1. Connections between heat and work

2. Connections Between Heat and
Work

3. In studying thermodynamics…
Surroundings
SYSTEM

4. Zeroth Law of Thermodynamics
 If objects A, B and C are in contact with each other,
and A is in thermal equilibrium with B and B is in
thermal equilibrium with C, then A is in thermal
equilibrium with C.
A
B
C

5. Zeroth Law at Work

6. First Law of Thermodynamics
Heat
(Q)
Internal
Energy (U)
Work
(W)

7. First Law of Thermodynamics
 The change in internal energy (ΔU) is equal to the
difference of the heat (Q) added/removed to/from the
system and the work (W) done on/by the system.
ΔU = Q – W

8. Sign Conventions for the First Law
For Heat (Q)
+ if the heat is added to the system
if the heat is removed from the system
For Work (W)
+ if work is done by the system
if work is done on the system

10. Sample Word Problems on 1st Law
 Suppose 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?
 You have a motor that absorbs 3000 J of heat while
doing 2000 J of work. What is the change in the
motor’s internal energy?
 Say that a motor does 1000 J of work on its
surroundings while releasing 3000 J of heat. By how
much does its internal energy change?

11. The First Law at Work

12. Thermodynamic Processes
 Adiabatic (constant heat)
 Isothermal (constant temperature)
 Isochoric (constant volume)
 Isobaric (constant pressure)

13. Second Law of Thermodynamics
 Review: How does heat flow?
 Would it be possible for heat to flow from an area of
lower to higher temperature?

14. Second Law of Thermodynamics
 Clausius Statement
Heat can flow spontaneously from a hot object; heat
will not flow spontaneously from a cold object to a hot
object.

15. Second Law and Heat Engines
 It is a machine that turns energy into mechanical
energy or motion, especially one that gets its energy
from a source of heat, such as burning of a fuel.
 Can be classified as external combustion and internal
combustion engines.

18. Water-Tube Type
 It is common with
stationary engines and
turbines.
 Water is allowed to pass
through tubes while the
flames and hot gaseous
products of combustion
follow a path over around
tubes.

19. Fire-Tube Type
 Used in steam
locomotives
 Flames and heat are
made to enter the tubes
which are horizontally
arranged in the boiler
and are surrounded by
water.

20. Other Classification of Steam
Engines
 Condensing type
 Non-condensing type

21. Gasoline Engines
 These are engines whose
working substance is
gasoline. It is internally
burned unlike steam
engines.

22. Parts of a Gasoline Engine

25. Intake Stroke

26. Compression Stroke

27. Power Stroke

28. Exhaust Stroke

29. Diesel Engines
 These are engines whose
working substance is
diesel. It is internally
burned unlike steam
engines.

30. Diesel Engine Structure

31. How efficient are heat engines?

32. Thermal Efficiency of an Engine
 It is defined as the ratio of the net work (W) done by
the engine during one cycle to the energy absorbed at
the higher temperature (QH) during the cycle.

33. Sample Problem on Thermal
Efficiency
 Find the efficiency of a heat engine that absorbs
2000 J of energy from a hot reservoir and exhausts
1500 J to the cold reservoir.
 Your car is powered by a heat engine and does 3.0 x
107 J of work getting you up a small hill. If the heat
engine is 80 percent efficient, how much heat did
it use and how much did it exhaust?

34. Sadi Carnot
 A French engineer who
established the concept of an
ideal engine known as the
Carnot engine.
 He developed the Carnot’s
theorem which states that No
real engine operating between
two energy reservoirs can be
more efficient than a Carnot
engine operating between the
same two reservoirs.

35. Basic Concept of the Carnot
Engine
 The ideal efficiency of an engine depends on the
difference of the hot and cold reservoirs.
 You can’t have it all.

36. Sample Word Problems on Carnot
Efficiency
 If an engine extracts heat from a 2730 K reservoir and
expels heat at 1730 K reservoir, what is its efficiency?
 How about if the engine extracts heat from a 10 730 K
reservoir instead?
 What if the engine reservoirs are working at the same
temperatures?

37. Wait a minute…
 It is possible to produce work from heat – that is heat
transferred from a hot reservoir to a cold reservoir.
Would it be possible to do the reverse?

38. Second Law of Thermodynamics
 Kelvin-Planck Statement
It is impossible to construct a heat engine that,
operating in a cycle, produces no effect other than the
absorption of energy from a reservoir and the
performance of an equal amount of work.

39. The Impossible Heat Pump

40. Heat Pumps
 These are heat engines running in reverse.
 Heat is transferred from a cold reservoir to a hot
reservoir by performing work.
 This is done through the aid of phase change.
 Examples are refrigerators and air conditioning units.

41. The Structure of a Heat Pump

42. Refrigerators

43. Questions to Ponder
 What do you notice with heat engines and heat
pumps?
 What do they have in common?
 What is its impact to the environment?

44. Entropy
 Natural processes tend to undergo increased state of
disorder.
 A measure of the amount of energy in a physical
system not available to do work.

45. Second Law of Thermodynamics
 On entropy
The total entropy of an isolated system that undergoes
a change can never decrease.

46. Entropy increases…

47. Third Law of Thermodynamics
 It is impossible to reach absolute zero.

48. Let’s wrap up…