Learning outcomes
By the end of this lesson, students should be able to:
- relate temperature to average particle kinetic energy
- distinguish temperature, thermal energy and internal energy
- explain the meaning of absolute zero
- convert between degrees Celsius and kelvin using T = θ + 273
- recognise that temperature differences have the same numerical size in kelvin and degrees Celsius
2.1 Temperature and random motion
Temperature indicates the degree of hotness of a body and is linked to the average kinetic energy of its particles. In a hotter sample, particles have a greater average kinetic energy and their random motion is more vigorous. Not every particle has exactly the same speed. At any instant there is a range of speeds, so the word average is essential.
Temperature does not measure the total energy stored in a body. A bath of warm water may contain more internal energy than a cup of hotter water because it contains far more particles. Temperature is an intensive quantity: it does not depend on how much material is present. Internal energy is extensive: it normally increases when the amount of material increases.
2.2 Thermal equilibrium and direction of transfer
When two bodies at different temperatures are placed in thermal contact, energy is transferred from the hotter body to the colder body. Transfer continues until both have the same temperature. They are then in thermal equilibrium and there is no net thermal energy transfer between them.
A thermometer must reach thermal equilibrium with the object whose temperature is being measured. If it is read too soon, it may show a value between its original temperature and the object temperature. The thermometer should be small enough not to change the object temperature significantly.
Figure 3. Original KG2UNI diagram.
2.3 Absolute zero
There is a lowest possible temperature, approximately -273 °C, called absolute zero. At absolute zero particles have the least possible kinetic energy. At this level, do not state that all particle motion necessarily becomes exactly zero; the syllabus wording is that particles have least kinetic energy.
The kelvin scale begins at absolute zero. A temperature of 0 K corresponds to about -273 °C. The kelvin is written K, not °K. Kelvin temperature cannot be negative in ordinary school-level thermodynamics.
2.4 Celsius-kelvin conversion
Use T(in K) = θ(in °C) + 273. To convert Celsius to kelvin, add 273. To convert kelvin to Celsius, subtract 273. Thus 25 °C is 298 K, while 310 K is 37 °C.
A temperature rise of 10 °C is equal to a rise of 10 K because the size of one division is the same on both scales. Only the zero points differ. This matters in equations: a temperature change can be expressed in °C or K with the same numerical value, but absolute gas-law temperatures must be in kelvin.
Figure 4. Original KG2UNI diagram.
2.5 Evidence for particle motion
Brownian motion, although not a separate recall point in this syllabus section, supports the particle model. Tiny visible smoke particles move irregularly because invisible air molecules collide with them unevenly. The observed smoke particle is not an air molecule. Higher temperature produces more vigorous molecular motion and therefore more vigorous bombardment.
Diffusion is also consistent with random motion: particles spread from a region of higher concentration to lower concentration. It occurs much faster in gases than in liquids because gas particles move more rapidly and are much farther apart.
Worked examples
Celsius to kelvin
A laboratory temperature is 22 °C. T = 22 + 273 = 295 K.
Kelvin to Celsius
A gas is at 350 K. θ = 350 – 273 = 77 °C.
Temperature change
Water warms from 18 °C to 63 °C. The increase is 45 °C, which is also 45 K.
Practical focus
Investigation
Place a thermometer in warm water and record readings every 20 s while it cools. Stir gently before each reading so the water has a more uniform temperature. Plot temperature against time. The curve approaches room temperature because the rate of net transfer decreases as the temperature difference becomes smaller.
Examination guidance
- Use kelvin, not Celsius, in gas-law relationships involving absolute temperature.
- Write average kinetic energy, not simply kinetic energy, when linking temperature and particles.
- Do not confuse a large amount of warm material with a high temperature.
Check your understanding
- What particle quantity is linked to temperature?
- Convert -20 °C to kelvin.
- Convert 300 K to degrees Celsius.
- Why must a thermometer be left in contact with an object before reading it?
- Is a rise of 15 °C equal to 15 K?
Answers
- Average kinetic energy.
- 253 K.
- 27 °C.
- So that the thermometer reaches thermal equilibrium with the object.
- Yes; temperature intervals have the same numerical size on both scales.
