IB Physics SL

Revision Notes

3.1.2 Temperature


  • Temperature is a measure of how hot or cold objects are
  • Temperature also determines the direction in which thermal energy will flow between two objects (or between an object and its surroundings)
  • When thermal energy is exchanged, the objects (or systems) involved are said to have a thermal interaction
  • The thermal energy exchanged during a thermal interaction is referred to as heat
  • During a thermal interaction:
    • Thermal energy always flows from the hotter object to the colder object
    • The energy transfer continues until the two objects are in thermal equilibrium (i.e. they both have the same temperature)
    • Thermal energy can be transferred via conduction, convection or radiation
  • Temperature is a scalar quantity and it is measured using a thermometer
  • It is measured in degrees Celsius (°C) or kelvin (K)
    • The kelvin is the SI base unit for temperature
  • The temperature of an object is a macroscopic measure of the average kinetic energy of the particles (atoms or molecules) that make up the object

Absolute temperature

  • Absolute temperature is temperature measured in kelvin (K)
  • Absolute zero is a temperature of zero kelvin (0 K) and corresponds to the temperature at which the average kinetic energy of the molecules is at its minimum
  • The conversion between the Kelvin and the Celsius scale is given by:

T(K) = T(°C) + 273.15

  • It is important to notice that differences in absolute temperatures correspond to differences in Celsius temperatures 

ΔT(K) = ΔT(°C)

    • Where ΔT stands for temperature change
  • The absolute temperature of a body is directly proportional to the average kinetic energy of the molecules within the body

Absolute temperature and kinetic energy of molecules, downloadable IB Physics revision notes

The ice point is determined by placing a thermometer in a beaker containing melting ice, while the steam point is determined by placing the thermometer in a beaker with boiling water

Worked Example

Give an estimate of room temperature in kelvin (K).

Step 1: State a reasonable value for room temperature in degree Celsius (°C)

room temperature (°C) ~ 20°C

Step 2: Write down the conversion between Celsius scale and Kelvin scale 

T(K) = T(°C) + 273.15

Step 3: Convert the room temperature value and express it in kelvin (K)

room temperature (K) ~ 293 K

Exam Tip

Remember that the lowest possible temperature on the Kelvin scale is absolute zero (0 K). Therefore, if you are calculating temperature in kelvin and you end up with a negative number, you need to check your work, since negative numbers do not exist on the Kelvin scale.


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