good insulator preventing conduction of heat

28
Q

How can you minimise the rate of energy transfer out of homes?

A

Fitting fibreglass loft insulation to reduce conduction
Cavity wall insulation that traps air in small pockets to reduce convection
Double glazing to reduce energy transfer by conduction through windows
Draught proofing to reduce convection

29
Q

How can you reduce the rate of energy transfer with a door?

A

You can have a draught excluder - the brush under the door.

30
Q

What is behind radiators and why?

A

Aluminium foil is behind radiators to reflect infra red radiation back into the room.

31
Q

Why is fibreglass a good insulator?

A

It contains trapped air

32
Q

Why are the pipes that contain the water in a solar panel often painted black?

A

The pipes that contain the water in a solar heating panel are often painted black because black is the best absorber of radiation.

33
Q

Which types of energy transfer involve particles?

A

Conduction, convection and evaporation

34
Q

How does energy from the sun reach the earth?

A

Energy from the sun reaches earth as radiation that travels through space.

35
Q

How does the colour of a surface affect the rate of conduction?

A

The colour of a surface has no effect on the rate of conduction

36
Q

Why are gases poor conductors

A

Their particles are far apart

37
Q

Explain in terms of particles how heat is conducted through a glass wall of the vacuum flask.

A

The particles are heated up and vibrate vigorously this kinetic energy is passed on from particle to particle, the particles are also moving rapidly so this helps with the movement of heat.

38
Q

Draughts transfer heat energy by?

A

Convection

39
Q

Explain how the air cavity between two walls reduces the heat transfer from the house.

A

Having an air cavity reduces the conducted heat because air is a good insulator and a poor conductor. Also a layer of air is trapped so it cannot flow to outside.

40
Q

Why do metal objects feel colder than wooden ones?

A

Because they conduct the heat away from your hand.

41
Q

How is thermal conduction shown through a bar with pins waxed to it?

A

At the heat travels along the bar the wax melts and the drawing pins fall off one by one.

42
Q

How to drink mats reduce the conduction from a hot cup?

A

They are made from good insulating materials to reduce the rate at which heat is conducted from a hot cup to the surface of a table. Hot water pipes are lagged with foam to reduce heat losses from the pipe.

43
Q

Why do kettles and saucepans have shiny outside surfaces?

A

To reduce heat losses by radiation

44
Q

What are thermals and what is their use?

A

When the sun heats the land during the day the hot air that rises above it is called a thermal. They can be used to provide the lift for gliders, para gliders and hang gliders.

45
Q

What is energy measured in ?

A

Joules

46
Q

What Is polystyrene?

A

A poor conductor, reducing heat conduction

47
Q

How does the heat travel from one end of a piece of metal to the other?

A

When you heat a piece of metal at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and on, the vibrations are passed along, the metal and so is the heat. We call this conduction

Physics (9 decks)

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