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Physics > Transfer Of Thermal Heat > Flashcards

Transfer Of Thermal Heat Flashcards

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1
Q

Define ‘Convection’

A

Process by which thermal energy is transferred from one place to another by movement of heated particles of gas or liquid. Main mode of heat transfer in fluids- liquids and gases.

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2
Q

What is convection current and explain it.

A

Explained by: heating beaker of water. Water at bottom that is heated expands, density decreases, warm water rises. Cool water at top being denser moves down, gets heated in turn. Thus convection current is set up with less dense warm fluids carrying thermal energy upwards. This circulation of liquid is called convection current. Eg electric heater.

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2
Q

Give examples of convection in everyday applications.

A

Electric heater,hot water system,cooling system in car(Thé Radiator). Air con,refrigerator,ventilation in kitchen.

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3
Q

Explain the formation of seas breezes during the day.

A

Land heats up faster than water, air above land heats up and rises, cool air above seas rushes in.

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4
Q

Explain the formation of land breezes during the night.

A

Land cools faster then sea, cool air above land moves to sea, warm air above sea rises.

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5
Q

Define ‘Conduction’

A

Process by which thermal energy is transmitted through a medium from one particle to another. Main mode of heat transfer in solids.

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6
Q

Simplify ‘thermal energy of a body’

A

It’s the total kinetic energy of atoms or molecules in body.

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7
Q

What is heat defined as?

A

Heat is energy in transit from high temperature object to lower temperature object. Recognized into as it crosses boundary of system.

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8
Q

Which states of matter are poor thermal conductors?

A

Liquid and gases.

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9
Q

How does conduction work? In general solids.

A

Takes place when TE transferred from 1 particle to next. When 1 rod end heated, molecules there gain energy, vibrate faster. These molecules collide with less energetic neighbors. Some of their energy transferred to these neighboring molecules which in turn gain kinetic energy. In this way,TE passes along rod by vibrating molecules. Transfer of energy from molecule to molecule continues till cold end reaches same temperature as hot end. No net movement of molecules during process of conduction.

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9
Q

Why are solids better conductors of thermal energy than liquids and gases?

A

Molecules in solids are closer together than in liquids and gases. Thus kinetic energy can be transferred more quickly.

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10
Q

How is thermal energy conducted through metals?

A

Conduction takes place through vibrating molecules and free electrons in metals. When heated, free electrons gain energy,move faster. These electrons free to travel in spaces between molecules before colliding with other electrons and molecules, transferring some of their energy to them. Process much faster than conduction by vibration of molecules in body. Hence metals conduct heat much faster than non metals which have no free electrons.

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11
Q

Why are insulators poor conductors of heat?

A

They contain pockets of STILL air. Air is very poor conductor thermal energy.

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12
Q

What are some examples of good conductors?

A 
Gold. 
Silver. 
Copper. 
Iron. 
Brass. 
Aluminum.
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13
Q

What are some examples of insulators?

A 
Glass
Stone
Water
Plastic
Wood
Polystyrene foam
Wool 
Fiberglass
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14
Q

When are good conductors used?

A

Situations where heat has to be quickly transmitted. Eg cooking utensils(made of good conductors)

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15
Q

How are insulators used?

A

They keep unwanted heat away, eg mittens. And prevent heat loss, eg metal flask, wool used in winter clothing.

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16
Q

Conduction and convection require a medium to transfer thermal energy. True or false?

A

True.

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17
Q

Define ‘Radiation’.

A

Transfer of thermal energy by electromagnetic waves, does not require material medium.

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18
Q

How does heat radiate from an object?

A

Heat is transferred by infra red waves. Eg heat from sun transferred by radiation to earth.

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19
Q

When does convection occur?

A

Only when objects are placed in path of convection current.

20
Q

What factors contribute to rate of energy transfer?

A
  1. Color and texture of surface of body. Dull black surface best heat radiator,shiny smooth surface worst heat radiator.
  2. Surface temperature of body. Rate increases as temperature increases.
  3. Surface area of body. Rate increases as surface area increases.
21
Q

When is an object a net emitter of radiation?

A

When temperature of object is higher than that of its surroundings.

22
Q

When is an object a net absorber of radiation ?

A

When temperature of object is lower than that of its surroundings.

23
Q

A rough dull black surface is both ..

A

Both a good emitter and a good absorber.

24
Q

A smooth polished surface is both..

A

Both a poor emitter and a poor absorber.

25
Q

Example of use of emitter of radiation ?

A

Inner surface of oven is rough and painted black.

26
Q

Example of good absorber of radiation?

A

Coils of solar heater painted dull black.

27
Q

Example of use of poor absorber of radiation?

A

Space suits of astrometry are shiny.

28
Q

Example of use of poor emitter of radiation?

A

Water kept warm longer in shiny chrome kettle compare to water in a kettle blackened with soot cause shiny chrome is poor emitter of heat.

31
Q

What happens if a hot object is places next to a cold object?

A

There’s transfer of thermal energy from object of higher temperature to object of lower temperature.

32
Q

Which is the most significant process of thermal energy transfer in fluids?

A

Convection.

33
Q

What’re convection currents caused by?

A

Conduction currents are caused by density changes. Hot fluids rise and cold fluids sink.

34
Q

Why are air conditioners best placed near high ceiling of room?

A

As hot air rises and cold air sinks, the cold air blown by air con sinks, and the warmer air travels upwards due to density changes. This creating a cycle.

35
Q

Why does convection occur more readily in gases than in liquids?

A

Gases expand much more in liquids when their temperature rises.

36
Q

How does radiation work?

A

Source of thermal energy transmit energy transmit energy in form of waves. These waves constitute to part of electromagnetic spectrum, known as infra red radiation. When absorbed, energy of waves transform into thermal energy of receiving body.

37
Q

How does the sun’s energy travel to earth?

A

By radiation. As there’s vacuum between earth and sun, conduction and convection can’t take place. Sun’s energy travels to earth as electromagnetic waves at speed of light.

38
Q

Why does convection not take place in solids?

A

Molecules in solid vibrate about fixed positions. In convection, atoms/ molecules move from one place to another.

39
Q

How does the vacuum flask reduce transfer of thermal energy?

A

By: CONDUCTION, CONVECTION AND RADIATION

40
Q

What’re the features of a vacuum flask?

A

1) Double walled glass container that contains vacuum.

2) Supported on foam plastic which is a poor heat conductor

41
Q

Explain how milk is heated on a saucepan.

A

Heat transferred from hot plate to saucepan by Conduction, convection currents are then set up so that milk is heated up.

42
Q

Explain how the refrigerator is cooled by convection currents.

A

Cold air sinks to be cooled by refrigerator, while hot air rises to freezer compartments to be cooled, this convection currents are created.

43
Q

Explain how the vacuum flask works to minimize heat loss.

A
  1. No thermal energy can leave/enter flask by conduction/convection across vacuum.
  2. Inner silvered surface reflects radiation from hot fluids back into flask.
  3. Outer silvered surface reflects radiation in external surroundings away from flask.
  4. Foam plastic support and plastic cap minimize thermal energy transmitted by conduction through thin glass walls of flask.
  5. Lastly, plastic cap stops convection and evaporation.
44
Q

Give 2 reasons why steel wired shelves are used rather than solid shelves in the centre of the refrigerator.

A

They allow convection currents to flow and conduct away any heat present in refrigerator.

45
Q

Explain how expanded polystyrene and aluminum foil reduce thermal energy entering the refrigerator.

A

Aluminum foil is poor heat absorber. Expanded polystyrene traps air, which is bad heat conductor to reduce heat gain due to conduction and convection.

46
Q

Explain how convection currents are created in an electric kettle.

A

Hot water rises and cool water sinks. Thus convection currents are created due to density differences.

47
Q

Give 2 reasons why energy used to heat water is always less than energy supplied to kettle.

A

Some energy lost to surrounding due to radiation. Kettle casing also absorbs some of the heat energy supplied.

48
Q

Why are most kettle bodies made from plastics instead of metals?

A

Help reduce heat loss to surrounding as plastic is better insulator.

49
Q

Explain why when a liquid evaporates, a cooling effect occurs.

A

Evaporation brings about cooling as the more energetic molecules leave the surface of the liquid, leaving behind the less energetic molecules.

50
Q

What happens when a beaker of water is heated up?

A
  1. Water being heated expands, density decreases, warm water rises.
  2. Cooler water at top being denser moves down and gets heated.
  3. Convection current set up with less denser lucid carrying thermal energy upwards.
51
Q

What happens when one end of a metal rod gets heated?

A
  1. Molecules there gains energy, vibrate faster.
  2. These molecules collide with their less energetic neighbors.
  3. Some of their energy transferred to neighboring molecules, gain KE
  4. Thermal energy is passed along rod by vibrating molecules.
  5. Energy transfer stopped when temp of cold end same as hot end

Physics (9 decks)

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