1.8.2Electricity, Energy and Waves
Absolute Zero and Temperature
5 min read 6 parts Lesson 42 of 54 Unit quiz
Section 1 of 6Temperature and Particle Motion
Temperature is linked to the average kinetic energy of particles.
Hotter gas
- In a hotter gas the molecules move faster on average, so they have more kinetic energy.
Collisions
- Faster molecules strike the container walls more often and with greater force.
Pressure
- This means that for a fixed volume, a rise in temperature increases the pressure.
Cooling
- As temperature falls, the molecules slow down and the pressure drops.
Section 2 of 6Absolute Zero
Absolute Zero
Idea
- Lord Kelvin recognised that there must be a temperature at which gas particles exert no pressure because they have stopped moving.
Value
- This temperature is called absolute zero, equal to −273 °C or 0 K.
Definition
- At absolute zero the particles have minimum kinetic energy and no more energy can be removed.
Lowest possible
- No temperature can be lower than absolute zero, so kelvin temperatures are never negative.
| Point | Celsius | Kelvin |
|---|---|---|
| Absolute zero | −273 °C | 0 K |
| Water freezes | 0 °C | 273 K |
| Water boils | 100 °C | 373 K |
Section 3 of 6The Kelvin Scale
The Kelvin Scale
Conversion
- Temperature in K = temperature in °C + 273, and °C = K − 273.
Equal steps
- A change of 1 K is the same as a change of 1 °C, so temperature differences are identical on both scales.
Why use it
- Gas relationships such as pressure and temperature being directly proportional only work using kelvin.
Check
- A negative answer in kelvin shows a mistake.
Section 4 of 6Converting Temperatures
1
Identify whether the starting temperature is given in °C or K.
2
To convert a temperature from °C to K, add 273 to it.
3
To convert a temperature from K to °C, subtract 273 from it.
4
Check the result is sensible; a kelvin value must be greater than zero.
5
Include the correct unit symbol, K or °C, in the answer.
Section 5 of 6Worked Example
Question
- A laboratory is at 295 K.
- Convert this to °C.
- A freezer is at −18 °C.
- Convert this to kelvin.
- For the laboratory, °C = K − 273 = 295 − 273 = 22 °C.
- For the freezer, K = °C + 273 = −18 + 273 = 255 K.
- Checkboth answers fit the reference points, since 0 K is −273 °C.
Answer 22 °C for the laboratory and 255 K for the freezer.
Section 6 of 6Exam Checklist & Tips
1.8.2 – Key Points for the Exam
- Absolute zero The lowest possible temperature, 0 K or −273 °C.
- Kelvin The SI unit of temperature, starting at absolute zero.
- Kinetic energy The energy a particle has because of its motion.
- Average speed The mean speed of the molecules in a gas, related to temperature.
Common Mistakes
- Adding 273 when converting from kelvin to Celsius.
- Writing kelvin with a degree sign, as °K.
- Saying particles disappear at absolute zero; they just have minimum energy.
- Believing that temperatures below −273 °C are possible.