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RC
Numerade Educator

# In thermodynamics, the first law of thermodynamics is an expression of the principle of conservation of energy. The law states that the change in the internal energy of a system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings.

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### Video Transcript

welcome to our fourth example video. Looking at the first law of thermodynamics in this video, we're going to consider our first law, which is change in e thermal is equal to Q minus W. And we're going to consider it for the processes that we've looked at so far. Where we have the constant volume, or is a coric the constant pressure, or is a barrack the constant temperature or is a thermal? We're also going to consider What does it mean to have Q equal to zero? Now, looking at these situations, you'll remember that work equals zero. Here, here, we're going to see that we have work is equal to a P Delta V. Notice that since we have a change in volume, this is going to imply that we're going to have a change in temperature. So api Delta V is going to be equal to an N R. Delta T, and then over here for is a thermal process. We have work is definitely greater than zero, especially if we're going to the right. Specify that as our direction. But we have a delta t equal to zero. Okay, so let's examine what all these things mean in the first case, we have a work that is going to be zero. And when we see that work is zero, we know that the chain any change in thermal energy is going to be equal to some heat. So this is heat either added to the system it be positive or taken away from the system. It would be negative just looking at the direction here. We can see that we have a pressure decreasing, which would imply that we're losing energy from the system. And so we would have a negative que here, so Delta e thermal would be less than zero jewels. Meanwhile, looking over here for the constant pressure process, Theis a barrack process. We see that we do have a work and it's going over some space here. And so our Delta E thermal is going to be Q minus W. And because it's going to the right, W will be positive. If it had been to the left, we would have had minus a negative W. In which case we would have had Q plus W as our total thermal energy. Remember, if it's to the left, that means that his work being done on the system. Energy is being transferred into the system via mechanical work and the third situation where we have a change in temperature that is equal to zero. That's the same thing is saying that we're on. We're going to be in thermal equilibrium, which means we have a zero change in thermal energy, which would imply that we need Q equal to our work so we can actually calculate the heat transferred in this system because it will be equal to how much work is done. So if we do it positive work by the system, that means we're going to be adding this heat here, okay, and lastly, we have the situation where Q equals zero again, this is known as an idiomatic process will look at thes several times as we continue to look at all these processes with an added a Batic process. It means that any change in thermal energy is going to be due entirely to work done either by or on the system. Remember, by the system means it's positive on the system means it's negative, and that will change. Sign for the delta, the thermal If the system does work. That means it loses thermal energy because we have minus W. If we have the system having work done on it, that's a negative W and negative times a negative is a positive. When we do work on the system, there is a positive change in thermal energy for the system.

RC
University of North Carolina at Chapel Hill

#### Topics

Kinetic Theory Of Gases

The Second Law of Thermodynamics

##### Top Physics 101 Mechanics Educators    ##### Aspen F.

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