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LC Physics > Heat and Temperature > Flashcards

Heat and Temperature Flashcards

1
Q

Conductor

A

A material which allows energy to pass through it
/
A material which is able to conduct heat and/or electricity

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2
Q

Heat

A

Energy possessed by an object due to the kinetic energy of its atoms or molecules

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3
Q

Temperature + how its measured

A

A measure of the ‘hotness’ of a substance

Measured in °C (celsius)

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4
Q

Thermometric property

A

A physical property that changes measurably with temperature

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5
Q

Standard Unit (SI) of temperature

A

Kelvin (K)

represented by T in questions

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6
Q

Practical unit of temp

A

The degree Celsius (°C)

represented as t in questions

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7
Q

converting celsius and kelvin

A

°C = K - 273.15

t = T - 273.15

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8
Q

Thermometric properties

A
  1. Length of a column of liquid
  2. E.m.f of a thermocouple
  3. Pressure of a gas at constant volume
  4. Resistance
  5. Colour
  6. Thermometers
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9
Q

length of a column of liquid eg. mercury thermometer

A

Liquid expands when heated, it’s trapped in narrow column up middle of the thermometer, so the only place it can go is either up when heated or down when cooled

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10
Q

E.m.f of a thermocouple

A
  • E.m.f is the electromotive force ( a small voltage)
  • Thermocouple is two diff metals joined together (copper + iron?)
  • One junction is kept in a hot area, + other one is the cooler area
  • The bigger the temp diff, the bigger the emf (measured in millivoltmeter)
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11
Q

Pressure of a gas at constant volume

A
  • Take a glass gas syringe, put it face down in a beaker w/ weights on top of piston
  • The greater the rise in temp, the more weights needed to keep the volume constant as gas expands when heated

or

Take a glass gas syringe, put an air tight seal on end
once a constant volume has been set
-As gas is heated, expands + piston is pushed out

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12
Q

Resistance

A
  • Electrical resistance of a conductor increases as temp increases
  • Resistance of a semiconductor decreases w/ increasing temp
  • A thermistor uses resistance
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13
Q

Colour

A

There are crystals in the strip that change colour with increasing temp

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14
Q

Thermometers

A
  • Thermometers measure temp
  • They will agree @ freezing + boiling point (0°C and 100°C) but two thermometers do not necessarily give the same reading at other temps (called Disagreement)
  • Therefore, there is a need for standard thermometers, we use a commercial laboratory mercury thermometer as school standard
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15
Q

Using a thermometric property to measure temperature

A
  • Graduate two thermometers at ice and steam points

- Compare values obtained for an unknown temp, using a straight-line graph between the reference points

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16
Q

Practical thermometers

A

-Practical thermometers eg. clinical thermometer
-Infra-red radiation thermometer
-Plastic strip thermometer
Oven thermometer
Boiler thermometer
Temperature gauge in a car

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17
Q

Heat capacity

A

The amount of heat energy needed to change the temp of an object by 1 C / 1 K

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18
Q

Symbol for heat capacity

A

C

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19
Q

Unit for heat capacity

A

joule per Kelvin (J K⁻¹)

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20
Q

Formula for heat energy

A

Heat energy = heat capacity x change in temperature

Q = CΔθ

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21
Q

Heat energy to raise and given out as temp falls

A

The amount of heat energy needed to raise the temp of an object by 1°C (1 K) is the same as the amount of heat energy given out if its temp falls by 1°C (1 K)

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22
Q

Specific Heat Capacity

A

amount of heat energy necessary to change the temp of 1kg of a subdtance by 1°C (or 1K)

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23
Q

Specific heat capacity symbol + unit

A

sumbol: c

Unit: joule per kilogram per Kelvin ( J kg⁻¹ K⁻¹ )

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24
Q

Heat energy added/lost formula

A

mass x specific heat capacity x rise in temp

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25
Q

Real world applications of specific heat capacity

A

Storage heaters use concept of SHC, are warmed at night when it is cheaper to use electricity, and give out heat gradually during day. Contain bricks which release heat stored in them. They have a large heat capacity.

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26
Q

Latent heat of a substance

A

the heat energy neeed to change its state without a change in temp.
eg. solid to liquid (dry melting ice)

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27
Q

Latent heat symbol + unit

A

Symbol: L

Unit: joule (J)

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28
Q

perspiration

A

As perspiration evaporates, it takes latent heat from our bodies + cools us down

29
Q

perfume

A
  • Same thing happens if you spray perfume on your skin, or spill methylated spirits on yourself
  • Liquid takes its latent heat from your body, making you feel cool in that area
30
Q

specific latent heat of a substance

A

the amount of heat energy needed to change the state of 1kg of that substance w//o a change in temp

31
Q

Heat pump

A

transfers energy from a cooler region to a warmer one

32
Q

use of heat pump

A

refrigerators + in air conditioning systems

33
Q

how heat pump works

A
  • work must be done to bring this about
  • circulating liquid: high specific latent heat of vaporisation + low boiling point
  • liquid pumped around closed circuit
  • in expansion valve: pressure drops + liquid evaporator + takes in its latent heat from inside of (fridge), thus cooling it
  • vapour reaches compressor: pressure increased + turns back into a liquid, giving out its latent heat
  • heat gives off to surroundings from black pipes + cooling fins at back of fridge
34
Q

conduction

A

Movement of heat energy through a substance by the passing on of molecular vibration from molecule to molecule. There is no overall motion of the substance

35
Q

convection

A

Transfer of heat through a fluid by means of circulating currents of fluid caused by the heat

36
Q

radiation

A

Transfer of heat energy from one place to another in the form of electromagnetic waves

37
Q

differences between conduction, convection, radiation

A

conduction: no net movement of medium
convection: circulation of a fluid
radiation: electromagnetic/through a vacuum

38
Q

what is the thermometric property of a thermocouple?

A

emf

39
Q

what is the thermometric property of a mercury thermometer?

A

length/volume

40
Q

distinguish between heat and temperature

A

heat: a measure of energy
temperature: a measure of hotness

41
Q

the temp of the water at the bottom of the niagra falls would be greater than the top due to potential energy being converted into heat energy. In reality, the temp change will be small. Why?

A

energy converted into other forms + energy lost to surroundings

42
Q

two desirable physical properties of the fluid used in a heat pump

A

high specific latent heat of vaporisation

low boiling point / volatile

43
Q

energy lost through a window formula

A

energy lost = (time)(area)(u-value)(temp change)

44
Q

why does heat not travel through solids by means of convection?

A

the particles cannot move freeely

45
Q

describe graph of temp vs energy of melting ice

A

temp of ice increased (From x to y)
ice temp stays at 0 while ice is changing state
latent heat
temp increases again

46
Q

how could energy be supplied to something at a constant rate?

A
  • heating coil
  • joulemeter

/

water bath

47
Q

why is the bottom of a flat-plate collector blackened?

A

good absorber of heat/energy/radiation

48
Q

1 litre to m³

A

1 litre = 0.001 m³

49
Q

why does the liquid in a vacuum-tube solar collector have a large specific latent heat of vaporisation?

A

more energy released/absorbed during change of state

50
Q

name three ways heat could be lost

A

conduction
convection
radiation

51
Q

how is the sun’s energy trapped in a vacuum-tube collector?

A

silvered walls prevent radiation

evacuated walls prevent conduction + convection

52
Q

advantage

A

geothermal system functions all the time

solar heating system works only during daytime

53
Q

describe operation of a heat pump in terms of heat transfer

A

energy taken from body/place (making it colder)

to another body/place (making it hotter)

54
Q

why is a filament bulb not an efficient source of light?

A

It converts most of its electrical energy input into heat rather than light.

55
Q

..In reality, the time taken to heat the water will be greater. Why?

A
  • energy will be lost to surroundings
  • lost by evaporation
  • converted to other forms of energy
56
Q

why do the thermistor and thermocouple thermometer give diff temp readings

A

each of the devices has a diff thermometric property that changes differently with temperature

57
Q

calculate the heat generated by a toaster in 2 mins if it has an efficiency of 96%

A

heat generated = power x time

then find 96%

58
Q

emf

A

voltage/work done in moving unit charge/electromotive force

59
Q

SI unit of temp

A

Kelvin / K

60
Q

advantage of using Kelvin in scientific measurements

A

always positive

61
Q

lab experiment to demonstrate principle of operation of a thermocouple

A
  • two diff metals
  • two junctions held at diff temps
  • voltage/emf measured
62
Q

advantage of using a thermocouple thermometer instead of a mercury-in-glass thermometer

A
  • less fragile
  • wider temp range
  • mercury is toxic
63
Q

why is it necessary to have a standard thermometer

A

diff thermometers have diff thermometric properties + give diff readings at same temperature

64
Q

state principle that underlies operation of storage heater

A

large heat capacity
heated when electricity is inexpensive (off peak) / store a large quantity of energy / release energy slowly (during the day)

65
Q

units for specific latent heat, etc

A

know, they are not in log table

66
Q

temp steam will condense

A

between 0 and 100, will not condense above 100 though

67
Q

when using energy for temp change formula

A

always have temp as positive, even if temp drops!

68
Q

U-value

A

of a structure is the amount of heat energy conducted per second through 1 m^2 of that structure when a temp difference of 1 C is maintained between its ends

69
Q

solar constant (solar irradiance)

A

average amount of the Sun’s energy falling per second perpendicularly on 1 metre squared of the Earth’s atmosphere. Value is about 1.35 kW M^-2

LC Physics (25 decks)

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