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Physics > Chapter 5: Thermal Effects > Flashcards

Chapter 5: Thermal Effects Flashcards

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1
Q

Char of solid

A

Does not flow
Fixed shape and volume
Cannot be compressed
Vibrates
Particles arranged in a lattice formation

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2
Q

Internal energy formula

A

Kinetic energy + potential energy

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3
Q

How is thermal energy transferred (in terms of bodies and energy)

A

From the hotter body to the colder one, till they are the same temperature.

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4
Q

Kelvin to Celsius

A

K = C + 273

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5
Q

Relationship between temp and average KE per particle

A

Objects at the same temp have the same KE per particle. The higher the temp, the higher the avg KE per particle

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6
Q

Internal energy def

A

Total KE and PE of all atoms and molecules in a material

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7
Q

What is temperature (and in terms of particle theory)

A

The degree of hotness or coldness of an object

the average kinetic energy of particles within a substance

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8
Q

Thermal expansion property of liquid

A

More volume of liquid present = more expansion

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9
Q

what does thermal expansion depend on for solids

A

Depends on the material

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10
Q

3 Modes of heat transfer

A

Conduction
Convection
Radiation

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11
Q

What is conduction

A

The transfer of thermal energy from the hotter side to the cooler side by the vibrations between particles in a material

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12
Q

what is convection

A

Convection is the main way that heat travels through liquids and gases, doesn’t occur in solids

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13
Q

how does convection occur

A

-The molecules push each other apart, making the liquid/gas expand
-This makes the hot liquid/gas less dense than the surroundings
-The hot liquid/gas rises, and the cooler (surrounding) liquid/gas moves in to take its place
-Eventually the hot liquid/gas cools, contracts and sinks back down again
-The resulting motion is called a convection current

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14
Q

what is radiation

a special property

A

Radiation is the flow of heat from one place to another by means of electromagnetic waves (infrared radiation - all objects emit this)

It does not require a medium.

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15
Q

how does radiation occur

what doe very hot objectives also give

A

Thermal radiation is mainly infra-red waves, but very hot objects also give out light waves.

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16
Q

what is the best and worst heat emitter

A

best: matt black
worst: silver

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17
Q

what is the best and worst reflector

A

best: silver
worst: matte black

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18
Q

what is the best and worst absorber

A

best: matte black
worst: silver

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19
Q

what is the specific heat capacity of a substance

A

Specific Heat capacity (c) is the amount of energy
required to raise the temperature of 1 kg of a certain
substance by 1 degree celsius

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20
Q

what is the formula for specific heat capacity

A

c = Q/MΔt

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21
Q

what is thermal capacity

A

Thermal Capacity (Q) is the amount of energy required to raise the temperature of an object by 1 degree celsius

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22
Q

what is the formula for thermal capacity

A

q = mc

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23
Q

What are the 2 latent heats

A

Latent heat of fusion
latent heat of vaporization

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24
Q

What is latent heat of fusion
what happens to temp when it’s at the melting point

A

Energy required to separate particles so that they can form the liquid.

temp doesn’t change while its at the melting point until state is fully converted

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25
Q

Formula for latent heat

A

Q = ML
Q = heat required to change state
M = mass of material
L = latent heat

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26
Q

What is latent heat of vaporization

A

Energy absorbed during change of state from liquid to gas. Most of it is needed to separate particles so that they can form a gas.

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27
Q

What is specific latent heat of fusion

A

Energy required to change 1 kg of a substance from solid to liquid

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28
Q

What is specific latent heat of vaporization

A

Energy required to change 1 kg of a substance from liquid to gas

29
Q

definition of evaporation

A

change from liquid to gas BELOW THE BOILING POINT

30
Q

factors that affect rate of evaporation

A

larger surface area
higher temp
more airflow over liquid to help diffusion

31
Q

where does evaporation occur

A

surface of liquid

32
Q

where does boiling occur

A

throughout the liquid

33
Q

why does evaporation cause a cooling effect?

A

the more rapidly moving particles/particles with higher kinetic energy. have enough energy to overcome attractive forces and escape the surface of the liquid as a gas.
temperature, as we know is the average kinetic energy. so if the particles with high kinetic energy have escaped, the average kinetic energy will obviously decrease.

34
Q

Explain, in terms of energy, the process which takes place as a solid at its melting point
changes into a liquid at the same temperature.

A

heat/thermal energy is required to break bonds between molecules/overcome attractive forces and turn from solid to liquid.

35
Q

Describe the motion of the molecules of the gas.

A

molecules move in random directions, at high speeds and molecules collide with eachother

36
Q

which state expands the most when heated up

A

gas

37
Q

why do substances expand

A

particles gain more kinetic energy when heated and start vibrating more rapidly, hence they gain separation from neighbouring particles

38
Q

uses of thermal expansion of liquid

A

thermometers, alcohol or mercury in the thermometer will expand when placed in hot climates and shrink back down in colder climates

39
Q

formula for energy

A

E = mass (m) x specific heat cap (C) x change in temp

E = MCΔT

40
Q

what is thermal capacity dependent upon

A

mass (more mass will have higher thermal capacity)

material

41
Q

a rise in the temperature of an object results in what

A

rise in temp –> increase in thermal energy –transfer–> increase kinetic energy therefore increased internal energy (ie = ke + pe)

42
Q

experiment to find out the specific heat capacity of a liquid

A

0.5kg of liquid is used in a container that has insulation
thermometer used to measure the temp of the liquid
electrical heater with known power is placed in liquid
initial temp reading is taken
heater is switched on and timer is started simultaneously
timer is stopped when temp increases by 10 deg cel
assume the heater is 100% efficient

use C = E/M x Δt (E = power x time)

43
Q

what makes a material a good conductor

A

if a material has delocalised electrons that are free to move around it makes them a better conductor as they will vibrate and hit the ions of the material

44
Q

metal conduction experiment

A

heat a copper bar on one end, on the other place wax blobs with drawing pins being hung on the wax, the heat transferred through the metal will cause the wax to start melting and will eventually drop the pin

45
Q

Describe thermal expansion of solids in terms of particles

A

When a solid is heated, particles speed up (thermal energy transfers to kinetic energy).

They start to vibrate more and thus vibrations take more space - so the solid expands slightly in all directions

opposite for when object is cooled

46
Q

Give real world applications of thermal expansion

A

Electricity cables
Thermometers
Bimetal strips
bridges
train tracks

47
Q

Describe use of expansion in electricity cables

A

often hung with some slack. If its hot - they expand even more. if its cold - they contract and won’t snap.

48
Q

Describe the use of thermal expansion in thermometers

A

Mercury expands and contracts to give an indication of the temperature

49
Q

Describe the use of thermal expansion in bimetal strips

A

strips of 2 metals are bonded together. When heated, one metal expands more than the other metal which causes the strip to bend. Used in thermostats (which are used to keep a constant temp)

50
Q

In thermostats
what do modern devices use

A

they use thermistors instead of bimetal strips to keep the temp constant

51
Q

Why do gasses expand so much in comparison to solids

A

Strength of attraction between particles.

In solids, the inter molecular force of attraction is extremely strong - which leads to little expansion since the particles are held so tightly together.

In gasses, they are very weak which leads to lots of expansion.

52
Q

Factors that help increase the amount of thermal energy transferred in conduction

A

Temp diff across bar/solid is increased

Cross section area of solid is increased

Length of bar is reduced

53
Q

How do non-metals conduct heat (particle theory)

A

When a material is heated, particles move faster and push on neighbouring particles as well, hence speeding those up. (transfer of KE)

54
Q

How do metals conduct heat

why does this also mean they are good conductors of electricity

A

they also use the same ‘system’ as non metals except they also have

free DELOCALISED electrons that speed up when the metal is heated. They move randomly throughout the metal and collide with atoms and make them vibrate faster

this also means they are good conductors of electricity since electricity is the flow of electrons and they have free electrons to conduct electricity

55
Q

What happens to particles at absolute zero (-273 deg Celsius)

A

the particles have least kinetic energy

56
Q

Define brownian motion

A

random collisions between the microscopic particles that are in suspension (eg dust) and the particles of the gas or liquid

57
Q

When is an object at constant temp

A

When energy transferred away from the object is at the same rate that it energy is received

58
Q

What happens if the rate at which an object receives energy > the rate at which energy is transferred away from the object

vice versa

A

object heats up

vice versa: object cools down

59
Q

How is the temperature on earth affected

why do we have global warming

A

by factors controlling the balance
between incoming radiation and radiation emitted from the Earth’s surface

a big example of a factor is the CO2 and H2O in the atmosphere which “insulate” the earth and reduces the rate of radiation. Increasing CO2 means reduced radiation from earth which causes global warming

60
Q

At home

where is the heater located
where the the AC located

why?

A

heater - located near the floor since the cooler and denser air is near the floor. It takes this air in and heats it to warm air

ac - located near the ceiling since the hotter less dense air is towards the ceiling. Takes this hot air in and cools it and sends in down

61
Q

does convection occur if the source of heat is above the container

A

No

you are heating the less dense fluid so then there is no displacement or convection current

62
Q

Thermal processes in burning wood or coal

A

Radiation - from the burning of the coal or wood itself

Convection - a convection current is formed and the room is heated by hot air which circulates

63
Q

How does a car radiator work

A

Unwanted thermal energy is carried away by a liquid coolant to an array of pipes.

here the heat is conducted to metal fins which is carried away by air flowing across the fins

64
Q

Is radiation used to transfer heat in car radiators

A

only very little energy is transferred by radiation

most of it is conduction

65
Q

Experiment to compare emitters

A

A metal cube is filled with boiling water

One surface of the cube is painted white, the other is painted black

A thermal radiation detector is placed at the same distance from either surface

meter readings are compraed

66
Q

Experiment to compare absorbers

A

Metal plates are placed at the same distance from a radiant heater

One plate is painted white
one plate is painted matt black

rise in temp of plates are compared to compare the absorbers

67
Q

Without of a supply of thermal energy what happens to

boiling
evaporation

A

boiling - stops

evaporation - continues

68
Q

Formula for thermal capacity
in terms of energy

A

Q = c * delta t

q = energy
c = specific heat capacity
t = temp

69
Q

If you had a diagram

with objects beside a source of heat

why wouldnt the objects be heated by convection

A

The heated air (convection currents) move upwards not sideways

Physics (25 decks)

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