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# More on internal energy

Getting more intuition of internal energy, heat, and work. Examples of using the first law to calculate work.  Created by Sal Khan.

## Want to join the conversation?

• What is the exact difference between enthalpy and heat? If there is a difference..
• Intuitively, enthalpy can be said to be the internal energy of a system as well as the energy it takes that system to establish itself in differentiation or displacement from its environment. For example, when considering the enthalpy of a puddle, it is the amount of internal energy in the puddle, as well as the amount of energy required for that puddle to establish its pressure and volume in relation to the air above it and the earth around and beneath it.
• I really get confused what is meant by work done by system and work done to the system ? i wanna a clear example illustrating that
• A diagram will help illustrate each version of the equation.

Imagine a container drawn as a box like in the video.
The container represents the system.
The boundaries of the container represent the boundaries of the system.
Write "U" inside the container. .This is the system's internal energy.
Draw an arrow starting from outside the system pointing toward the the system and crossing the system boundary. Label it "Q".
This is the heat added to the system.
Draw an arrow starting from inside the system pointing out and crossing the system boundary and label it as "W".
This is the work done by the system across the boundary to external systems.
So the system starts with an initial internal energy U.
Heat Q is added to the system, thus increasing the internal energy U.
Work W is "taken" from the system or done by the system, thus decreasing the internal energy U.
The change in U is just:
a) the heat Q added to (crossing the boundaries of) the system; and
b) the work W done by ("taken from" and crossing the boundaries of) the system.

Change the direction of the arrow representing Q, and now heat is "taken from" the system.
Change the direction of the arrow representing W, and now work is done on the system.

In short, always draw a picture.
A picture really is worth a thousand words.
• Im kind of stuck but will you answer me this question....?
What will be the internal energy of hot tea in a thermos flask after it is vigorously shaked?
what i think is:
since, Del U= q-w
or, Del U= Q - P(Del V)
now, since there is no change in volume so workdone will be zero.
now whats left is, U = Q
and thermos flask is nearer to an adiabatic(isolated) system so Q will also be zero because no heat can flow in or out.therefore,
Del U= 0
which means there is no change in internal energy that is remains constant.
Is this correct?
• Assume the temperature of the tea was T1 before you shake it. The internal energy before you shake it will depend (mostly) on the mass of the tea and the temperature. When you shake it you inject a lot more kinetic energy into the tea, but initially, that kinetic energy is not molecular level, random, kinetic energy. It is represented by complex and chaotic and turbulent flow patterns in the tea that will subside when you set the flask down. Until it subsides, it is not part of what is called the internal energy. Until it subsides, it is kinetic energy of macroscopic motion. Once it subsides, it will have become randomized molecular motion and will add to the internal energy. This will be manifested in an increase in the temperature to T2 > T1. The longer and more vigorously you shake it, the greater the final value of T2 and the greater the increase in the internal energy.
• my text book says that there is no intermolecular force between gas molecules in an ideal gas and therefore the potential energy of the molecules is 0 . why is that ? whats causes a molecule to have potential energy ?
• Well, potential energy comes from interaction with a charge, be it mass (gravity), electric charge (electric potential), weak or strong nuclear forces... by reducing intermolecular interaction to 0, and recognizing that the mass of a gas molecule is so tiny as to render gravity negligble (by mgh), the molecules have no potential energy.
• Are heat and work the only two ways to add/subtract energy of a system? Can't there be any more?
• nice question
doing work on a system or exchanging heat. yes, they seem o me to be the only ways,
looking at the second law of thermodynamics i think confirms your idea
• If your question involves Cal, do you need to convert that to joules first?
• what do you think? How do we determine what units we need to use in a particular problem?
• Is heat at molecular level work?
• Hello Ayudh,

You are very close. Heat is a form of energy. By doing work on an object you can increase its temperature and consequently its energy.

For example, rubbing your hands together would increase the temperature of your hands. Your muscles are doing the work to move your hands against friction causing them to heat up.

On a related note, know that heat can be converted to other forms of energy. For example, the heat from burning cola can be converted into electricity...

Regards,

APD
• why is it that work done by the system is negative in a chemistry text book and positive in a physics textbook?
• It's probably because the aforementioned examples are different kinds of work. I'm not sure, really.
(1 vote)
• At Sal said that change in interal energy=Q+W is the definition for internal energy can you give me a more precise definition for it?
• Internal energy is all the energy contained in an object, including both kinetic energy and potential energy.
• I am still confused with the difference between U = Q + W, and U = Q - W.
Also what does Q and W mean in both equation?
Can anybody explain it in a easier way?