HEAT, WORK AND
INTERNAL ENERGY
Objectives
What is the ideal gas
GAS-PROPERTIES
• https://phet.colorado.edu/en/simulation/lega
cy/balloons-and-buoyancy
https://www.youtube.com/watch?v=BTyQj39ZbMs
Work done on a gas
Won gas =-pV
•
•
•
•
IN SI system of units,
P : Pascal (Pa),
V : m3,
W : Joule (J)
Problem 1
The volume of an ideal gas changes from 0.40 to 0.55 m3 although its
pressure remains constant at 50 000 Pa. What work is done on system(gas) by its
environment?
a.
7 500 J
b.
200 000 J
c.
7 500 J
d.
200 000 J
Problem 2
The volume of an ideal gas changes from 0.40 to 0.55 m3 although its
pressure remains constant at 50 000 Pa. What work is done by the gas on its
environment?
a.
7 500 J
b.
200 000 J
c.
7 500 J
d.
200 000 J
PROBLEM2
• In an isobaric process 4.5  104 J of work is
done on a quantity of gas while its volume
changes from 2.6 m3 to 1.1 m3. What is the
pressure during this process?
a.
1.2  104 Pa
b.
2.4  104 Pa
c.
3.0  104 Pa
d.
4.1  104 Pa
problem3
• Calculate the work done by a gas that expands from 0.020 m3
to 0.80 m3 at constant atmospheric pressure.
•
How much work is done by the environment on the gas
when it expands this much?
Work Done by a gas
from PV-graph
• Area under pressure-volume curve is the
work done
• W = P∆V
• W = Area under
PV graph.
The figure given below represents p-V diagram of different stages
of a thermodynamic process. Calculate the work done in each
stage and also the net work done in the complete cyclic process.
Calculate the work done on the gas
from D to E to F
1-from D to E
2-from E to F
3-from F to D
4-total work
Example 12.2
Find the numeric value of the work done
on the gas in (a) Figure 12.4a and (b)
Figure 12.4b.
Solution to Example 12.2
Quick Quiz
By visual inspection, order the PV
diagrams shown in Figures below from the
most negative work done on the system to
the most positive work done on the
system.
(a)a, b, c, d
(b) a, c, b, d
(c) d, b, c, a
(d) d, a, c, b
Quick Quiz
Objectives
Gas processes
 Gas process: describes how a gas gets from one state to another
 Isothermal: process occurs at constant temperature
 Isobaric: process occurs at constant pressure
 isovolumetric/Isometric/Isochoric: process occurs at constant
volume
 Adiabatic: process is insulated, no heat energy enters or leaves
system (Q = 0)
Isobaric Process
(constant pressure)
P
T1 T2 T
3
Isobaric Expansion
Isobaric Contraction
P = 0 (constant P)
V
Isothermal Process
(constant temperature)
P
T1 T2 T
3
Initial State of Gas
Isothermal Process
Final State of Gas
Gas “isotherms”
T = 0 (constant T)
V
Isometric Process
(constant volume)
P
T1T2 T
3
V = 0 (constant V)
V
Adiabatic process
(insulated)
P
Temperature, pressure,
and volume all change
in an adiabatic process.
T
isotherm
slope of an adiabatic curve steeper
than an isothermal curve.
adiabat
Q = 0 (no heat enters or leaves)
V
Why the slope of an adiabatic curve steeper
than an isothermal curve.?
• For an adiabatic compression, the pressure change is due to
- reduction in volume
- increase in temperature (since there's no heat lost from the
system, the temperature rises when you do work on the system);
this means an increase in average particle speed.
•
For an isothermal compression, the pressure change is due to
- reduction in volume alone.
The temperature does not increase, heat energy is removed from
the system (to keep the temperature constant). So the particles
aren't moving as fast, compared to the adiabatic change.
The change in pressure (for the same volume change) is therefore
higher for an adiabatic changes, making the graph steeper.
Problem 2
• During an isobaric process which one of the
following does not change?
a.
volume
b.
temperature
c.
internal energy
d.
pressure
Problem 2
• During an isothermal process which one of the
following does not change?
a.
volume
b.
temperature
c.
internal energy
d.
pressure
Problem 4
•
On a P-V diagram, an ____ process is
represented by a horizontal line.
a.
isobaric
b.
isothermal
c.
isovolumetric
d.
adiabatic
Problem 4
•
On a P-V diagram, an ____ process is
represented by a vertical line.
a.
isobaric
b.
isothermal
c.
isovolumetric
d.
adiabatic
Problem 3
• Area on a P-V diagram has units associated
with:
a.
energy.
b.
momentum.
c.
temperature.
d.
change in temperature.
The first law of Thermodynamic
• https://www.youtube.com/watch?v=qVAvmie
RM1E
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)
U = 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
15-1 The First Law of Thermodynamics
An amount of heat equal to 2500 J 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?
Q = +2500 J
W = -1800 J
Δ U = Q – W = +2500 – (-1800) = 4300 J
15-1 The First Law of Thermodynamics
What would be the internal energy change if
2500 J of heat is added to the system and
1800 J of work is done by the system
(i.e. as output)?
Q = +2500 J
W = +1800 J
Δ U = Q – W = +2500 – (+1800) = 700 J
First law and thermodynamic
processes
https://edpuzzle.com/media/59d282df9750b4
4afb418d45
problem1
• A system is acted on by its surroundings in
such a way that it receives 50 J of heat while
simultaneously doing 20 J of work. What is its
net change in internal energy.
a.
70 J
b.
30 J
c.
zero
d.
30 J
Problem2
• According to the first law of thermodynamics,
the sum of the heat gained by a system and the
work done on that same system is equivalent to
which of the following?
a.
entropy change
b.
internal energy change
c.
temperature change
d.
specific heat
Problem2
• In an isothermal process for an ideal gas
system (where the internal energy doesn't
change), which of the following choices best
corresponds to the value of the work done on the
system?
a.
its heat intake
b.
twice its heat intake
c.
the negative of its heat intake
d.
twice the negative of its heat intake
Problem4
In an isovolumetric process by an ideal gas,
the system's heat gain is equivalent to a change in:
a.
temperature.
b.
volume.
c.
pressure.
d.
internal energy.
Internal combustion engine
Sample problem
Problem1
A heat engine receives 6 000 J of heat from its combustion process
and loses 4 000 J through the exhaust and friction.
What is its efficiency?
( ANSWER:A)
a.
33%
b.
40%
c.
67%
d.
73%
Problem2
• If a heat engine has an efficiency of 30% and
its input energy is 6000J , what is useful work
done by it ?
a.
1 800 J
b.
2 400 J
c.
2 000 J
d.
3 000 J
Problem3
• A heat engine exhausts 3 000 J of heat while
performing 1 500 J of useful work. What is the
efficiency of the engine? (ANSWER :B)
a.
15%
b.
33%
c.
50%
d.
60%