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Flashcards in THERMO1 Deck (32):
1

0th Law of Thermodynamics

Defines temperature

2

1st Law of Thermodynamics

Conservation of energy

3

2nd Law of Thermodynamics

The entropy of an isolated system can never decrease

4

3rd Law of Thermodynamics

The entropy of a perfect crystal tends to zero at absolute zero

5

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

6

What is thermal equilibrium?

When heat flow ceases

7

What is heat?

Flow of energy caused by a difference in temperature

8

What is temperature?

A measure of the average translational kinetic energy per molecule

9

What happens when an object absorbs heat?

It increases its internal energy

10

What is a thermal reservoir?

An object with such a large specific heat capacity that any transfer of heat will not change its temperature

11

What does the amount of heat flow depend on?

Mass
Specific heat capacity
Temperature difference

12

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

13

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

14

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

15

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

16

What kinetic energy do atoms have at absolute zero?

Zero translational kinetic energy

17

What is the lowest temperature ever achieved?

100pK

18

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

19

Thermal Expansion
Linear Expansion

ΔL = LαΔT

L = initial length
α = coefficient of linear expansion

20

Coefficient of Linear Expansion

α = (ΔT->0)lim 1/L * dL/dT

21

Thermal Expansion
Volume Expansion

ΔV = VβΔT

V = initial volume
β = coefficient of volume expansion

22

What is the relationship between the coefficient of linear expansion and the coefficient of volume expansion?

β = 3α

23

At what temperature does water have maximum density?

4°C

24

Thermal Current

I = ΔQ/Δt = kA * ΔT/Δx

25

Thermal Conduction
V=IR

ΔT = IR = I*Δx/kA

26

Thermal Resistance

R = ΔX/kA

27

Thermal Resistors
Series

Rt = R1 + R2 + R3 + ...

E.g. double glazing

28

Thermal Resistors
Parallel

1/Rt = 1/R1 + 1/R2 + 1/R3 + ...

29

What is convection?

The transfer of thermal energy by direct mass transport

30

What is radiation?

The transfer of thermal energy in the form of electromagnetic radiation
No medium is necessary

31

Stefan-Boltzmann Law

Pnet = eσA (T⁴ - T0⁴)

32

Wien's Displacement Law

λmax = 2.898mmK / T