Lesson summary
14.2.2 The Kelvin Scale — AS PHYSICS 9702 notes: Use the thermodynamic (Kelvin) scale and explain absolute zero.Thermodynamic scale — it does not depend on the properties of any particular substance, so it is an absolute scaleTemperature is a scalar quantity and the kelvin is the…
- Celsius scale
- Definition of the unit
- Conversion
- Size of the unit
- Memory aid
- Direction of approach
Section 1 of 10A scale independent of any substance
Thermodynamic scale
- Thermodynamic scale it does not depend on the properties of any particular substance, so it is an absolute scale
- Temperature is a scalar quantity and the kelvin is the SI base unit
Celsius scale
- Celsius scale it was based on the properties of water, with the freezing point 0 °C and the boiling point 100 °C
- It is measured with a liquid-in-glass thermometer whose liquid does not always expand linearly
Definition of the unit
- Definition of the unit the kelvin is defined using the Boltzmann constant
- Historically 273.16 K was assigned to the triple point of water, where ice, liquid and vapour coexist
Section 2 of 10Converting between scales
Conversion
- T/K = θ/°C + 273.15 and θ/°C = T/K − 273.15
Size of the unit
- Size of the unit one kelvin equals one degree Celsius, so temperature differences are identical
- A change of 30 °C equals a change of 30 K
Examples
- 0 °C = 273 K, 100 °C = 373 K, 25 °C = 298 K and −196 °C = 77 K
Notation
- Notation write the kelvin without a degree sign
- Equations in thermal physics, including the ideal gas law, must use kelvin
Memory aid
- 0 °C = 273.15 K, so add 273.15 to go from Celsius to kelvin
Section 3 of 10Absolute zero
Definition
- Definition absolute zero, 0 K or −273.15 °C, is the lowest possible temperature
- A temperature in kelvin is never negative
Energy
- Energy it is where the average kinetic energy of the particles is at its minimum
- No more energy can be removed from a system at 0 K
Direction of approach
- Direction of approach laboratories reach within billionths of a kelvin but never exactly zero
Space
- Space intergalactic space is at about 2.7 K, just above absolute zero
Section 4 of 10Evidence from gas pressure
Experiment
- Experiment measure the pressure of a fixed mass of gas at constant volume at several temperatures in °C
Graph of p against θ
- Graph of p against θ a straight line, which extrapolates to zero pressure at about −273 °C for any gas at low density
Meaning
- Meaning an ideal gas would have no pressure because its particles would have no kinetic energy
- Real gases liquefy first, but the same intercept for every gas supports a universal absolute zero
Graph of p against T
- Graph of p against T a straight line through the origin, so p ∝ T
Section 5 of 10Temperature and kinetic energy
Link
- Link the absolute temperature is directly proportional to the average kinetic energy of the molecules
- For an ideal gas the mean translational kinetic energy is (3/2)kT
Ratios
- Ratios doubling the kelvin temperature doubles the mean kinetic energy
- 600 K is twice as hot as 300 K, but 60 °C is not twice as hot as 30 °C
Section 6 of 10Reference temperatures
| Temperature | Value in K |
|---|---|
| Absolute zero | 0 (−273 °C) |
| Liquid nitrogen boiling point | 77 |
| Water ice point | 273 |
| Triple point of water | 273.16 |
| Room temperature | about 293–298 |
| Steam point | 373 |
| Surface of the Sun | about 5800 |
Section 7 of 10Why kelvin matters in calculations
Why kelvin matters in calculations
Gas heated from 27 °C to 54 °C
- Gas heated from 27 °C to 54 °C the Celsius value doubles but the kelvin value goes from 300 K to 327 K, only 9% more
- The pressure at fixed volume rises by 9%, not 100%
Rule for proportional relations
- Rule for proportional relations for any relation quantity ∝ T, such as the ideal gas equation, mean kinetic energy and internal energy, use kelvin
Temperature differences
- Δθ in °C and ΔT in K are interchangeable, as in specific heat capacity calculations
Section 8 of 10More from the specification
More from the specification
Thermodynamic and empirical temperature scales
- Thermodynamic and empirical temperature scales shifting absolute zero by 273.15 degrees links the thermodynamic Kelvin and empirical Celsius temperature scales.
Thermodynamic temperature and absolute zero
- The Kelvin scale is the absolute temperature scale independent of any substance properties,
- where zero kelvin corresponds to minimum internal energy.
- (0 K = -273.15 °C)
Thermocouples and thermometric properties
- Thermocouples generate an emf proportional to the temperature difference between hot and cold junctions, calibrated across wide thermal ranges.
Resistance thermometers and thermistors
- Resistance of metallic RTDs increases linearly with temperature,
- whereas negative temperature coefficient (NTC) thermistors display steep non-linear resistance drops.
Thermodynamic temperature and absolute zero
- The Kelvin scale is the absolute temperature scale independent of any substance properties,
- where zero kelvin corresponds to minimum internal energy.
- (0 K = -273.15 °C)
Thermocouples and thermometric properties
- Thermocouples generate an emf proportional to the temperature difference between hot and cold junctions, calibrated across wide thermal ranges.
Resistance thermometers and thermistors
- Resistance of metallic RTDs increases linearly with temperature,
- whereas negative temperature coefficient (NTC) thermistors display steep non-linear resistance drops.
Resistance
- Opposition to current, R = V / I, in ohms (Ω).
Thermocouple
- Two dissimilar metals joined at two junctions producing an e.m.f. from a temperature difference
Thermistor
- Resistor whose resistance falls as temperature rises.
Calibrated
- Set to known, accurate values
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Thermodynamic temperature scale | a scale independent of any particular substance, measured in kelvin |
| Kelvin | the SI base unit of temperature, equal in size to one degree Celsius |
| Absolute zero | 0 K, the lowest possible temperature, where particles have minimum kinetic energy |
| Triple point of water | the temperature, 273.16 K, where ice, liquid and vapour coexist |
| Celsius scale | a scale based on the ice and steam points of water |
| Constant-volume gas thermometer | a gas thermometer using pressure at fixed volume |
Section 10 of 10Exam technique
1
Using °C in a ratio or proportionality — convert to kelvin first.
2
Writing a degree sign with kelvin — write 300 K, not 300 °K.
3
Adding 273 to a temperature difference — a change in °C equals the change in K.
4
Thinking absolute zero can be reached or be negative in kelvin — it is a lower limit that cannot be passed.