Thermodynamics - A-level Physics

Summary

This 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

00:00

😅 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.

05:01

📊 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.

10:02

📈 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.

Keywords

💡Thermodynamics

Thermodynamics is the study of the relationships between heat, work, and energy. In the video, thermodynamics is used to explain the behavior of gases under various conditions. The script delves into the first law of thermodynamics, which is central to understanding how energy is transferred and converted within a system, such as a gas.

💡First Law of Thermodynamics

The First Law of Thermodynamics, also known as the Law of Energy Conservation, states that energy cannot be created or destroyed, only converted from one form to another. In the script, this law is introduced with the equation \( \Delta U = Q - W \), where \( \Delta U \) is the change in internal energy, \( Q \) is the heat added to the system, and \( W \) is the work done by the system. This law is fundamental to understanding the energy transformations in gases.

💡Heat (Q)

Heat, denoted by \( Q \), refers to the energy transferred between a system and its surroundings due to a temperature difference. In the video, heat is discussed as the energy supplied to a gas, which can cause the gas particles to move faster and potentially do work, thus affecting the internal energy of the gas.

💡Work (W)

Work, represented by \( W \), is the energy transferred by a force acting through a distance. In the context of the video, work is done by a gas when it expands, which results in a loss of energy from the gas. The script explains that if a gas does work, it loses some of the energy supplied to it in the form of heat.

💡Internal Energy

Internal energy is the total energy contained within a system, which includes the kinetic and potential energies of its particles. The script mentions that when heat is supplied to a gas, some of this energy is retained as internal energy, which is often associated with the temperature of the gas.

💡Isothermal Process

An isothermal process is one in which the temperature of the system remains constant. The script explains that during an isothermal change, the internal energy of the gas does not change, and all the heat supplied to the gas is converted into work done by the gas, as described by the equation \( PV = \text{constant} \).

💡Adiabatic Process

An adiabatic process is one in which no heat is exchanged between the system and its surroundings. The video script describes this as a scenario where the change in internal energy is equal to the negative of the work done by the gas, since no heat is added or removed (\( Q = 0 \) and \( \Delta U = -W \)).

💡PV Diagram

A PV (Pressure-Volume) diagram is a graphical representation of the relationship between the pressure and volume of a gas during various thermodynamic processes. The script uses PV diagrams to illustrate isothermal and adiabatic processes, showing how the pressure and volume of a gas change with work done and heat transfer.

💡Constant Volume

In the context of the video, constant volume refers to a scenario where the volume of the gas does not change, implying that no work is done by or on the gas. This is exemplified in the script where it is stated that if the volume is constant, then by definition, no work is being done.

💡Constant Pressure

Constant pressure is another condition discussed in the script, where the pressure of the gas remains unchanged during a process. This is illustrated by a horizontal line in a PV diagram, indicating that the work done is equal to the product of the constant pressure and the change in volume (\( W = P \Delta V \)).

💡Four-Stroke Engine

A four-stroke engine, such as those found in cars, is briefly mentioned in the script. It operates through a cycle of four distinct strokes: intake, compression, power, and exhaust. The video script uses the four-stroke engine as an example to demonstrate how PV diagrams can represent the complex series of thermodynamic processes that occur within an 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