The 1st Law of Thermodynamics
• Warm-up
1. State the law of conservation of energy
as stated in the textbook (Chapter 8),
2. Describe how heat engines work
answer
1. The law of conservation of energy states
that;
– In a closed system (isolated system), energy
can neither be created nor destroyed.
Energy can change form but it is always
conserved.
2. heat engine:( steam-powered
trains)
• In a heat engine, an input of heat causes an increase in the temperature of
the working substance (water),
• At high temperature, gaseous water (steam) pushes a piston (perform
work), which causes the wheel to turn.
question
• Looking at the heat engine in the previous
slide, how would you figure out the amount
of work done to turn the wheel?
solution
d
workdone = heat added at high Temp
i - heat lost(at low Temp)
Problem
• 100 Joules of heat are added to a drier to dry the
clothes. If the exhaust heat is about 40 Joules, how
much heat energy was used to dry the clothes?
• Solution:
First law of thermodynamics
• This law states that:
• “Whenever heat is added to a system, it
transform to an equal amount of some other
form of energy”
• More specifically, this means:
heat added =
increase in
interna energy

external workdone
by the system
1st Law of thermodynamics
• ∆U = Q + W(on the system)
• ∆U = Q - W(by the system)
• Where;
• ∆U – change in internal energy of system
• Q –Quantity of internal energy of a system
• W- work done
Increase in internal energy
Heat
added
External work done
Questions
1.
If 10 J of energy is added to a system that does no
external work, how much will the change in internal
energy be?
2.
1000 joules of heat are added to a system consisting
of a gas in a cylinder with movable piston top. If the
internal energy of the system increases by 700 joules,
the work done by the gas on the pistons is:
________________
Class work
• Textbook page 368 # 1-7
Second Law of Thermodynamics
1.
1.
Warm-up
How much heat is lost to the environment in calories
from 400 g water that cools from 40oC to 23oC.
(specific heat capacity of water is 1c/goC)
If 10J of energy is added to a system that does 4J of
external work, by how much will the internal energy of
that system be raised?
solution
1. Heat lost (Q calories) = M •c •∆T
= 400g •1c/goC •17oC
=6800 calories
2.
Second Law of Thermodynamics
• The second law of thermodynamics states
that:
– Heat flows from hot to cold objects.
– Heat will not of itself flow from cold to hot objects.
And;
– Whenever energy transforms, it degenerates into
waste.
– Organized energy (concentrated and
therefore Usable energy) degenerates into
disorganized energy (non usable energy)
Second law explained.
• usable is organized.
• After work, the energy
released is
disorganized and
unusable.
• The unusable energy
becomes air pollution
which is waste.
Order tends to disorder
• Natural systems tend to proceed toward a state of greater disorder.
question
A
B
C
Nitrogen molecules within a glass tube are
allowed to move randomly. Which figure
shows
the molecules in a state of greatest entropy?
Order tend to disorder
Entropy
• Entropy is the measure of the amount of
disorder.
• The second law of thermodynamics says
that the total entropy of the universe can
never decrease.
• Organized structure always become
disorganized messes over time.
Entropy
• Organized structure always become
disorganized messes.
Class work/Homework
• Textbook pg 369 # 19-25