Thermal Physics Flashcards
What is internal energy?
The sum of the potential energy due to attractions between particles, and the kinetic energy due to random motion of the particles.
What is the connection between energy and temperature?
ΔQ = m c ΔT
Energy required/absorbed = mass x SHC x temp change
What is…
Specific heat capacity?
Thermal capacity?
- The amount of energy needed to raise the temperature of a unit mass of a substance by 1K. It is a property of a material. (J/kg/K)
- The amount of energy needed to raise the temp of an OBJECT by 1K. C x ΔT = Q, C is in J/K
What is Latent heat? How can you work it out?
It is the energy required to change the state of a unit mass of a substance at constant temperature (J/kg). Temp is constant because the KE is constant, but work is done to move particles apart.
Latent heat of vaporisation (boiling/condensing), Lf > Latent heat of fusion (freezing/melting), because more energy is required for a larger change in potential energy.
Q = m L
What is an experiment to find the SHC of a substance?
With an electric heater, heat a sample of substance. Make sure it is well insulated. Measure the temperature change, the time, and the current and amplitude. Plot a graph of ΔT against t, and measure the gradient.
What is temperature?
What is thermodynamic temperature?
What is a thermometer?
What happens when two objects touch?
- It is the measure of thermal energy in an object, and depends only upon the KE of the particles of the object.
- It is measured using the Kelvin scale, 273K=0C
- It contains a substance that has a physical property which changes consistently with temperature.
- Thermal energy will be transferred in both directions between objects in thermal contact. When there is no resultant energy transfer, the objects are in thermal equilibrium.
What are the three gas laws?
How can they be combined?
- Boyles law p ∝ 1/V at constant temperature
- Charles’ law V ∝ T (K) at constant pressure
- Pressure law p ∝ T (K) at constant volume
- p₁V₁/T = p₂V₂/T or p₁V₁/T = constant
- purely experimental laws
What is the ideal gas equation?
pV = nRT
where n is the number of moles of gas
R is the molar gas constant
Note: one mole of gas (in g) = molar mass
How can you investigate Boyle’s law?
How can you investigate the pressure law?
- Vary the volume of a fixed mass of gas at constant temperature, perhaps with a gas syringe. Measure the temperature and pressure. Plot results in a graph, p against 1/V
- Heat a fixed mass and volume of gas in a sealed container in a water bath, and measure the pressure and temperature.
What is an ideal gas? When do gases act like ideal gases? [5+1]
- Particles only interact when they collide - no inter-particle forces.
- Molecules only have kinetic energy.
- Collisions are elastic and time taken is negligible.
- Particles move randomly, obey newtons laws.
- The volume of particles is negligible.
- Gases are like ideal gases at low density and high temp
What does kinetic theory tell us about the pressure and volume of a gas?
- pV = 1/3 N m c²bar
N is the no. of molecules
m is the mass of one molecule
c²bar is the average of the squares of the velocities of all the particles “mean square speed”
What is the Maxwell Boltzmann distribution?
- In an ideal gas, it is a bar chart of the number of molecules at different speeds. The peak is the modal speed. The area under the graph is always the same because ΣN is always constant. Cooler gases have a higher peak closer to 0 m/s
What is the mean molecular energy in a gas?
Where does the equation come from?
From this, how can you find the internal energy of a gas?
- The average KE that each molecule has in an ideal gas = 1/2 m c²bar = 3/2 k T
- equate formulas for pV, and rearrange:
n R T = 1/3 N m c²bar - Multiply by N, the no. of molecules.
