Heat refers to the hotness of an object, while temperature measures both hotness and coldness. The document discusses how heat can be converted into work through machines and engines, and how doing work releases heat. It defines key terms like heat, work, thermal equilibrium, and internal energy. The First Law of Thermodynamics states that the change in a system's internal energy equals the heat added minus any work done by the system. Sample problems demonstrate using this law to calculate internal energy changes.

1. Review: Heat and Temperature
If the answer is ‘True’, give a
thumbs up, otherwise, give a thumbs down.
1. Heat refers to the hotness of an object.
2. Temperature is the measure of hotness and
coldness.
3. In a car’s engine, heat provides energy to do work.
4. Machines can create and destroy energy.
5. Heat can be caused by friction.

3. OBJECTIVES:
explain how heat can be turned into work and
how doing work releases heat.
discusses on the spontaneous processes where
heat flows from an object of higher temperature
to an object of lower temperature.

4. LET’S DEFINE,
FIRST!

8. Heat, Q, energy caused by
temperature difference

9. Systems (or objects) are said to be in thermal
equilibrium if there is no net flow of thermal
energy from one to the other. A thermometer
is in thermal equilibrium with the medium
whose temperature it measures, for example.
If two objects are in thermal equilibrium, they
are at the same temperature.
Thermal Equilibrium

10. Work, W, energy caused by
physical motion

11. WORK
W is positive if work is
done by system.
Air does work on the
environment: W > 0.
W is negative if work is done on the
system.
Environment (man) does work on
system: W < 0
(Alternative: system does negative
work because force by air pressure
on thumb is opposite to the direction
of motion of the thumb.)

12. RELATIONSHIP BETWEEN HEAT AND WORK
Why does the volume of gas expands when it is heated?
W = F x d
Pressure (P) = (Force) F or F = P A
(Area) A
Volume (V) = L x W x H or A x d
d = V
A
W = P A V = P V
A

13. INTERNAL ENERGY (U or E)
is the total of the kinetic energy due to the motion of
molecules (translational, rotational, vibrational) and
the potential energy associated with the vibrational
and electric energy of atoms within molecules or
crystals.

15. The First Law of Thermodynamics states that :
The internal energy of a system changes from an
initial value Ui to a final value Uf due to heat
added (Q) and work done by the system (W)
DU = Uf – Ui = Q – W
Q is positive when the system gains heat, and
negative when the system loses heat.
W is positive when it is done BY the system, and
negative when it is done ON the system

16. SAMPLE PROBLEM:
If 150J of energy is added to a system
when no external work was done, by
how much will the thermal energy of
the system raised?

17. Example: 1000 J of thermal energy flows into a
system (Q = 1000 J). At the same time, 400 J of
work is done by the system (W = 400 J).
What is the change in the system's internal
energy U?
----------------------------------------------------------
Solution:
DU = Q - W
= 1000 J - 400 J
= 600 J

18. Example: 800 J of work is done on a system (W = -
800 J) as 500 J of thermal energy is removed from
the system (Q = -500 J).
What is the change in the system's internal energy
U?
-----------------------------------------------------Solution:
DU = Q - W
= -500 J - (-800 J)
= -500 J + 800 J
= 300 J

19. QUIZ:
1. 800 J of work is done on a system (W = -800 J) as 500 J of
thermal energy is removed from the system (Q = -500 J). What
is the change in the system's internal energy U? Explain.
2. 1000 J of thermal energy flows into a system (Q = 1000 J). At
the same time, 400 J of work is done by the system (W = 400 J).
3. What is the change in the system's internal energy U?

20. Assignment:
Cite at least five (5) practical
applications related to the
concepts of heat and work.

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