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. Thanks for watching, guys. Subscribe to my channel for more tutorials, support me on patreon so I can keep making content, and as always feel free to email me:
