5. Thermodynamics Flashcards
What is the difference between a closed system and an isolated system?
closed system - matter cannot leave or come into the system
isolated system - matter AND energy cannot leave or come in.
What is thermal energy? What is temperature? What is heat (Q)?
Thermal energy is the internal energy of a system caused by the aggregate microscopic motion of its particles. Temperature is a macroscopic measure of thermal energy, given as per molecule. Hence, thermal energy depends the on quantity of particles.
Heat (Q) is the transfer of thermal energy between system and surroundings
What is the zeroth law of thermodynamics?
objects in thermodynamic equilibrium have no heat transfer, and have the same internal temperatures.
i.e. heat flows from higher temperatures to colder ones.
What are the three mechanisms by which heat transfer can occur? Explain all 3.
conduction - transfer through contact.
convection - transfer of heat from some source to a fluid causing that fluid (fluids can be gases) to expand and rise.
radiation - heat transfer through EM waves.
Why is the heart pumping warmer blood considered forced convection?
Natural convection occurs when there is heat transfer through fluid with no external force on the fluid.
The heart pumps metabolically-warmed blood to the extremities where the blood cools and circulates back to the heart. This is forced convection.
What is the first law of thermodynamics, what is its equation?
Energy of the universe is constant. Energy cannot be created or destroyed, only transferred.
ΔE = Q - W
Q = heat W = work
Think Edward Qui
The first law of thermodynamics equation is ΔE = Q - W. Explain the sign conventions of heat and work relative to the system.
Heat:
heat is added TO the system, Q is +
heat is lost FROM the system, Q is -
Work:
work is done BY the system, W is +
work is done ON the system, W is -
t or f, if an ideal gas is in a sealed container, then KE = PE = 0. Thus internal energy of the system must equal the thermal energy of the system alone.
True, and thus, in systems where the internal energy is only thermal energy, ΔE = Q - W (as long as its not an isolated system in which ΔE = 0)
If an ideal gas is in a container with a piston locked in place and we add a flame to the bottom of the container, what happens?
The piston is locked in place, no work can be done. As the flame heats the gas, the average kinetic energy of particles increases and thus its temperature rises. Since heat is being added, Q = + and ΔE = Q
when we let the gas cool down, Q = -, ΔE = -Q
If an ideal gas is in a container with a piston locked in place and we add a flame to the bottom of the container. After heating the gas we lock unlock the piston. What happens?
No heat is being added any more so Q=0.
the hot gas will expand and do work on the piston lifting it upwards. Since the system is doing work, W=+ and ΔE = -W. Once all of the hot gas transfers its heat into work (i.e. its cooling down) if we add more weight ontop of the pistion, the gas will be compressed. Negative work is being done (W= -) and thus ΔE = W
t or f, W = PΔV.
True, work also equals Fdcos0
We know that ΔE = Q - W and that W = PΔV.
Explain an isobaric system. Explain the Pressure vs volume (x) graph.
pressure is constant, so , W = PΔV, work varies with a change in volume. (recall an isobaric process = Charles’s law)
On a P vs V graph, there is a straight horizontal line. Thus P x (V2 - V1) gives the AUC which is work.
If ΔV is positive, then work is positive (which makes sense since a positive ΔV means that the system is doing work)
We know that ΔE = Q - W and that W = PΔV.
Explain an isochoric system. Explain the Pressure vs volume (x) graph.
An isochoric system, volume is constant, so W=PΔV, and ΔV = 0, work must be 0.
In systems where the volume cannot change, ΔE = Q. On the graph, this is seen as a straight vertical line and AUC is zero.
We know that ΔE = Q - W and that W = PΔV.
Explain an isothermal system. Explain the Pressure vs volume (x) graph.
If heat is allowed to pass freely between system and environment, then an isothermal process can occur in which the system remains a constant temperature (recall isothermal process = Boyle’s law)
We know that E is directly proportional to temperature. IF temperature is not changing then ΔT = 0 and ΔE = 0. Therefore Q = W.
According to W = PΔV, on the P vs ΔV graph, they must be inversely proportional. e.g. as gas expands and volume goes up, pressure must go down.
What is the difference between an isothermal process and an adiabatic process?
isothermal process = temperature does not change. Heat transfer can occur, but will be compensated by work to keep T the same.
adiabatic process = there is no heat transfer (Q = 0)
