Unit 3.4 - Thermal Physics Flashcards
What is temperature associated with?
The motion of microscopic particles
What is heat?
A fluid-like substance
What can heat do and what does this mean?
Can do work
Is a form of energy
How come heat is a form of energy?
Can do work
What does it prove due to the fact that heat can do work?
Heat is a form of energy
For a as behaving ideally, what can the total energy of the system be considered to be?
The total kinetic energy of all the particles of which it comprises
Why is there zero potential energy for a gas behaving ideally?
No forces between the molecules
When does a gas stop behaving ideally and what does it do instead in this circumstance?
When it is either under high pressure or at low temperatures
Start to condense
When do gases start to condense?
Under high pressure or at low temperatures
What is the total internal energy of any system (solid, liquid, gas)?
Is equal to the sum of all the individual molecular kinetic and potential energy
What is internal energy?
The total energy
What does internal energy act as?
An energy store
What type of energy are heat and work done energy?
Energy in transit
Name 2 types of energy in transit
Heat
Work done
Difference between internal energy and heat and work done
Internal energy = energy store
Heat and work done = energy in transit
What are the ways of changing internal energy?
Doing work on the gas
Heat transfer
Where should work be done to change internal energy?
On the gas
What do all atoms and molecules that make up a system have?
Some kinetic energy
What types of kinetic energy can be in all atoms or molecules that make up a system?
Translational (gases and liquids)
Vibrational (solids)
Rotational (liquids and gases)
What type of substances have rotational kinetic energy?
Liquids and gases
What type of substances have vibrational kinetic energy?
Solids
What type of substances have translational kinetic energy?
Gases and liquids
What type of kinetic energy don’t ideal gases have and why?
No rotational kinetic energy as they’re monoatomic
What may all real atoms and molecules have between them?
Potential energy (electrostatic)
What is the internal energy of a system?
The sum of all of the individual kinetic and potential energy of the particles that make up the system
What is internal energy measured in?
Joules
Why does an internal energy of zero not need to be defined?
In general, we are interested in changes in internal energy
Symbol for change in internal energy
ΔU
Can the internal energy of a system change?
Yes
How does energy enter or leave a system to cause a change in internal energy?
Either by heat transfer or by work done
Heat symbol
Q
ΔU
Change in internal energy
Q
Heat
Heat
Energy in the process of moving into or out of a system
2nd law of thermodynamics
Heat flows from hot to cold objects
What o we have if we consider an object to have a different temperature to its surroundings?
A temperature gradient
If a system is at a higher temperature to its surroundings, describe the temperature gradient and the direction of heat flow
Gradient from the system to the surroundings
Heat will flow out of the system down a temperature gradient
If a system is at a lower temperature to its surroundings, describe the temperature gradient and the direction of heat flow
The gradient is to the system from the surroundings
Heat will flow into the system
If a system is at the same temperature to its surroundings, describe the temperature gradient and the direction of heat flow
Both are at thermal equilibrium
No heat flow can take place
when does thermal equilibrium occur?
When the system is at the same temperature as its surroundings
what can’t happen at thermal equilibrium?
Heat flow
What do we say the heat flow is when heat flows into a system?
Positive
(+Q)
What do we say the heat flow is when heat flows out of a system?
Negative
(-Q)
How can the internal energy of a system change apart from through heat flow?
By doing work against the surroundings or by having work done upon it
Why is work done by a system or on a system energy in transit?
It’s a process over time
Work
The product of force and displacement caused by that force
W = Fxcos(theta)
Pressure
Force/area
What happens to gas under pressure?
Expands
isochoric
Constant volume
Constant volume
Isochoric
What is the work done when the pressure and temperature increase in an isochoric system? Why?
Work done is zero
Work done is pressure x change in volume
How does the system do work in the example of apistion?
The gas in the piston is pushing outwards against the surrounding pressure
When does positive work occur?
Gas expands
Work done by the gas
When is there no change in internal energy?
Same temperature throughout
When would a system have work done upon it?
The system contracts
When does a system contract?
When work is done upon it
When is there negative work in a system?
Gas compressed
Work done on the gas
Describe the work done when a gas expands
Work done by the gas
Positive work
Describe the work done when a gas is compressed
Work done on the gas
Negative work
What do we say when work is done by the system?
Work done is positive (+W)
What Dow e say when work is done on a system?
Work done is negative (-W)
What can’t liquids do and what happens because of this?
Can’t do work = volume doesn’t change
Internal energy has to rise
What happens to the volume when gas does work against its surroundings?
Changes
Explain W = fx
Work done is applied force multiplied by the displacement in the direction of said force
Describe the pressure in the case of a gas
Equal pressure in all directions
If the pressure is the same in all directions for a gas, what does this mean?
Force per unit area is the same in all directions
Pressure equation
Force/area
Substitute pressure equation into work definition
W = pA x x
What is work also equal to except for the normal definition?
Pressure multiplied by change in volume
W = pΔV
Derive W = pΔV
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
Derive using units work done = pressure x volume
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
Constant pressure
Isobaric
Isobaric
Constant pressure
Work done on a graph
Area under the pressure-volume graph
what is the area under a pressure-volume graph?
Work done
What will the area under the p-v graph be if the pressure is changing?
Still be equal to the work done
What is doing work if a gas is expanding?
Work done by the gas
Constant temperature
Isothermal
Isothermal
Constant temperature
How could we prove that something is isothermal using a p-v graph?
Using PV = nRT at different points
What is the first law of thermodynamics really?
A conservation of energy expression
Under which circumstance can we calculate the change in internal energy with changing volume?
If the temperature change is zero
When do we use the first law of thermodynamics?
When the change in internal energy is caused by both heat flow and work done
First law of thermodynamics
ΔU = Q - W
Explain
ΔU = Q-W
ΔU = change in internal energy
Q = the heat flow into the system
W = work done by the system
What is W in the first law of thermodynamics?
Work done by the system
When is heat positive?
When it flows into a system
When is heat negative?
When it flows out of a system
When is work positive?
When done by the system
When is work negative?
When done by the system
Describe the work done when a system expands
Positive
Describe the work done when a system contracts
Negative
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)
ΔU = (+Q) - (+W) = Q - W
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)
ΔU = (+Q) - (-W) = Q + W
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)
ΔU = (-Q) - (+W) = -Q - W
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)
ΔU = (-Q) - (-W) = -Q + W
What does the value and sign of ΔU depend on?
The values and signs on the right hand side
What two things does it mean if ΔU is negative?
Internal energy has decreased
The temperature has decreased
What two things does it mean if ΔU is positive?
Internal energy has increased
Temperature has increased
What must have also increased if the internal energy has increased?
Temperature
Special cases of the first law of thermodynamics
Isothermal change
Abiatic change
Solids and liquids
Isothermal change
When a change happens at a constant temperature
When a change happens at a constant temperature
Isothermal change
What happens if there is no temperature change and the system is composed of an ideal gas?
There is no change in internal energy
What type of expansion is an isothermal change?
Slow
What does it mean due to the fact that an isothermal change is a slow expansion?
There’s lots of time for heat to flow
Describe what happens during an isothermal change
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
When can an isothermal change occur?
If the expansion is very slow
If the vessel walls and very heat conducting/thin
Describe how work is transferred during an isothermal change
Work done is transferred minimally to the internal energy since there’s lots of heat flow
Does a constant temperature mean that Q = O?
Not necessarily
Does the flow of heat into a system mean that the temperature has to rise?
No
Two points that arise form an isothermal change
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
Abiatic change
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
Q during an Abiatic change
Zero
What is the change in internal energy during an Abiatic change and why?
W
Since Q = O
Describe how work is transferred during an Abiatic change
Mainly to internal energy
What type of system does an Abiatic change usually occur in?
A system with thick walls
What happens to the temperature during an Abiatic change?
Increases
Why can’t solids or liquids significantly expand or contract?
There is no empty space between the molecules
How does the first law describe the internal energy of solids and liquids and why?
ΔU = Q
(W = O since they can’t significantly expand or contract)
What do we usually look at when considering a system undergoing changes?
The cycles that the system (gas) goes through
Examples of gas cycles
Gas that cools a refrigerator
Gas that drives a piston in an engine piston
Work done if there’s no change in volume
Zero
If the volume is constant but the pressure is dropping, what must be happening?
The temperature must be dropping
If the volume is constant but ΔU is positive, what must be happening?
Heat must be flowing into the system (+Q)
How do we work out the total work done in a cycle?
Enclosed by the loop on the p-v graph
How do we know that a system has ended up with the same internal energy in a cycle?
Reaches the same point
If a cycle ends up on the same internal energy, what is the total work done by the gas equal to?
The total heat supplied to the gas over the cycle
Thermodynamic scale
Kelvin scale
Kelvin scale
Thermodynamic scale
What is the thermodynamic scale defined by?
The properties of substances
Is it possible to reduce the internal energy of a system to zero?
No
What would the temperature of the gas be if someone reduced the internal energy of a system to zero?
The temperature of the gas would also be zero
Equation for the internal energy of an ideal gas
U = 3/2nRT
What would happen in terms of energy at absolute zero?
There would be zero potential energy and zero kinetic energy = no movement at all
Even the electrons in orbit around the nucleus would freeze
What would freeze at absolute zero? why?
Even the electrons in orbit around the nucleus
There would be zero potential energy and zero kinetic energy = no movement at all
Has absolute zero ever been achieved?
Very low temperatures, but absolute zero cannot physically be achieved
How can we define absolute zero?
Using Charles’ law
How would we know that the work done in one of the circumstances in a cyclic process is higher/lower than another?
Based off of the area under the curve
(Think - the area is all the way to the axis)
What do we do if we’re asked to “estimate” the difference in the net work done between two cycles?
Count squares
(Can work out the area of an individual square and compare)
How do we justify the number of significant figures used in a practical?
The resolution of the measuring equipment should be used
How do we justify if the results of an experiment are consistent with an equation?
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)
Define the specific heat capacity of a substance
The amount of thermal energy required to raise the temperature of 1kg of a substance by 1k
Explain how the converse is true for the definition of specific heat capacity
By removing c joules of energy from the system, 1kg will cool by 1k
What does the value for the specific heat capacity of a substance determine?
The amount of energy needed to change its temperature
Units of specific heat capacity
Jkg-1K-1 or Jkg-1degreesc-1
Do we use K or Celsius in specific heat capacity calculations? Explain
Since we’re just measuring the change in temperature, we can use either and they’ll give the same value
Symbol of specific heat capacity
C
What are specific heat capacities specific to?
A substance, which is where it derives its name
Where does specific heat capacity derive it’s name?
Specific heat capacities are specific to a substance
What is specific heat capacity used for in mainly?
Liquids and solids
Describe the energy needed to raise the temperature of a heavier material
Heavier materials need more thermal energy to raise their temperatures
What type of materials require more thermal energy to raise their temperatures?
Heavier ones
If the change in temperature is higher, describe the thermal energy needed
Higher
When is a high amount of thermal energy needed to achieve a change in temperature?
For a large change in temperature
What is needed for a large change in temperature?
A high amount of thermal energy
Specific heat capacity equation
ΔQ = mcΔtheta
ΔQ in specific heat capacity equation + explanation
The energy required to change the temperature of a substance is mcΔtheta
As this is heat, we use Q
(J)
m in specific heat capacity equation
Mass of substance being heated (kg)
c in specific heat capacity equation
Specific hat capacity of the substance
Δθ in specific heat capacity calculation + unit
Change in temperature (K or Celsius)
Give two features describing materials with low specific heat capacities
Heat up and cool down quickly
Takes much less energy to change its temperature
Give two features describing materials with high specific heat capacities
Warms up and cools down slowly
Take much more energy to change its temperature
Example of a material with a high specific heat capacity
Water
specific heat capacity of water
4200Jk-1K-1
Describe the specific heat capacity of metals compared to water
Metals are much lower
What does it mean that water has a high specific heat capacity?
Has a high capacity to store internal energy
What is water used in and why?
Radiators (is a good heat store - once it’s heated, it stores the heat)
What do the different heat capacities of different substances give us information on?
How useful they would be for specific purposes
example of a material with a low specific heat capacity
Metal
What are metals good at and why in terms of specific heat capacity?
God electrical conductors since they’re good conductors of heat since they have low specific heat capacities
Give reasons why the mass of heating gas may be higher in practice than calculated
-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
Explain what happens when mixing hot and cold liquid
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
How do we make sure that we don’t get confused on whether to use + or - with W?
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
Why do all real atoms (some example, in liquids, not just ideal gases) have potential energy between them?
Due to the intermolecular forces between molecules
