1. First Law of Thermodynamics
Work:
Total energy transferred to a system by macroscopic forces
exerted on it by other systems
Work
done by a gas on a piston A
Work done by a fluid as it expands
from V0 to Vf
Fluid (gas) F
=P A x x
Energy leaves the system x A
Energy feed in the system

2.  Q A First law of thermodynamics:
All systems have an internal energy state function that is changed
by heat transferred to or from the system and by the work
done by or on the system.
nothing but conservation of energy
closed system
internal energy
state function 
Energy transferred by
macroscopic mechanical
means
Energy transferred by
non-macroscopic mechanical
means A Q

3. Joule’s experiment: the mechanical equivalent of heat
What do we learn from the experiment 1
-dissipative work Wdis done
-no heat flow Q
-change in
measured
by change in temperature
(V=const.) 2
-same change in
could be created by a
heat flow Q
Joule's Paddle-Wheel Apparatus 1849
Change of internal energy U in a dissipative process is the same as if
there had been a heat flow Q into the system of equal magnitude to Wd.
click for virtual experiment

4. Application of 1. law in a free expansion experiment
V1 ,Ts
V2 ,Tf
Thermal insulation
gas
no heat flow
=:adiabatic process
fixed walls
no work done by the container
W=0
1. law:
ideal gas
U=U(T) for the ideal gas only