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A level Physics > Unit 3.4 - Thermal Physics > Flashcards

Unit 3.4 - Thermal Physics Flashcards

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1
Q

What is temperature associated with?

A

The motion of microscopic particles

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2
Q

What is heat?

A

A fluid-like substance

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3
Q

What can heat do and what does this mean?

A

Can do work
Is a form of energy

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4
Q

How come heat is a form of energy?

A

Can do work

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5
Q

What does it prove due to the fact that heat can do work?

A

Heat is a form of energy

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6
Q

For a as behaving ideally, what can the total energy of the system be considered to be?

A

The total kinetic energy of all the particles of which it comprises

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7
Q

Why is there zero potential energy for a gas behaving ideally?

A

No forces between the molecules

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8
Q

When does a gas stop behaving ideally and what does it do instead in this circumstance?

A

When it is either under high pressure or at low temperatures
Start to condense

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9
Q

When do gases start to condense?

A

Under high pressure or at low temperatures

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10
Q

What is the total internal energy of any system (solid, liquid, gas)?

A

Is equal to the sum of all the individual molecular kinetic and potential energy

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11
Q

What is internal energy?

A

The total energy

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12
Q

What does internal energy act as?

A

An energy store

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13
Q

What type of energy are heat and work done energy?

A

Energy in transit

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14
Q

Name 2 types of energy in transit

A

Heat
Work done

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15
Q

Difference between internal energy and heat and work done

A

Internal energy = energy store
Heat and work done = energy in transit

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16
Q

What are the ways of changing internal energy?

A

Doing work on the gas
Heat transfer

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17
Q

Where should work be done to change internal energy?

A

On the gas

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18
Q

What do all atoms and molecules that make up a system have?

A

Some kinetic energy

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19
Q

What types of kinetic energy can be in all atoms or molecules that make up a system?

A

Translational (gases and liquids)
Vibrational (solids)
Rotational (liquids and gases)

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20
Q

What type of substances have rotational kinetic energy?

A

Liquids and gases

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21
Q

What type of substances have vibrational kinetic energy?

A

Solids

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22
Q

What type of substances have translational kinetic energy?

A

Gases and liquids

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23
Q

What type of kinetic energy don’t ideal gases have and why?

A

No rotational kinetic energy as they’re monoatomic

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24
Q

What may all real atoms and molecules have between them?

A

Potential energy (electrostatic)

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25
Q

What is the internal energy of a system?

A

The sum of all of the individual kinetic and potential energy of the particles that make up the system

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26
Q

What is internal energy measured in?

A

Joules

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27
Q

Why does an internal energy of zero not need to be defined?

A

In general, we are interested in changes in internal energy

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28
Q

Symbol for change in internal energy

A

ΔU

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29
Q

Can the internal energy of a system change?

A

Yes

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30
Q

How does energy enter or leave a system to cause a change in internal energy?

A

Either by heat transfer or by work done

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31
Q

Heat symbol

A

Q

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32
Q

ΔU

A

Change in internal energy

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33
Q

Q

A

Heat

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34
Q

Heat

A

Energy in the process of moving into or out of a system

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35
Q

2nd law of thermodynamics

A

Heat flows from hot to cold objects

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36
Q

What o we have if we consider an object to have a different temperature to its surroundings?

A

A temperature gradient

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37
Q

If a system is at a higher temperature to its surroundings, describe the temperature gradient and the direction of heat flow

A

Gradient from the system to the surroundings
Heat will flow out of the system down a temperature gradient

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38
Q

If a system is at a lower temperature to its surroundings, describe the temperature gradient and the direction of heat flow

A

The gradient is to the system from the surroundings
Heat will flow into the system

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39
Q

If a system is at the same temperature to its surroundings, describe the temperature gradient and the direction of heat flow

A

Both are at thermal equilibrium
No heat flow can take place

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40
Q

when does thermal equilibrium occur?

A

When the system is at the same temperature as its surroundings

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41
Q

what can’t happen at thermal equilibrium?

A

Heat flow

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42
Q

What do we say the heat flow is when heat flows into a system?

A

Positive
(+Q)

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43
Q

What do we say the heat flow is when heat flows out of a system?

A

Negative
(-Q)

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44
Q

How can the internal energy of a system change apart from through heat flow?

A

By doing work against the surroundings or by having work done upon it

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45
Q

Why is work done by a system or on a system energy in transit?

A

It’s a process over time

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46
Q

Work

A

The product of force and displacement caused by that force
W = Fxcos(theta)

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47
Q

Pressure

A

Force/area

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48
Q

What happens to gas under pressure?

A

Expands

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49
Q

isochoric

A

Constant volume

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50
Q

Constant volume

A

Isochoric

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51
Q

What is the work done when the pressure and temperature increase in an isochoric system? Why?

A

Work done is zero
Work done is pressure x change in volume

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52
Q

How does the system do work in the example of apistion?

A

The gas in the piston is pushing outwards against the surrounding pressure

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53
Q

When does positive work occur?

A

Gas expands
Work done by the gas

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54
Q

When is there no change in internal energy?

A

Same temperature throughout

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55
Q

When would a system have work done upon it?

A

The system contracts

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56
Q

When does a system contract?

A

When work is done upon it

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57
Q

When is there negative work in a system?

A

Gas compressed
Work done on the gas

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58
Q

Describe the work done when a gas expands

A

Work done by the gas
Positive work

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59
Q

Describe the work done when a gas is compressed

A

Work done on the gas
Negative work

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60
Q

What do we say when work is done by the system?

A

Work done is positive (+W)

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61
Q

What Dow e say when work is done on a system?

A

Work done is negative (-W)

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62
Q

What can’t liquids do and what happens because of this?

A

Can’t do work = volume doesn’t change
Internal energy has to rise

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63
Q

What happens to the volume when gas does work against its surroundings?

A

Changes

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64
Q

Explain W = fx

A

Work done is applied force multiplied by the displacement in the direction of said force

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65
Q

Describe the pressure in the case of a gas

A

Equal pressure in all directions

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66
Q

If the pressure is the same in all directions for a gas, what does this mean?

A

Force per unit area is the same in all directions

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67
Q

Pressure equation

A

Force/area

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68
Q

Substitute pressure equation into work definition

A

W = pA x x

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69
Q

What is work also equal to except for the normal definition?

A

Pressure multiplied by change in volume
W = pΔV

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70
Q

Derive W = pΔV

A

Pressure = force/area
Substitute into work definition
W = pA x x
In the piston example, the distance moved is denoted by Δx
W = pA x Δx
We can clearly see that
A x Δx = ΔV
The change in volume.
This gives us an expression for the work done by a gas under constant pressure
W = pΔV

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71
Q

Derive using units work done = pressure x volume

A

Pa = Nm-1
P = f/A = kgms-2/m2
Kgms-1s-2 x V
Kgms1s-2 x m3
Kgm2s-2
Work = J
J = kgm2s-2

72
Q

Constant pressure

A

Isobaric

73
Q

Isobaric

A

Constant pressure

74
Q

Work done on a graph

A

Area under the pressure-volume graph

75
Q

what is the area under a pressure-volume graph?

A

Work done

76
Q

What will the area under the p-v graph be if the pressure is changing?

A

Still be equal to the work done

77
Q

What is doing work if a gas is expanding?

A

Work done by the gas

78
Q

Constant temperature

A

Isothermal

79
Q

Isothermal

A

Constant temperature

80
Q

How could we prove that something is isothermal using a p-v graph?

A

Using PV = nRT at different points

81
Q

What is the first law of thermodynamics really?

A

A conservation of energy expression

82
Q

Under which circumstance can we calculate the change in internal energy with changing volume?

A

If the temperature change is zero

83
Q

When do we use the first law of thermodynamics?

A

When the change in internal energy is caused by both heat flow and work done

84
Q

First law of thermodynamics

A

ΔU = Q - W

85
Q

Explain
ΔU = Q-W

A

ΔU = change in internal energy
Q = the heat flow into the system
W = work done by the system

86
Q

What is W in the first law of thermodynamics?

A

Work done by the system

87
Q

When is heat positive?

A

When it flows into a system

88
Q

When is heat negative?

A

When it flows out of a system

89
Q

When is work positive?

A

When done by the system

90
Q

When is work negative?

A

When done by the system

91
Q

Describe the work done when a system expands

A

Positive

92
Q

Describe the work done when a system contracts

A

Negative

93
Q

What does the equation for the first law of thermodynamics change to if…
Heat flows into the gas and the gas expands (work done by the gas)

A

ΔU = (+Q) - (+W) = Q - W

94
Q

What does the equation for the first law of thermodynamics change to if…
Heat flows into the gas and the gas contracts (work done on the gas)

A

ΔU = (+Q) - (-W) = Q + W

95
Q

What does the equation for the first law of thermodynamics change to if…
Heat flows out of a gas and the gas expands (work done by the gas)

A

ΔU = (-Q) - (+W) = -Q - W

96
Q

What does the equation for the first law of thermodynamics change to if…
Heat flows out of the gas and the gas contracts (work done on the gas)

A

ΔU = (-Q) - (-W) = -Q + W

97
Q

What does the value and sign of ΔU depend on?

A

The values and signs on the right hand side

98
Q

What two things does it mean if ΔU is negative?

A

Internal energy has decreased
The temperature has decreased

99
Q

What two things does it mean if ΔU is positive?

A

Internal energy has increased
Temperature has increased

100
Q

What must have also increased if the internal energy has increased?

A

Temperature

101
Q

Special cases of the first law of thermodynamics

A

Isothermal change
Abiatic change
Solids and liquids

102
Q

Isothermal change

A

When a change happens at a constant temperature

103
Q

When a change happens at a constant temperature

A

Isothermal change

104
Q

What happens if there is no temperature change and the system is composed of an ideal gas?

A

There is no change in internal energy

105
Q

What type of expansion is an isothermal change?

A

Slow

106
Q

What does it mean due to the fact that an isothermal change is a slow expansion?

A

There’s lots of time for heat to flow

107
Q

Describe what happens during an isothermal change

A

For every small increase in volume, the temperature will drop slightly
As a result, heat will flow into the system from the surroundings , which will prevent any further drop in temperature
The effect of this is that the expansion appears to take place at the same temperature - it is isothermal

108
Q

When can an isothermal change occur?

A

If the expansion is very slow
If the vessel walls and very heat conducting/thin

109
Q

Describe how work is transferred during an isothermal change

A

Work done is transferred minimally to the internal energy since there’s lots of heat flow

110
Q

Does a constant temperature mean that Q = O?

A

Not necessarily

111
Q

Does the flow of heat into a system mean that the temperature has to rise?

A

No

112
Q

Two points that arise form an isothermal change

A

A constant temperature does not necessarily mean that Q = O
A flow of heat into the system does not mean that the temperature has to rise

113
Q

Abiatic change

A

If a gas expands or contracts very quickly, there is no time for heat to flow in or out of the system, so the change is Abiatic

114
Q

Q during an Abiatic change

A

Zero

115
Q

What is the change in internal energy during an Abiatic change and why?

A

W
Since Q = O

116
Q

Describe how work is transferred during an Abiatic change

A

Mainly to internal energy

117
Q

What type of system does an Abiatic change usually occur in?

A

A system with thick walls

118
Q

What happens to the temperature during an Abiatic change?

A

Increases

119
Q

Why can’t solids or liquids significantly expand or contract?

A

There is no empty space between the molecules

120
Q

How does the first law describe the internal energy of solids and liquids and why?

A

ΔU = Q
(W = O since they can’t significantly expand or contract)

121
Q

What do we usually look at when considering a system undergoing changes?

A

The cycles that the system (gas) goes through

122
Q

Examples of gas cycles

A

Gas that cools a refrigerator
Gas that drives a piston in an engine piston

123
Q

Work done if there’s no change in volume

A

Zero

124
Q

If the volume is constant but the pressure is dropping, what must be happening?

A

The temperature must be dropping

125
Q

If the volume is constant but ΔU is positive, what must be happening?

A

Heat must be flowing into the system (+Q)

126
Q

How do we work out the total work done in a cycle?

A

Enclosed by the loop on the p-v graph

127
Q

How do we know that a system has ended up with the same internal energy in a cycle?

A

Reaches the same point

128
Q

If a cycle ends up on the same internal energy, what is the total work done by the gas equal to?

A

The total heat supplied to the gas over the cycle

129
Q

Thermodynamic scale

A

Kelvin scale

130
Q

Kelvin scale

A

Thermodynamic scale

131
Q

What is the thermodynamic scale defined by?

A

The properties of substances

132
Q

Is it possible to reduce the internal energy of a system to zero?

A

No

133
Q

What would the temperature of the gas be if someone reduced the internal energy of a system to zero?

A

The temperature of the gas would also be zero

134
Q

Equation for the internal energy of an ideal gas

A

U = 3/2nRT

135
Q

What would happen in terms of energy at absolute zero?

A

There would be zero potential energy and zero kinetic energy = no movement at all
Even the electrons in orbit around the nucleus would freeze

136
Q

What would freeze at absolute zero? why?

A

Even the electrons in orbit around the nucleus
There would be zero potential energy and zero kinetic energy = no movement at all

137
Q

Has absolute zero ever been achieved?

A

Very low temperatures, but absolute zero cannot physically be achieved

138
Q

How can we define absolute zero?

A

Using Charles’ law

139
Q

How would we know that the work done in one of the circumstances in a cyclic process is higher/lower than another?

A

Based off of the area under the curve
(Think - the area is all the way to the axis)

140
Q

What do we do if we’re asked to “estimate” the difference in the net work done between two cycles?

A

Count squares
(Can work out the area of an individual square and compare)

141
Q

How do we justify the number of significant figures used in a practical?

A

The resolution of the measuring equipment should be used

142
Q

How do we justify if the results of an experiment are consistent with an equation?

A

Analyse the graph
e.g:
-straight line
-intercept is consistent
-passes through all error bars
-vales linked to values in equation (e.g - using l for V in PV = nRT and V is proportional to T, so it gives a straight line)

143
Q

Define the specific heat capacity of a substance

A

The amount of thermal energy required to raise the temperature of 1kg of a substance by 1k

144
Q

Explain how the converse is true for the definition of specific heat capacity

A

By removing c joules of energy from the system, 1kg will cool by 1k

145
Q

What does the value for the specific heat capacity of a substance determine?

A

The amount of energy needed to change its temperature

146
Q

Units of specific heat capacity

A

Jkg-1K-1 or Jkg-1degreesc-1

147
Q

Do we use K or Celsius in specific heat capacity calculations? Explain

A

Since we’re just measuring the change in temperature, we can use either and they’ll give the same value

148
Q

Symbol of specific heat capacity

A

C

149
Q

What are specific heat capacities specific to?

A

A substance, which is where it derives its name

150
Q

Where does specific heat capacity derive it’s name?

A

Specific heat capacities are specific to a substance

151
Q

What is specific heat capacity used for in mainly?

A

Liquids and solids

152
Q

Describe the energy needed to raise the temperature of a heavier material

A

Heavier materials need more thermal energy to raise their temperatures

153
Q

What type of materials require more thermal energy to raise their temperatures?

A

Heavier ones

154
Q

If the change in temperature is higher, describe the thermal energy needed

A

Higher

155
Q

When is a high amount of thermal energy needed to achieve a change in temperature?

A

For a large change in temperature

156
Q

What is needed for a large change in temperature?

A

A high amount of thermal energy

157
Q

Specific heat capacity equation

A

ΔQ = mcΔtheta

158
Q

ΔQ in specific heat capacity equation + explanation

A

The energy required to change the temperature of a substance is mcΔtheta
As this is heat, we use Q
(J)

159
Q

m in specific heat capacity equation

A

Mass of substance being heated (kg)

160
Q

c in specific heat capacity equation

A

Specific hat capacity of the substance

161
Q

Δθ in specific heat capacity calculation + unit

A

Change in temperature (K or Celsius)

162
Q

Give two features describing materials with low specific heat capacities

A

Heat up and cool down quickly
Takes much less energy to change its temperature

163
Q

Give two features describing materials with high specific heat capacities

A

Warms up and cools down slowly
Take much more energy to change its temperature

164
Q

Example of a material with a high specific heat capacity

A

Water

165
Q

specific heat capacity of water

A

4200Jk-1K-1

166
Q

Describe the specific heat capacity of metals compared to water

A

Metals are much lower

167
Q

What does it mean that water has a high specific heat capacity?

A

Has a high capacity to store internal energy

168
Q

What is water used in and why?

A

Radiators (is a good heat store - once it’s heated, it stores the heat)

169
Q

What do the different heat capacities of different substances give us information on?

A

How useful they would be for specific purposes

170
Q

example of a material with a low specific heat capacity

A

Metal

171
Q

What are metals good at and why in terms of specific heat capacity?

A

God electrical conductors since they’re good conductors of heat since they have low specific heat capacities

172
Q

Give reasons why the mass of heating gas may be higher in practice than calculated

A

-thermal energy was lost to the walls of the room
-thermal energy was lost to other objects in the room
-some mass of air may escape the room

173
Q

Explain what happens when mixing hot and cold liquid

A

If hot liquid is introduced into cold liquid, the heat lost by the hot material cooling down is equal to the heat gained by the cold liquid + calorimeter + any heat loss to surroundings
So ΔQ is the same

174
Q

How do we make sure that we don’t get confused on whether to use + or - with W?

A

The minus shown here is always there, but then you need to consider whether the work is done…
On the gas (+W)
By the gas (-W)
Two negatives might end up making a positive

175
Q

Why do all real atoms (some example, in liquids, not just ideal gases) have potential energy between them?

A

Due to the intermolecular forces between molecules

A level Physics (29 decks)

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