Lecture3_IdealGasProcesses_deliver2
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3/04/2014 1 Thermal Physics 3 PV diagram, heat and work PHS1011 Physics Dr Lincoln Turner [email protected] Rm G06, Bldg 82 9905 1726 Last time Phase changes – Not just for solid/liqu id/gas – Any physical chang e that happ ens at a particula r – … and absorb s/emits heat Intensive and Extensive variables State variables and the Equation of State Equation of State for an ideal gas Note: Ma stering Ass ignment 2 open now Closes Wednesday 9 April at 2pm Today’ s ai ms Chapter 17 of Knight: Lecture 1 of 2. 1. Unde rsta nd p ress ure-volume () diagrams 2. Rela te eq uati on of stat e to diagrams 3. Know what ‘heat’ and ‘thermal e nergy’ are 4. Unde rsta nd the rmod ynamic wo rk Ideal-Gas Processes Many important gas processes take place in a container of constant, unchanging volume. A constant-volume process is called an isochoric process. Consider the gas in a closed, rigid container. Warming the gas with a flame will raise its pressure without changing its volume. Ideal-Gas Processes Many important gas processes take place in a container of constant, unchanging volume. A constant-volume process is called an isochoric process. Consider the gas in a closed, rigid con tainer. Warming the gas with a flame will raise its pressure without changing its volume. Ideal-Gas Processes Other gas processes take place at a constant, unchanging pressure. A constant-pressure process is called an isobaric process. Consider a cylinder of gas with a tight-fitting piston of mass M that can slide up and down but seals the container so that no atoms enter or escape. In equilibrium, the gas pressure inside the cylinder is
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Isobaric ProcessPoll: Which PV path for expansion at
constant temperature?
A. B. C. D.
52%
4%
39%
5%
A B
CD
Important DefinitionsPoll: A gas is sealed in a container. By what factor
does the gas pressure change if the volume is
doubled and the temperature is tripled?
1. 2. 3. 4. 5. 6. 7. 8. 9.
1% 1% 1%0%
5%
68%
10%
0%
13%
1. 0.125
2. 0.25
3. 0.5
4. 0.667
5. 1.25
6. 1.333
7. 1.5
8. 1.667
9. 2
A gas is sealed in a container. By what factor
does the gas pressure change if the volume isdoubled and the temperature is tripled?
Poll: Imagine if this lecture theatre is filled
with oxygen (32 g/mol). What is the mass of
this oxygen?
1. 2. 3. 4. 5. 6. 7. 8. 9.
2%
12%
18%
6%
5%
11%
14%
20%
14%
1. ~0.01 kg
2. ~0.1 kg
3. ~1 kg
4. ~10 kg
5. ~100 kg
6. ~1000 kg
7. ~10,000 kg
8. ~100,000 kg
9. ~1,000,000 kg
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Problem:
• What is the volume of this room?
• Can estimate this by taking strides of about 1m
length ways.
• But then we’ve got a slope to think about as well.
Treat the room as half a rectangular box.
• Also use: R= 8.3144 J/mol K
Imagine if this lecture theatre is filled with oxygen
(32 g/mol). What is the mass of this oxygen?
Poll: Imagine if this lecture theatre is filled
with oxygen (32 g/mol). What is the mass of
this oxygen?
1. 2. 3. 4. 5. 6. 7. 8. 9.
3%4%
16%
3%
0%
11%
27%
18%18%
1. ~0.01 kg
2. ~0.1 kg
3. ~1 kg
4. ~10 kg
5. ~100 kg
6. ~1000 kg
7. ~10,000 kg
8. ~100,000 kg
9. ~1,000,000 kg
Things we need to know: volume of this room; room
temperature and pressure; gas constant
Then: pV=nRT, so n=pV/RT
1.0 10 Pa
8.3 Jmol K 290 K
So
42
If Volume ~ 1000’s of cubic m, then n~10,000’s mol.
At 32 g/mol, oxygen would then have a mass of >100,000 g.
So an answer of 100’s or even 1000’s of kg
seems about right.
Imagine if this lecture theatre is filled with oxygen
(32 g/mol). What is the mass of this oxygen?
Poll: A gas is undergoing an isochoric process in
which the temperature changes from 300 K to 1200
K. The initial pressure was 2 atm.
What is the final pressure?
1. 2. 3. 4. 5. 6. 7. 8. 9.
0%
100%
0% 0%0%0%0%0%0%
1. 2 atm
2. 4 atm
3. 6 atm
4. 8 atm
5. 10 atm
6. 12 atm7. 14 atm
8. 16 atm
9. 18 atm
A gas is undergoing an isochoric process in whichthe temperature changes from 300 K to 1200 K.
The initial pressure was 2 atm.
What is the final pressure?
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What is heat? What is heat?
T e m p e
r a t u r e
T h e r m a l E n
e r g y
E n e r g y
T r a n s f e r
S p e e d o f P a
r t i c l e s
1%
27%
48%
24%
1. Temperature2. Thermal Energy
3. Energy Transfer
4. Speed of Particles
What is heat?
1. Temperature
2. Thermal Energy
3. Energy Transfer
4. Speed of Particles
From now on, we’ll distinguish between the words
“thermal energy”, “temperature” and “heat”.
Heat is the transfer of energy due only to a difference in
temperature.
Heat has the symbol . It is a form of energy, so units are
Joules
What do those different terms mean
from the previous slide then?
1. Temperature: how hot or cold something is,
tells us which way heat will flow.
It is related to “thermal energy per molecule”
2. Thermal Energy: this is a measure of the non-
mechanical energy of a whole system.
3. Energy Transfer : this is an interaction betweena system and an environment. Heat is a type of
energy transfer, labeled Q.
4. Speed of Particles: Just an average speed.
Related to kinetic energy per particle.
Sun shine warms me up. Energy is transferred.
As work or heat?
Sun doesn’t act on me mechanically,
or electrically, or magnetically, etc.
Doesn’t seem like work.
The Sun is a lot hotter than me.
Energy moves spontaneously, due to the temperature difference.
The sunshine is heat.
No physical contact required!
Heat can be conduction or radiation. More on this later.
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Heat is energy transfer due to a temperature difference.
(Temperature is the thing that is different that makes heat flow)
You don’t do anything to make it happen. It is spontaneous.
In fact, you can’t stop it happening – only slow it down.
Heat is not stored in things.
There is no “heat in a system”
Work is not stored in things (not in thermodynamic systems)
Energy can be stored as thermal energy .Heat and work are the two ways that energy can enter or leave a system
Poll: This gas cylinder and piston are well
insulated. You push the piston into the cylinder.
What happens to the gas temperature?
1. 2. 3. 4.
1% 1%
73%
25%1. Decreases.
2. Stays the same.
3. Increases.
4. Need more info.
This gas cylinder and piston are well insulated.
You push the piston into the cylinder.
What happens to the gas temperature?
Answer: Increases
(option 3).
1. You do work on the gas.2. Mechanical energy doesn’t change. No heat flows (insulated)
3. … so the thermal energy change of system = work done
Δ , and so Δ 0.
4. Number of particles didn’t change. So Δ/ increased.
5. This means the temperature increased.
Poll: In which process would the work done be zero?
1. 2. 3. 4.
35%
13%
9%
44%1. Isothermal
2. Isochoric
3. Isotonic
4. Isobaric
In which process would the work done be zero?
1. Isothermal
2. Isochoric
3. Isotonic
4. Isobaric
Answer: Isochoric
since the volume is constant.
Recall:
