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Heat Work And First Law - Example 2

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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Christina K.

Rutgers, The State University of New Jersey

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Video Transcript

welcome to our second example video. Looking at the first law of thermodynamics in this video, we're going to consider the same system we had in the last video. But we're then going to ask ourselves, what was the change in the thermal energy of the system? Remember Delta E Thermal? He's going to be equal to queue minus W. If we assume that there is zero Q here, a condition that will call Adia Batic in the future, then we'd be able to calculate that the change in thermal energy would be equal to either work one or work to, depending on the route that we take, whether we take Route one or route to. Now. If we were to say that we had some added energy to the system during this same time that we added 100 jewels or 100 calories to the system at this point, then we would say Delta E Thermo is going to be equal to 100 calories, minus in the case of work to we could put work to here. Now the question does arise. What if we went the opposite direction with our path here? What if instead of going from our first volume tow our final volume. We went from our final volume back to our first. Once we switched the direction of our arrows Here, what occurs in this case? Well, remember that the integral were taking here is the integral from the initial to the final of P Devi. So when we do either of these, what we're going to find here is that the initial volume will be greater than the final volume. This means we're going to end up with a negative value here. So when we have a arrow to the left going from right to left from an in ah, high volume to a low volume, that means we're going to end up with a negative work. This means this is work done by the system on the gas. In which case, when we went to calculate Delta E thermal, we would end up with Q minus a negative w one, for example, if we went along Path one, but in the opposite direction. So it's important that you keep track not just of what the shape of the path is, but what the direction of the path is