Thermal Equilibrium
Two objects are at thermal equilibrium when:
- They are at the same temperature
- There is no net transfer of thermal energy between them
Disadvantages of constant volume gas thermometer to measure temperature
- Large time to reach thermal equilibrium. It therefore has a large response time and cannot measure rapidly changing temperatures.
- Has a large heat capacity and reaching equilibrium requires significant transfer of energy. It therefore changes the temperature of environment being measured.
- Cannot measure temperature of solid objects.
Use cases of constant volume gas thermometer
- To measure a temperature that is constant over time
- To calibrate other thermometers
How the internal energy of a system may be changed according to the first law of thermodynamics
- Work done on the system
- Thermal energy supplied to the system
Use the first law of thermodynamics to explain why a bicycle pump gets hot when it is used to pump up a tyre == quickly==
- Increase in internal energy = work done on system + thermal energy supplied to system
- No thermal energy is transferred to system due to lack of time.
- Work is done on gas to compress it
- Internal energy increases so temperature increases
- As temperature is proportional to internal energy
With reference to molecular energies, explain why the temperature of water remains at when it vaporises in a kettle, even though it is being heated.
- During vaporisation molecular separation increases.
- Potential energy of molecules increases.
- Kinetic energy of molecules unchanged so temperature unchanged.
Define specific heat capacity
- Thermal energy per unit mass per unit change in temperature.
Define specific latent heat
- Thermal energy per unit mass to cause change of state
- at constant temperature
Why specific latent heat of vaporisation is usually greater than specific latent heat of fusion.
- Vaporisation involves a much greater increase in molecular separation than fusion.
- More work is done by molecules against intermolecular forces, causing a greater increase in potential energy.
- Because molecular kinetic energy is unchanged (as there is no temperature change during change of state) , more thermal energy is required.
Important: Why specific latent heat of vaporisation is larger than specific latent heat of fusion
- Vaporisation involves a greater increase in molecular spacing and volume of substance.
- More work is done by molecules against atmospheric pressure and intermolecular forces.
- Potential energy increases more.
- Kinetic energy is unchanged.
- Increase in internal energy is more positive
- Work done on system is more negative.
- More thermal energy is required.
Important: Why specific latent heat of fusion is smaller than specific latent heat of vaporisation.
Fusion involves a smaller increase in molecular spacing and negligible increase in volume.
Less work is done by molecules against intermolecular forces and negligible work is done against atmospheric pressure.
Potential energy increases less.
Kinetic energy is unchanged.
Internal energy increases less.
Less thermal energy is required.
Warning
For latent heat of fusion use “negligible increase volume” and not “increase in volume is smaller”. Because for water, volume actually decreases, so saying increase in volume is smaller would be physically incorrect.
Info
Explain how Fig. 2.2 shows that platinum is a suitable metal for use in a resistance thermometer.
- Resistivity varies linearly linearly with temperature.
- Each value of temperature has a unique value of resistivity.
Criteria for a property to be thermometric
- Varies linearly
- one-to-one mapping, each value of the property has a unique value of temperature.
Info
Suggest a reason why a platinum resistance thermometer is not suitable for measuring a rapidly changing temperature
- Thermometer has a high heat capacity
- Would take a long time to reach thermal equilibrium with environment whose temperature is to be measured.
Thermometer suitable for measuring rapidly changing temperatures
- Thermocouple
Internal energy of an ideal gas
- Total kinetic energy associated with the random motion of molecules of the ideal gas.
- plus total potential energy of molecules but potential energy is zero.
- As there are no intermolecular forces of attraction in an ideal gas.