3.4 Thermal Physics Flashcards
What is heat?
The flow of energy from a region at higher temperature to a region at lower temperature due to the temperature difference.
What is the internal energy?
The sum of the kinetic and potential energies of the particles of a system
What is work in terms of a gas that is in a cylinder fitted with a piston and in general for a gas?
The gas does work of amount pΔV when it exerts a pressure p and pushed the piston out a small way, so the gas volume increases by ΔV.
Work in general is the pΔV of a gas. It is energy in transit from or to the system.
What is the first law of thermodynamics?
The increase, ΔU, in internal energy of a system is ΔU = Q-W in which Q is the heat entering the system and W is the work done by the system.
Q and W can either be positive or negative
If the gas causes the volume to increased (e.g. pushing a piston) then the work is positive and the internal energy decreases
What is specific heat capacity?
The heat required, per kilogram, to increase the temperature of a substance by a degree or kelvin
Jkg^-1(K^-1) or Jkg^-1(C^-1)
What is absolute zero?
The temperature where the internal energy of a system is at a minimum.
What is the difference between gas and liquids/solids in terms of change in internal energy?
The work is negligible so ΔU = Q for liquids and solids
What is heat flow equal to?
Q = mcΔθ
What is work done by a gas equal to?
W = pΔV
and the area under a pV curve
What is isothermal expansion?
A slow expansion where work is done by the system as ΔU = 0 and Q=W
How do you derive the formula work done by a gas?
1) F = pA
2) ΔV = AΔx
Δx = ΔV/A
W = FΔx
W = FΔV/A
W = pΔV
It is also the area under a PV graph
What happens in rapid expansion of an ideal gas?
There is not enough time for heat to flow so Q=0 and ΔU = -W
In the end the temperature falls and the internal energy in the system is reduced.
What happens in slow (isothermal) expansion of an ideal gas?
This is when a gas slowly expands so that the ΔU = 0
The walls of the piston are conductive on heat so any flow of temperature in causes the gas to do an equal amount of work. Q = W.
In a clockwise cycle, why is more heat taken in altogether than are given out?
When the gas is expanding, heat flowing in has to supply the work to increase the volume as well as internal energy (heat it up so there is a thermal equilibrium).
When decreasing in volume, the work is done against the gas (the air is doing work) so not as much energy is needed.
Work done in a clockwise system is positive so the heat flow into the system, Q, must be positive for ΔU = 0.
Work done in an anti-clockwise system is negative so the heat flow into the system, Q, must be negative for ΔU = 0.
Clockwise = work done by gas
Anticlockwise = work done on gas
What is the experiment for estimating the absolute zero?
Equipment- Capillary tube, sulfuric acid, beaker of water, thermometer and heat source.
1) Add a drop of sulfuric add into the capillary tube to trap air and place it into the beaker
2) Heat water in beaker
3) Reduce the temperature of the water in the beaker and measure the temperature and the corresponding values of trapped air length (measure every 10 degrees
4) Plot a graph of length (y-axis) against temperature (x-axis)
5) Extrapolate the graph to find (temperature) x-intersect, the value of temperature when the gas has no volume which is absolute 0.
