Thermal physics

A-level Physics

Unit G484: The Newtonian World

Temperature

Thermal physics

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.

Temperature LOs

Thermal physics

Learning objectivesAt 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.

Lesson focus• Temperature

Thermal physics

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

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 higher temperature to one of lower temperature

Heat and temperature LOs

Thermal physics

Temperature

Temperature is a measure of how or cold a body is and is

related to the mean, translational kinetic energy of its particles.

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.

Thermal physics

What happens next? LOs

Thermal physics LO 2: explain that regions of equal temperature are in thermal equilibrium

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.

A and C are in thermal equilibrium with B.A and C are also in thermal equilibrium.

B

A C

A is in equilibrium with B

B is in equilibrium with C

A and C are also, therefore, in equilibrium. If they are brought into contact there will be no net heat transfer.

To do - textbooks Write a definition of thermal equilibrium.

Thermal physics

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

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

Constant volume gas thermometer LOs

Thermal physics

Since for a particular mass of an ideal gas,

LO 3: describe how there is an absolute scale of temperature that does not depend on the property of any particular substance

The absolute scale of temperature LOs

(𝑝𝑉 )𝒖𝒏𝒌𝒏𝒐𝒘𝒏 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒𝒖𝒏𝒌𝒏𝒐𝒘𝒏 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

(𝑝𝑉 )𝒌𝒏𝒐𝒘𝒏𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒𝒌𝒏𝒐𝒘𝒏𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒¿

The known temperature used is that of the ‘triple point of water’ (273.16 K)

(𝑝𝑉 )𝑢𝑛𝑘𝑛𝑜𝑤𝑛𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒𝑢𝑛𝑘𝑛𝑜𝑤𝑛𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

(𝑝𝑉 )𝑎𝑡𝑡𝑟𝑖𝑝𝑙𝑒𝑝𝑜𝑖𝑛𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟273.16¿

• Kelvin scale video

Thermal physics

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 which a substance has minimum internal energy

The absolute scale of temperature LOs

Thermal physics

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.

Temperature LOs