Thermodynamics - A-level Physics
Summary
TLDRThis educational video script delves into the complexities of thermodynamics, focusing on the first law and its implications for gases. It explains the concepts of heat supply (Q), work done by gas (W), and internal energy changes. The script simplifies these ideas through discussions on isothermal and adiabatic processes, using the gas laws PV=nRT and PV relationships. It further illustrates these principles with PV diagrams, explaining how they represent different thermodynamic processes such as compression, combustion, and expansion in engines. The goal is to clarify thermodynamics for viewers, making it less daunting and more understandable.
Takeaways
- 🔍 The first law of thermodynamics is introduced as \( Q = \Delta U + W \), where \( Q \) is heat supplied to a gas, \( \Delta U \) is the change in internal energy, and \( W \) is the work done by the gas.
- 🌡️ The script explains that thermodynamics principles apply to gases, focusing on the relationship between heat, work, and internal energy.
- 🔥 Heat supplied to a gas (Q) increases the kinetic energy of its particles, but if the gas expands, it does work (W), not retaining all the supplied heat.
- 🔄 The concept of isothermal processes is discussed, where the temperature remains constant, and thus the internal energy doesn't change, implying all supplied heat is lost as work.
- 📉 In an isothermal process, the product of pressure and volume (PV) remains constant, as temperature dictates the relationship between these variables.
- 🔙 The script touches on adiabatic processes, where no heat is exchanged with the surroundings, leading to a change in internal energy equal to the negative work done by the gas.
- 🔢 The adiabatic constant, which is different for various gases, is introduced, showing how it affects the relationship between pressure and volume in adiabatic changes.
- ⚖️ Work done by or on a gas is defined as \( P \Delta V \), and the script clarifies that no work is done if the volume remains constant.
- 📈 The script describes how to represent thermodynamic processes on PV diagrams, illustrating isothermal and adiabatic changes, and how work is represented by the area under the curve.
- 🚗 The concept is applied to the operation of a four-stroke engine, detailing the intake, compression, combustion, power, and exhaust strokes, and how they relate to PV diagrams.
Q & A
What is the first law of thermodynamics?
-The first law of thermodynamics, also known as the law of energy conservation, states that the change in internal energy (ΔU) of a system is equal to the heat added to the system (Q) minus the work done by the system (W), expressed as ΔU = Q - W.
What does the term 'Q' represent in the context of thermodynamics?
-In thermodynamics, 'Q' represents the heat supplied to a system. It's the energy transferred to the system due to a temperature difference.
What is meant by 'W' in thermodynamics?
-In thermodynamics, 'W' stands for work done by the system. It refers to the energy transferred by the system as it expands or contracts, typically in relation to its surroundings.
Why does a gas do work when it expands?
-A gas does work when it expands because the kinetic energy of its particles increases as they move faster due to added heat. This increased kinetic energy is converted into mechanical work as the gas pushes against its surroundings.
What is an isothermal process in thermodynamics?
-An isothermal process is one in which the temperature of the system remains constant. In this process, any heat added to the system is entirely used to do work, resulting in no change in internal energy.
How does the relationship between pressure and volume change during an isothermal process?
-During an isothermal process, the relationship between pressure and volume is inversely proportional, as described by Boyle's Law, which states PV = constant.
What is an adiabatic process in thermodynamics?
-An adiabatic process is one in which there is no heat exchange with the surroundings, meaning Q = 0. In this process, the change in internal energy is equal to the negative of the work done by the system, ΔU = -W.
What is the significance of the adiabatic constant in thermodynamics?
-The adiabatic constant, often represented by the Greek letter gamma (γ), is used in the equation relating pressure, volume, and temperature during an adiabatic process. It varies depending on the type of gas and is crucial for calculating the work done or the change in temperature in adiabatic processes.
What does it mean when work is done by a gas at constant volume?
-When work is done by a gas at constant volume, it means that the volume does not change, and therefore, no work is being done on or by the gas. This situation is represented by W = 0 in the thermodynamic equations.
How is work done related to the pressure and volume of a gas?
-The work done by a gas is directly related to the pressure and volume changes. The work done (W) is equal to the pressure (P) times the change in volume (ΔV), expressed as W = PΔV.
What is a PV loop and why is it important in thermodynamics?
-A PV loop is a graphical representation of the cyclic processes that a gas undergoes, such as in an engine. It shows the relationship between pressure and volume at different stages of the cycle, and the area under the curve represents the work done by or on the gas during these processes.
Outlines
😅 Admitting the Complexity of Thermodynamics
The speaker confesses their dislike and confusion with thermodynamics during university studies, suggesting the subject might confuse others as well. They begin by introducing the first law of thermodynamics, focusing on the concepts of heat (Q) and work (W) and their application to gases. The explanation highlights how heat added to a gas can cause the gas to do work, thereby reducing the internal energy gained from the heat. The paragraph discusses how the internal energy of the gas relates to its temperature, and mentions the difficulty of applying these concepts to real-world situations.
📊 Exploring Isothermal and Adiabatic Processes
This section explains the isothermal process, where temperature remains constant, and how that affects internal energy and work done by the gas. It dives into gas laws, specifically the ideal gas law (PV = nRT), and how pressure (P) and volume (V) interact when temperature is constant. The speaker also introduces the adiabatic process, where no heat is exchanged, leading to a change in internal energy that is equal to the negative work done by the gas. The concept of adiabatic constants and specific conditions under which work is done, like constant volume and constant pressure, is discussed in detail.
📈 Graphical Representation of Thermodynamic Processes
This paragraph covers how to visualize thermodynamic processes on a pressure-volume (PV) graph. The speaker explains isothermal compression and expansion, showing how P and V are inversely proportional and how changes in temperature affect the graph's appearance. The role of the area under the curve as a representation of work done by or on the gas is introduced. The paragraph contrasts isothermal and adiabatic changes, showing how adiabatic processes approach zero pressure more steeply. The importance of recognizing constant volume and constant pressure conditions is emphasized, along with their graphical representation.
🛠 Understanding Engines through PV Diagrams
The focus here shifts to applying thermodynamic principles to engines, specifically a four-stroke engine, which is common in cars. The speaker describes how the piston’s motion is represented on a PV diagram, starting with air intake, followed by compression, combustion, and exhaust. The description covers how these stages of the engine cycle correspond to changes in pressure and volume, and how they can be visualized graphically. The speaker emphasizes the importance of understanding work done by and on the gas during these phases, ultimately connecting the diagram to the practical workings of engines.
🚗 Four-Stroke Engine Operation and Summary
This final paragraph continues the explanation of the four-stroke engine process, detailing the interaction between pistons, air intake, and combustion within a cylinder. The speaker describes how the piston compresses the air-fuel mixture, ignites it with a spark plug, and then expands as combustion forces it back down. The process finishes with the expulsion of exhaust gases. The paragraph concludes with a summary of thermodynamics concepts and PV diagrams, encouraging viewers to leave comments or questions if they need further clarification.
Mindmap
Keywords
💡Thermodynamics
💡First Law of Thermodynamics
💡Heat (Q)
💡Work (W)
💡Internal Energy
💡Isothermal Process
💡Adiabatic Process
💡PV Diagram
💡Constant Volume
💡Constant Pressure
💡Four-Stroke Engine
Highlights
Introduction to the first law of thermodynamics with Q and W representing energies.
Explanation of Q as heat supplied to a gas and W as work done by the gas.
Clarification that when heat is supplied to a gas, it may not retain all of it due to work done.
Discussion on the conversion of leftover energy into kinetic energy or internal energy of gas molecules.
Application of the first law of thermodynamics to different situations, particularly gases.
Definition of isothermal change where the temperature and thus internal energy of the gas remain constant.
Explanation of how heat supplied to a gas during isothermal change is entirely converted to work.
Introduction to the ideal gas law PV=nRT and its simplification for isothermal processes.
Description of adiabatic processes where no heat is exchanged with the surroundings.
Explanation of internal energy change in adiabatic processes being equal to the negative work done.
Mention of the adiabatic constant and its dependency on the type of gas.
Clarification that work done by a gas is zero if the volume is constant.
Introduction to the concept of PV diagrams for visualizing gas processes.
Description of isothermal compression and expansion on a PV diagram.
Explanation of how the area under a PV graph represents the work done by or on a gas.
Discussion of adiabatic changes on a PV diagram and their characteristics.
Introduction to the concept of PV loops and their significance in understanding multiple gas processes.
Application of thermodynamics to a four-stroke engine and the explanation of its cycle.
Description of the intake, compression, combustion, expansion, and exhaust strokes in a four-stroke engine.
Conclusion summarizing the introduction to thermodynamics and PV diagrams.
Invitation for feedback and further engagement with the channel's content.
Transcripts
okay I have an admission I hate
thermodynamics it was one of those
things that always confused me in
university and it might confuse you to
select try and make sense of it all
let's start off with the first law of
thermodynamics and that is Q he calls
you you might see a delta in front of
that plus W now what are all these these
are all energies this rule is true for
gases so we need to be thinking about
these energies in terms of gases so if
you have a gas and you put heat into it
you heat a gas that is what this Q is
heat supply to gas that's fairly easy
but you know that if you supply heat to
a gas well the particles are going to
move faster and if a gas is contained
well great but if it's free to move and
expand or whatever then it is going to
exert a force and so that's what this is
this a W indicate work done by gas now
if work is done by gas then that means
that it's losing some of that energy and
so even though you're supplying maybe 10
joules of heat to a gas the gas might
then be doing eight joules of work and
so it's not keeping all 10 joules of
that heat what happened to the extra 2
joules left over well that is turned
into well kinetic energy of the
particles or the molecules that make up
the gas but we just say that this is
gained or rather change if it's positive
it's again in internal energy we know
the that is proportional that's
dependent on temperature isn't it so if
you supply heat to a gas then it will do
some work and so will not keep all of
that heat but it will keep some of that
heat some of that energy in the form of
kinetic energy internal energy of the
molecules
now that might make sense but the
difficulty is knowing how to apply that
to different situations let's think
about when a gas and the
an isothermal change of process ISO
means same thermal means wealth
temperature basically and so in this
case if the temperature is staying the
same then that means that the internal
energy of the gas is not changing at all
so that means that the heat that you are
supplying to the gas is all lost as it
were by the gas again by the work that
it does and knowing from thermal physics
that PV is equal to NRT if you don't
know that then have a look at my gas
laws video then we can say that PV is
proportional to T or in other words PV
over T is a constant so you can compare
before and after PV over T equals PV
over T but we know that temperature is
constant and so that means that we can
take it out of the equation and so PV is
equal to a constant as well so we can
say that p1 v1 equals p2 v2 I'll write
that down in a second let's go for
another one
adiabatic some people say adiabatic I
like saying adiabatic those when no heat
is lost or gained by the gas at all no
heat is supplied to the gas and no heat
is lost by the gas and so if that's the
case we know that Q is equal to zero so
if we take Q out of the equation put W
over the other side or Delta U of the
other side we know that the change in
energy internal energy is going to be
equal to minus W and that makes sense
because if a gas does work then that
means that it has to lose energy and it
has to be losing the energy from the
internal energy of its particles now for
this one we can also say that p1 v1 is
equal to p2 v2 however we have this
little thing here that we raise the
power of the volume by this is called
the adiabatic constant and you'll always
be told what that is that changes from
gas to gas for a monatomic gas let's say
argon that's equal to 5/3 I'm going to
add one thing in here as well work done
by a gas this
is equal to 0 if V is constant so if a
volume of a gas is constant then by
definition it can't do any work you
think about it work done is well force
times distance and so if a gas isn't
expanding or contracting then no work is
being done on all by the gas and of
course if work done is zero then that
means that all of the heat supplied is
being turned into the internal energy of
the gas so Q equals Delta U there's one
more thing that that is equal to P Delta
V so pressure times the change in volume
if constant pressure so those are the
four rules that you have to remember for
thermodynamics before you get started
with anything else an isothermal change
that means that the change in internal
energy is zero so we can say P vehicle's
PV adiabatic or adiabatic process no
heat supply so that means internal
energy is minus the work done if volume
is constant work done is 0 if constant
pressure work done is P Delta V what you
can do is draw a graph of P against V
and show what is happening to both
pressure and volume for a gas during a
change let's start off with our
isothermal compression well we know that
it's a constant temperature and so
therefore P and V are well they're
inversely proportional aren't they so if
T constant is inversely proportional to
V and so we get this shape graph now
whichever way the pressure and volume
are going we draw an arrow shown which
way is going so
here we go what's going on here well
volume of the gas is decreasing the
pressure is increasing so this is an
isothermal because it's happening to
constant temperature compression if the
arrow is going the other way
isothermal expansion and it would be
just exactly the same line with the
arrow going in the opposite direction
pretty much anyway now what about if
this happened at a different temperature
though if this gas was at a colder
temperature it's still a constant
temperature but at a colder temperature
then we know that the pressure and
volume would be less and so basically
the graph would look similar but it gets
closer to the origin so this would
colder temperature now we said that work
done is equal to P Delta V and so times
in pressure and volume together so what
part of this graph gives us work done on
or by the gas it's the area under the
graph so the area under the PV graph is
equal to work done
however you have to think clearly and so
I'm just doing it for the orange one
here at the heart of temperature you
just got to think is work being done or
by the gas
well it's compressing and so that means
that work is being done on the gas
that's what a PV graph looks like for an
isothermal change what about an
adiabatic change what is very similar
it's just the characteristic really of
an adiabatic change is that it gets
closer to zero pressure okay so what if
I had a PV graph and all I had was a
straight line going up so we can see
that the volume is staying the same or
what do we say we know is the case if
volume stays the same no work is being
done at all so W equals zero so we can
say that Q is equal to tell to you what
about if I had a horizontal line well
that's a constant pressure but changing
volume and just like we said before work
done is equal to P Delta V so again it
is the area under the graph now you can
see what I've done here is I've actually
got two lines joined together kind of
like vectors in a way and that's what
happens usually when we have maybe an
engine we don't just have one process
happening we have lots of processes
happening and they're all linked
together the gas undergoes work and then
it undergoes no work that the internal
energy is changing etc etc so let's have
a look at a PV loop so let's say that we
have a gas here and what we do we do
work on the gas like so let's say that
it undergoes compression could be
isothermal could be adiabatic so work is
done by the gas and then it undergoes
expansion and then work is done on the
gas
keep drawing arrows at the end you've
got a drawing in the middle and then
finally it undergoes maybe isothermal
compression could be adiabatic let's
extend this so let's just have a look at
the area under the graph for when the
volume it's increasing so that's from
here to here so this area under the
graph gives you work done by the gas
okay we can say it's system but I like
to think about it work done by the gas
don't forget if the volume is increasing
work is done by the gas but then on the
way back when the volume is decreasing
we have this area under the graph this
is the work done on the gas or on the
system and so if we have this much work
done by the gas and then this work done
on the gas then that means that we have
a certain amount of Network resultant
work done by the gas and this is kind of
what happens with an engine we're gonna
look at specifically a four-stroke
engine and that's usually what you have
in your car so what happens first of all
is that well you probably know what
happens you have a piston inside a
cylinder and that goes up and down now
what you can do is that you can suck in
air and then you can exhaust the fumes
out as well and we have an explosion as
it were happening inside of the cylinder
so what happens first is that the
cylinder comes down and it comes down
because it wants to suck in air and fuel
into the cylinder ready to be combusted
but the pressure isn't changing it's
just sucking the air in so we represent
that as just a straight line going
across like that again we can say that
the work is the area under this line
we're not too concerned with out of the
minute then what happens is that the
piston is then pushed back upwards to
compress the air and the fuel and then
halfway up we have a spark that's why we
need spark plugs and it causes
combustion to happen
and usually the spark happens about far
something like here I thought I could
draw sparks turns out I can't let me see
that sort of dogleg going on there
because the pressure is increasing very
rapidly of course it can't stay like
that can it because if the pressure is
too great and the piston is allowed to
move the piston is going to come back
down again and that's what we see but it
doesn't come down exactly the same way
because of course we know that happens
only if it's the same temperature but
it's going to be at a hotter temperature
so we're going to get further away from
the origin like that so this is
compression
this is combustion and then we have our
expansion as it explodes and that pushes
the piston down that's what drives your
car and then last but not least pistons
down here we have all the exhaust fumes
still in the cylinder we need to get rid
of those that's what happens the piston
goes up one more time and pushes just
all of the air out again that's just at
a constant pressure and so we have a
straight line going across there that's
supposed to be a straight line
horizontally as we push the gas out is
that a slightly higher pressure than
what we suck the air & fuel mixture in
with so there you go that's an
introduction to thermodynamics and PV
diagrams I hope that in my endeavor to
unconfuse myself I've hopefully
unconfused you a little bit as well if I
have and you found this helpful please
leave a like and if you think I've
missed anything you've got any questions
to put it in a comment down below don't
forget to check out the rest of my
videos on my channel for more help and
I'll see you next time
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