THE FIRST LAW OF
THERMODYNAMICS
“Conservation of Energy Law”

SECTION OBJECTIVES:
1.State the First Law of Thermodynamics.
2.Solve Problems on the First Law Of Thermodynamics.

What is ENERGY?
Energy can be changed from one form to
another, but it cannot be created or destroyed. The
total amount of energy and the matter in the
Universe remains constant, merely changing from
one form to another.
Energy is closely associated with physical as
well as chemical changes.

Forms of Energy:
Potential Energy (stored)
Kinetic Energy (in motion)
Mechanical Energy (at work)
Thermal Energy (heat)
Electrical Energy (mobile electrons)

Internal Energy - the sum of the kinetic and
potential energies of its constituent atoms.
Heat – thermal energy transferred from a
hotter system to a cooler system that are in
contact.
System work- the quantity of energy
transferred from one system to another.

• The First Law of Thermodynamics:
• The increase in the internal energy of a system is equal to the amount of
heat added to the system, plus the amount of work done on the system.
U = Q + W
• The increase in the internal energy of a system is equal to the amount of
heat added to the system, minus the amount of work done by the
U = Q - W
• Q = heat added to system -Q = heat added by the system
• W = work done by the system -W = work done on the system

EVALUATION:
1.) 2500 J of heat is added to a gas under a piston in a
closed cylinder, and 1800 J of work is done by the system
as its piston expands. What is the change in internal
energy of the system?
2.) 2500 J of heat is added to a system, and 1800 J of
work is done on the system. What is the change in
internal energy of the system?

Some Vocabulary for Thermodynamic Processes:
Isobaric Process (Pressure is constant)
P = constant W=P V
Isovolumetric Process ( Volume is constant, Work is 0)
V = constant Q = [Cv/R (v)][(P2-P1)]
W = 0
Isothermal Process( Temperature is constant, I.H is 0)
T = constant PV = nRT
U = 0 (ideal gas)
Adiabatic Process( Heat is 0)
Q = O P = v + w