Thermal Physics Flashcards
Kinetic model of matter
describing the structure of solid, liquid and gases
Describe the structures of solid, liquid and gas
Solid: particles are close together, strong bonds, vibrate in fixed positions
Liquid: particles are relatively close together, relatively strong bonds, can move past each particle
Gas: particles are free to move, weak bonds, widely spread apart and move irregularly
Describe the cooling curve
y-axis = temperature
x-axis = time
the temperature drops rapidly at first, and then more slowly as it reaches room temperature
at BC, temperature is steady, the liquid slowly turns to solid -> energy is still being lost even though temperature is not decreasing
once no liquid remains, temperature starts dropping again in CD
Describe the cooling curve when heating a substance
y axis = temperature
x axis = time
at AB, the temperature starts to rise to 0°C
at BC, the temperature is constant and the substance changes from solid to liquid
at CD, the temperature starts to increase to 100°C
at DE, the temperature is constant and the substance changes from liquid to gas (steam)
at EF, the temperature starts increasing to above 100°C
Describe the motion of particles when heat is applied in relation to kinetic energy
if particles move around more freely and faster, their KE has increased
if they break free from neighbours and become more disordered, the electrical potential energy increased
Describe the relation of potential energy and the seperation of particles
Work must be done to seperate two atoms
The electrical potential energy of two atoms is large and negative
As the seperation of the atoms increases, their potential energy also increases
When atoms are completely seperated, their potential energy also increases
The more the potential energy, the larger the negative number
Describe the heating curve in terms of potential energy
AB = increase in KE, little change in seperation hence little change in PE
BC = ice starts to melt, molecules become more disordered and PE increases
CD = increase in KE, little change in seperation hence little change in PE
DE = water is boiling and molecules are greatly disordered so large change in PE
EF = KE is increasing, maximum potential energy = 0
Evaporation
when a liquid changes into gas without going through boiling
- molecules that move faster than others break free from the liquid (net outflow of energetic molecules from the liquid)
- leaves molecules with below average KE
- temperature falls
Internal energy
the sum of the random distribution of KE and PE within a system of molecules
Factors of internal energy
- temperature
- random motion of molecules
- phases of matter
Increasing and decreasing internal energy
Increasing:
- doing work to it by compression
- adding heat to it
Decreasing:
- losing heat to its surroundings
First law of thermodynamics
Internal energy = energy supplied by heating TO the system + work done ON the system
△U = q + W
How does principle conservation of energy apply to first law of thermodynamics?
Principle of conservation of energy -> energy cannot be created nor destroyed, only transferred to one another
This means that all energy put into a gas by heating it and doing work must end up inside the gas
- hence total internal energy remains unchanged
What does positive value of △U mean?
- internal energy of △U increases
- heat q is added TO the system
- work W is done ON the system
What does negative value of △U mean?
- internal energy of △U decreases
- heat q is taken AWAY from the system
- work is done BY the system
Bonus: what does “work done BY a gas” mean? what does “work done ON a gas” mean?
- it means the expansion of gas
- it means the compression of gas
How to find the work done in a pressure volume graph
work done = area under the line of a p-v graph
Constant pressure graph
- straight line from v1 to v2
- increasing arrow means increase in internal energy and work is done BY the gas
- decreasing arrow means decrease in internal energy and work is done ON the gas
Constant Volume graph
- straight line from p1 to p2
- area of the line is zero
- no work done, volume stays the same
Explain how gases do work
Gases exert pressure on the walls of their container. If a gas expands, the walls are pushed outwards therefore the gas has done work on its surroundings
Equation for work done by gases
W = p △V
work done when the volume of gas changes at constant pressure
What does q = 0 mean?
gas is compressed very quickly
What does △U = W mean?
all internal energy increases due to work done by piston
the KE increases and the temperature increases unless there is a change in state
What happens if gas is compressed VERY slowly?
- temperature stays at constant room temperature
- ISOTHERMAL CHANGE -> KE of molecules stay the same
- 0 = q + W because U constant = △U that is 0. this means if you push the piston in and do positive work W, then q is negative, and heat is lost from the syringe
Thermal energy
energy is transferred from a region of higher temperature to a region of lower temperature
Thermal equilibrium
when two substances in physical contact with each other no longer exchange any heat energy and both reach an equal temperature
- the final temperature of thermal equilibrium depends on the initial temperature difference between them
Kelvin scale
- absolute scale that is not defined in terms of a property of any particular substance
Absolute zero
- the lowest temperature possible at which atoms have KE =0 and PE =0
- 0K or -273.25°C
Kelvin scale and degree celsius scale
0K -> -273°C
100K -> -173°C
200K -> -73°C
300K -> 27°C
400K -> 127°C
the change in a temp of 1K is equal to the change in a temp of 1°C
Specific heat capacity
the amount of thermal energy required to raise the temperature of 1kg of substance by 1°C
c = E / m △ θ
What is SHC in a temperature-work done graph?
the gradient of the graph
the steeper the gradient, the faster the substance heats up, hence the lower the SHC
Factors of SHC
- the heavier the material, the more thermal energy required to raise temperature
- the larger change in temperature, the higher the thermal energy required to achieve this change
Describe high and low SHC
High:
- warms up and cools down slowly
- takes more energy to change its temperature
Low:
- warms up and cools down fast
- takes less energy to change its temperature
Specific latent heat
the thermal energy required to change the state of 1kg of mass of a substance without any change in temperature
- the energy required to change the state of a substance
E = mL
SLH for melting and boiling
Melting: specific latent heat of fusion
Boiling: specific latent heat of vaporization
Which process (melting or boiling) requires more energy?
Boiling; energy is required to completely seperate the molecules until there are no more forces of attraction
For melting, energy is required to just increase the molecular seperation until they can flow freely over each other
Thermometer
any device that can be used to measure temperature
Liquid in glass thermometer
depends on change of density of a liquid
A thermometer must…
- be calibrated at two or more known temperatures (0°C and 100°C)
- scale must be divided into equal divisions
