The First Law of Thermodynamics: Internal Energy, Heat, and Work

Summary

Professor Dave explains the first law of thermodynamics, detailing its relation to internal energy, work, and heat. He outlines processes like isovolumetric, isothermal, and adiabatic, emphasizing their significance in thermodynamics. The tutorial also covers the importance of understanding the signs of heat and work in calculations, using an example to illustrate energy transfer as heat and work.

Takeaways

• 🔄 The first law of thermodynamics, also known as the law of energy conservation, describes the relationship between internal energy, work, and heat.
• ⚖️ The law is mathematically expressed as ΔU = Q - W, where ΔU is the change in internal energy, Q is the heat transferred, and W is the work done.
• 🔊 In an isovolumetric process, there is no change in volume, meaning no pressure-volume work is done, so ΔU = Q.
• 🌡️ An isothermal process is characterized by no change in temperature, implying no change in internal energy (ΔU = 0), and thus Q = W.
• 🌀 An adiabatic process involves no heat transfer (Q = 0), so any change in internal energy is due to work done on or by the system (ΔU = -W).
• 🏔️ In Earth's atmosphere, adiabatic processes can be observed as air masses move due to pressure differences without heat exchange.
• 🔒 An isolated system, where neither heat nor work is exchanged, will have no change in internal energy (ΔU = Q = W = 0).
• ✅ The signs of Q and W are crucial: Q is positive when heat is absorbed and negative when released; W is positive when work is done by the system and negative when work is done on the system.
• 📐 To solve for heat transfer in a system, rearrange the first law equation to Q = ΔU + W and substitute the appropriate values for ΔU and W.
• 📚 Understanding these principles and calculations is fundamental for further studies and applications in thermodynamics.

Q & A

What is the first law of thermodynamics?

The first law of thermodynamics, also known as the law of energy conservation, outlines the relationship between internal energy, work, and heat. It can be expressed by the equation ΔU = Q - W, where ΔU is the change in internal energy, Q is the heat transferred, and W is the work done.

What does the equation ΔU = Q - W represent?

The equation ΔU = Q - W represents that the change in internal energy (ΔU) of a system is equal to the energy transferred to or from the system as heat (Q) minus the energy transferred as work (W), with all quantities measured in joules.

What is an isovolumetric process?

An isovolumetric process is a process where there is no change in volume for the system, meaning no pressure-volume work can be done on or by the system. In such a case, ΔU equals Q, and any change in internal energy must be the result of heat transfer.

What is an example of an isovolumetric process?

An example of an isovolumetric process is a bomb calorimeter, where a combustion reaction produces a change in temperature, but the rigid walls result in no change in volume.

What is an isothermal process?

An isothermal process is a process where there is no change in the temperature of the system, implying no change in internal energy (ΔU = 0), and thus any heat transferred into the system is used to do work (Q = W).

What is an adiabatic process?

An adiabatic process is a process where there is no heat transfer (Q = 0), and the change in internal energy (ΔU) is solely due to work done on or by the system (W).

How is the sign of Q determined?

The sign of Q (heat) is positive when heat is absorbed by the system and negative when heat is lost by the system.

How is the sign of W determined?

The sign of W (work) is positive when work is done by the system (like an expanding gas) and negative when work is done on the system (like gas compression).

What is the significance of the equation ΔU = Q - W in thermodynamics calculations?

The equation ΔU = Q - W is significant in thermodynamics calculations as it allows us to determine the changes in internal energy, the amount of heat transferred, and the work done in various processes, which is crucial for understanding energy transformations.

What does it mean if both Q and W are zero in a thermodynamic process?

If both Q (heat transfer) and W (work done) are zero in a thermodynamic process, it indicates an isolated system where there is no exchange of energy with the surroundings, and thus no change in internal energy can occur.

Outlines

00:00

🔍 The First Law of Thermodynamics

Professor Dave begins by discussing the first law of thermodynamics, also known as the law of energy conservation. This law describes the relationship between internal energy, work, and heat, represented by the letter Q. The law is mathematically expressed as ΔU = Q - W, indicating that the change in internal energy (ΔU) is equal to the heat transferred (Q) minus the work done (W), all measured in joules. The video explains different types of processes: isovolumetric (no volume change, work is zero), isothermal (no temperature change, Q = W), adiabatic (no heat transfer, ΔU = -W), and isolated systems (no heat or work transfer, ΔU = 0). The video also emphasizes the importance of understanding the signs of Q and W for correct calculations, providing an example where 100 joules of compression work results in an increase of 74 joules in internal energy, with 26 joules lost as heat. The summary concludes with a call to action for viewers to subscribe for more content.

05:13

📧 Contact and Support Information

The second paragraph provides viewers with ways to support the content creator and stay connected. It encourages viewers to subscribe to the channel for more tutorials and to support the creator on Patreon to help fund the production of more content. Additionally, the creator invites viewers to reach out via email for any questions or further discussions.

Keywords

💡First Law of Thermodynamics

The First Law of Thermodynamics, also known as the Law of Energy Conservation, is a fundamental principle in physics that states the total energy in a closed system remains constant. It is central to the video's theme as it sets the stage for understanding the relationship between internal energy, work, and heat. The law is mathematically represented as ΔU = Q - W, where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done by the system. The video uses this law to explain various thermodynamic processes.

💡Internal Energy

Internal energy refers to the total energy contained within a system. It includes all the kinetic and potential energies of the particles within the system. In the context of the video, internal energy is a key variable in the First Law of Thermodynamics, as changes in internal energy (ΔU) are directly related to the heat added to or removed from a system and the work done by or on the system.

💡Work

In thermodynamics, work (W) is the energy transferred to or from a system by the application of force over a distance. It is a form of energy transfer and is crucial in the First Law of Thermodynamics. The video explains that if work is done by the system, it is positive, and if work is done on the system, it is negative. An example given is a bomb calorimeter where no work is done due to the rigid walls, hence W = 0.

💡Heat

Heat (Q) is the energy transferred between systems due to a temperature difference. In the video, heat is described as a form of energy transfer that affects the internal energy of a system. When heat is absorbed by a system, Q is positive, and when heat is lost, Q is negative. The video uses the example of a bomb calorimeter to illustrate heat transfer as a change in temperature due to a combustion reaction.

💡Isovolumetric Process

An isovolumetric process is one in which the volume of the system does not change, meaning no work is done in terms of pressure-volume work. This concept is important in the video as it illustrates a scenario where ΔU = Q, indicating that any change in internal energy is solely due to heat transfer. The bomb calorimeter is used as an example where the rigid walls prevent volume change, thus making the process isovolumetric.

💡Isothermal Process

An isothermal process is one in which the temperature of the system remains constant. According to the video, during an isothermal process, ΔU is zero because there is no change in internal energy, and Q = W, meaning all the heat transferred into the system is used to do work. The video suggests that an ideal car engine would operate isothermally, converting all heat energy into work.

💡Adiabatic Process

An adiabatic process is one in which there is no heat transfer into or out of the system (Q = 0). The video explains that in such a process, the change in internal energy is solely due to work done on or by the system. An example from the video is the movement of air masses in Earth's atmosphere due to pressure differences, which can be considered adiabatic.

💡Isolated System

An isolated system is one that does not exchange heat or work with its surroundings. In the video, it is mentioned that in an isolated system, both Q and W are zero, and therefore, there can be no change in internal energy. This concept is used to illustrate a system that is completely closed off from any external energy interactions.

💡Sign Convention

The sign convention in thermodynamics is a set of rules for assigning positive or negative values to quantities like heat and work based on the direction of energy transfer. The video emphasizes the importance of this convention for correctly applying the First Law of Thermodynamics. For instance, heat absorbed by the system is positive (Q > 0), while heat lost is negative (Q < 0), and work done by the system is negative (W > 0), while work done on the system is positive (W < 0).

💡Energy Conservation

Energy conservation is a fundamental principle stating that energy cannot be created or destroyed, only transformed from one form to another. This principle is encapsulated in the First Law of Thermodynamics discussed in the video. It is illustrated through the equation ΔU = Q - W, showing that energy is conserved as it changes forms within a closed system.

Highlights

The first law of thermodynamics is also known as the law of energy conservation.

It outlines the relationship between internal energy, work, and heat.

The law is represented by the equation ΔU = Q - W.

Change in internal energy equals heat transferred minus work done.

All quantities are measured in joules.

An isovolumetric process occurs with no change in volume, making work zero.

A bomb calorimeter is an example of an isovolumetric process.

In an isothermal process, there is no change in temperature, making ΔU zero and Q equal to W.

An ideal car engine operates as an isothermal process.

An adiabatic process has no heat transfer, making Q zero and ΔU equal to -W.

Adiabatic processes can be observed in Earth's atmosphere with air masses changing position.

In an isolated system, both Q and W are zero, resulting in no change in internal energy.

The signs of Q and W must be defined for proper calculations: Q is positive when absorbed, negative when lost; W is positive when done by the system, negative when done on the system.

Example calculation: If 100 J of work is done on a system and ΔU increases by 74 J, 26 J is transferred as heat.

Heat dissipation is a result of work done on a system not entirely converting to internal energy.

Thermodynamics calculations frequently use the first law of thermodynamics equation.

Correct use of signs for Q and W is crucial for accurate thermodynamics calculations.

Transcripts

Professor Dave here, let's discuss the

first law of thermodynamics.

The first law of thermodynamics, popularly

known as the law of energy conservation,

when examined more rigorously actually

outlines the relationship between

internal energy, work, and heat, which from

now on will be represented by the letter Q.

The law can be stated as an equation

where Delta U equals Q minus W.

This means that the change in the internal

energy of a system will be equal to the

energy transferred to or from the system

as heat minus the energy transferred to

or from the system as work, and all of

these quantities will be measured in

joules. Because of this law, we can

outline a few different types of

processes that can occur. If a process

occurs where there is no change in

volume for the system, that means that no

pressure-volume work can be done on or

by the system, so work is zero in such a

case. Delta U equals Q and any change in

internal energy must be the result of

heat transfer in or out. This will be

called an isovolumetric process, meaning

no change in volume. An example would be

a bomb calorimeter where a combustion

reaction produces a change in

temperature but the rigid walls results

in no change in volume. If there is no

change in the temperature of the system

there cannot have been any change in the

internal energy of the system, since

these two values are proportional. Delta

U will be zero which makes Q equal to W.

This means that any heat transferred

into the system is used by the system to

do work rather than increasing the

internal energy of a system. This is

called an isothermal process, meaning no

change in temperature. An ideal version

of a car engine would be an example of

this as the pistons ought to convert all

of the heat energy from the combustion

reaction directly into expansion

work that moves the car. If there is no

heat transferred, Q will be 0 and Delta U

will equal negative W. This means that

the internal energy of a system changes

as a result of doing work on its

surroundings or the surroundings doing

work upon the system. Such a process will

be called an adiabatic process, meaning

no heat transfer. We can see this in

certain processes in Earth's atmosphere

as masses of air change position due to

pressure differences. And if Q and W are

both zero, meaning there is no heat

transfer and no work done, there can be

no change in internal energy and this

must be an isolated system. Hopefully

these scenarios make some intuitive

sense because we will frequently use

this equation to do calculations. We will

also need to define the signs of these

quantities in order to use this equation

properly so let's note that when heat is

absorbed by the system, Q will be

positive. If heat is lost by the system, Q

will be negative. If work is done by the

system, like an expanding gas, W will be

positive. If work is done on the system

by the surroundings, like gas compression,

W will be negative. If there is no

transfer of heat or no work done these

values can also equal 0 as we have

previously discussed. When doing

calculations make sure that you use the

correct signs for these values or the

math will be incorrect. For example, if

100 joules of compression work is done

on a system and as a result the internal

energy of the system increases by 74

joules, how much of the energy is

transferred as heat and in which

direction? Let's take our equation and

rearrange to solve for Q, which will be

Delta U plus W, then we can plug in

positive 74 joules for Delta U, since

internal energy increases, and negative

100 joules for work since work is being

done on the system, and we should get

negative 26 joules for heat. This means

that as 100 joules of work is applied to

the system,

only 74 go towards increasing the

internal energy of the system while 26

joules are lost as heat dissipates out

of the system. These kinds of

calculations will happen a lot in

thermodynamics so let's check comprehension.

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