Thermal Physics 2 - 1st Law

People, you have got to start listening in class. That goes for all classes - tutorials, lectures, labs etc.

So anyway, stuff we went through this week:

First Law of Thermodynamics

The internal energy of a system depends only on its state; the increase in the internal energy of a system is the sum of the work done on the system and the heat supplied to the system.

ΔU = Q + W

(Note that when defining the first law, you have to include the underlined phrases)

This is one of the first laws you've learnt so far that can be applied to a calculation question. Hence when you attempt any of the 1st Law thermal questions, please follow the standard format:

(start with...)

By the 1st law of thermodynamics,
ΔU = Q + W

(followed by substituting in the values before presenting your final answer)


Note that ΔU depends on the state hence as long as a process involving a move from point 1 to 2, ΔU will be the same regardless of path taken.

Work Done (W) however depends on the path taken as shown below.
The areas under the p-V graph represent the amount of work done in getting from point 1 to 2.

** For cyclic processes, magnitude of net work done is actually the area enclosed by the cycle and there is no change in internal energy (because you go back to the same point!).


Conventions

ΔU positive: increase in internal energy
negative: decrease in internal energy

Q positive: heat supplied to the system/gas
negative: heat is lost from the system/gas

W positive: work is done on the gas (compression occurs)
negative: work is done by the gas (expansion occurs)


Processes & p-V graphs

(1) Isothermal Processes
The shape of an isotherm is as such because, since pV=nRT, when T is a constant and you plot p vs V. You essentially end up with a y = 1/x type of graph.
So what happens when T increases? the "constant" increases so your graph gets transposed upwards. Hence T₁>T₂>T₃>T_4.

(2) Isochoric Process

(3) Isobaric Process
(2) Adiabatic Process