1. A-level Physics
Unit G484:
The Newtonian
World
Temperature
Thermal physics

2. Temperature LOs
To do
1. Define ‘internal energy’.
2. Write an expression for the mean k.e ( Ek ) of an atom in an ideal gas.
3. Explain how Ek is related to the internal energy (U) of an ideal gas.
4. Draw a graph to show the variation of pressure with absolute
temperature for an ideal gas.
Thermal physics

3. Lesson focus
• Temperature
Learning objectives
At the end of the lesson you will be able to:
• explain that thermal energy is transferred from a region of higher
temperature to one of lower temperature;
• explain that regions of equal temperature are in thermal equilibrium;
• describe how there is an absolute scale of temperature (the
‘thermodynamic scale’) that does not depend on the property of any
particular substance;
• state that absolute zero is the temperature at which a substance has
minimum internal energy.
Thermal physics

4. Learning outcomes
All of you should be able to
• explain the meaning of thermal equilibrium;
• explain in simple terms how absolute temperatures are measured;
• explain the meaning of absolute zero;
• convert the temperatures from celsius to kelvin (or vice versa).
Some of you may be able to
• explain why a substance at absolute zero cannot be said to have
zero energy.
Thermal physics

5. Heat and temperature LOs
Heat (symbol: Q)
Heat is defined as thermal energy in transit (flowing) – usually – from
a hot region to a cooler region.
The study of heat energy travelling in this way led to the science of
thermodynamics.
Question
Can you think of a situation in which thermal energy flows from a cool
region a warm region?
LO 1: explain that thermal energy is transferred from a region of
Thermal physics higher temperature to one of lower temperature

6. Heat and temperature LOs
Heat (symbol: Q)
Heat is defined as thermal energy in transit (flowing) – usually – from
a hot region to a cooler region.
Temperature
Temperature is a measure of how or cold a body is and is
related to the mean, translational kinetic energy of its particles.
Thermal physics

7. What happens next? LOs
Thermal physics

8. The zeroth law of thermodynamics LOs
When two objects are in thermal equilibrium (at the same temperature)
with a third object, they are also in thermal equilibrium with each other.
B is in
A is in equilibrium
equilibrium B with C
with B
A C A and C are also, therefore,
in equilibrium. If they are
brought into contact there
will be no net heat transfer.
A and C are in thermal equilibrium with B.
A and C are also in thermal equilibrium.
To do - textbooks Write a definition of thermal equilibrium.
Thermal physics LO 2: explain that regions of equal temperature are in thermal equilibrium

9. How is temperature measured? LOs
Temperature is usually measured by taking advantage
the thermometric property of a substance, e.g
• the expansion of a liquid (such as mercury or
alcohol)
• the resistance of a metal (e.g platinum)
Unfortunately, it is found that different types of
thermometer do not agree with each other.
Thermal physics

10. The thermodynamic scale of temperature LOs
To solve this problem, Lord Kelvin proposed an
absolute scale of temperature (the thermodynamic
scale) based, not on the behaviour of a particular
substance, but on the theoretical performance of a
heat engine.
A more practical scale of temperature is based on
the behaviour of an ideal gas.
Thermal physics

11. Constant volume gas thermometer LOs
Thermal physics

12. The absolute scale of temperature LOs
The known temperature used is that of the ‘triple point of water’ (273.16 K)

• Kelvin scale video
LO 3: describe how there is an absolute scale of temperature that
Thermal physics does not depend on the property of any particular substance

13. The absolute scale of temperature LOs
This scale of temperature gives rise to the concept of absolute zero.
pV0 0
=
pVTP 273.16
Where pV = 0 because an ideal gas at this temperature exerts no pressure.
At this temperature, substances have a minimum (but not zero) internal
energy (‘zero point energy’).
To do - research
Why it is not possible for the particles of a gas to have zero energy?
LO 4: state that absolute zero is the temperature at
Thermal physics which a substance has minimum internal energy

14. Temperature LOs
To do
Decide whether each of these statements is true or false. For each, say whether it is
always true (or false). Provide correct versions of false statements.
1. Water boils at 273 K.
2. The pressure of a fixed volume of gas decreases at its temperature falls.
3. Objects in thermal equilibrium have the same temperature.
4. One degree Celsius and one degree kelvin are equal in size.
5. When a substance is heated its particles increase in size.
6. Heat flows from a hotter region to a cooler region.
7. The thermometric property of a thermocouple is the change of electrical resistance
with temperature.
8. Water boils at 100 °C.
9. The resistance of a NTC thermistor increases with increasing temperature.
Thermal physics