C14 - Thermal Physics Flashcards
What is the triple point of a substance?
A specific temperature and pressure where three phases of matter (solid, liquid and gas) can exist in thermal equilibrium - there is no net transfer of thermal energy between the phases.
What’s thermal equilibrium?
When there is no net transfer of thermal energy between the phases.
What’s the zeroth law of thermodynamics?
If two objects are each in thermal equilibrium with a third, then all three are in thermal equilibrium with each other.
E.g. If A and C are in thermal equilibrium with B then A and C must also be in thermal equilibrium.
What’s the internal energy of a substance?
The sum of the randomly distributed kinetic and potential energies of atoms or molecules within the substance.
Why can’t internal energy be zero?
Even when there is no kinetic energy, there is still electrostatic potential energy stored between the particles.
How does increasing temperature effect the internal energy of a substance?
Increasing temperature increases the average kinetic energy of the atoms or molecules, increasing internal energy.
What can cause internal energy to increase?
Increase in temperature
Change in states
What effect does changing states have on the internal energy of a substance?
When a substance changes phases, e.g. From solid to liquid, it’s temperature remains constant however it’s electrostatic potential energy increases.
When the melting/boiling points of a substance are reached, it’s electrostatic potential energy changes since electrical forces between atoms change.
Only once this change is completed will kinetic energy continue to increase.
What are the electrostatic energies of the different phases?
Gas - it’s zero due to negligible electrical forces between atoms/molecules.
Liquid - it’s negative (meaning energy must be supplied to break the atomic/molecular bonds).
Solid - very large negative as there are very strong electrostatic forces between atoms/molecules.
(Lowest in solids, highest in gases)
How can the specific heat capacity of a material be identified by ‘method of mixtures’?
Method of mixtures - known masses of two substances at different temperatures are mixed together and their final temp at thermal eq’ is recorded. This allows one ‘c’ to be determined if the other is known.
What’s latent heat of vaporisation?
The energy required to change 1kg of a substance from liquid to gaseous phase (Lv).
Latent heat of vaporisation is usually greater than latent heat of fusion as there’s a greater difference in internal energy between gases and liquids and solids and liquids.
How can Lv be determined?
Latent heat of vaporisation can be found using a condenser to collect and measure the mass of liquid changing phase.
Lv = IVt / m
(m is the mass of the substance that changed phase during heating).
What’s specific latent heat?
Energy required to change the phase of a substance per unit mass.
There are two forms, depending on the phase change.
What’s specific latent heat of fusion?
The energy required to change 1kg of a substance from solid to liquid phase (Lf).
How can Lf be determined?
Latent heat of fusion can be determined by using a heating circuit.
Using ice a thermometer is needed to ensure that the ice is at its melting point (not lower) before switching the heater on.
By measuring the potential difference and current in the heater and time during which he heater is used, the energy he ice can be determined by E = IVt (E=Pt).
Specific latent heat of fusion can then be determined by: Lf = IVt / m (energy per unit mass).
