THERMO1 Flashcards
(32 cards)
0th Law of Thermodynamics
Defines temperature
1st Law of Thermodynamics
Conservation of energy
2nd Law of Thermodynamics
The entropy of an isolated system can never decrease
3rd Law of Thermodynamics
The entropy of a perfect crystal tends to zero at absolute zero
Zeroth Law of Thermodynamics
If two objects are in thermal equilibrium with a third, then the two objects are also on thermal equilibrium with each other
What is thermal equilibrium?
When heat flow ceases
What is heat?
Flow of energy caused by a difference in temperature
What is temperature?
A measure of the average translational kinetic energy per molecule
What happens when an object absorbs heat?
It increases its internal energy
What is a thermal reservoir?
An object with such a large specific heat capacity that any transfer of heat will not change its temperature
What does the amount of heat flow depend on?
Mass
Specific heat capacity
Temperature difference
Bose-Einstein Condensate
When the de Broglie wavelength of atoms becomes bigger than the atomic spacing
Occurs when a solid is cooled to a very low temperature
Cooling to Lower than the Surroundings
Magnetic Cooling
- atoms with magnetic dipoles can be aligned with a magnetic field
- if the magnetic field is slowly decreased then the dipoles can overcome the field and become randomly orientated by taking thermal energy from their motion
- this leads to sample cooling as the average translational kinetic energy per molecule is decreasing
Cooling to Lower than the Surroundings
Laser Cooling
- a laser photon of momentum p = hf is fired at the sample
- the frequency of each photon is slightly less than the frequency corresponding to the transition energy of the atoms
- this means that it can only be absorbed by atoms travelling towards it by the Doppler shift effect
- this collision reduces the velocity of the atom
- average kinetic energy and therefore temperature are reduced
Cooling to Lower than the Surroundings
Evaporative Cooling
- atoms are held in a magnetic trap
- only atoms with the highest velocity are able to escape
- as these high energy atoms leave the average kinetic energy of the atoms in the magnetic trap decreases
- the magnetic field is decreased so that atoms can continue to escape
- this continues until there are far fewer but much colder atoms remaining
What kinetic energy do atoms have at absolute zero?
Zero translational kinetic energy
What is the lowest temperature ever achieved?
100pK
Cooling to Absolute Zero
- removing all possible energy would require an infinite amount of time
- it would contradict the Heisenberg Uncertainty Principle which doesn’t allow no motion as then it would be possible to specify both position and momentum precisely
- zero-point energy remains
Thermal Expansion
Linear Expansion
ΔL = LαΔT
L = initial length α = coefficient of linear expansion
Coefficient of Linear Expansion
α = (ΔT->0)lim 1/L * dL/dT
Thermal Expansion
Volume Expansion
ΔV = VβΔT
V = initial volume β = coefficient of volume expansion
What is the relationship between the coefficient of linear expansion and the coefficient of volume expansion?
β = 3α
At what temperature does water have maximum density?
4°C
Thermal Current
I = ΔQ/Δt = kA * ΔT/Δx
