 Let's talk about the first law of thermodynamics. Written out as an equation, you'll see it written this way quite often. Looking at what each of those terms mean, we can break this down a little bit more. Now, this delta E int stands for the change in internal energy. Some textbooks are going to show it a little bit different. Rather than using an E, it could be a capital U. And it may or may not have the subscript of the int. When we look at those two things, we can realize that we're talking about a change in internal energy, not the internal energy itself. So a positive value means the internal energy is increasing, while a negative value means the internal energy is decreasing. We ended up with less internal energy than we started with. As opposed to the positive, we ended up with more internal energy than what we started with. For an ideal gas, the internal energy is directly related to the temperature. And therefore, a positive change in internal energy means the temperature is increasing, and a negative change in internal energy means the temperature is decreasing. We can really only say this for an ideal gas, because when we get into solids and liquids, the internal energy involves the temperature, but also some bonding between the atoms for the phases. Q is the heat. And by heat, we mean the transfer of thermal energy. So positive values for Q mean that thermal energy is added to the system. And negative Q means thermal energy is removed from the system. Now, this is only talking about the transfer of thermal energy. We have to be careful that we're not strictly talking about a change in temperature. It's just, are we adding energy, or are we removing energy from the system? W is the work, and the work is a transfer of mechanical energy. So similarly, a positive W value means that mechanical energy is being added to the system. And a negative value for work means mechanical energy is removed from the system. For talking about an ideal gas again, positive work can be thought of as work being done on the gas by an outside force. And generally, this is a compression of the gas. A force outside of the gas is pushing in on the gas. A negative value for work can be expressed as work being done by the gas. And it's by the gas on the outside environment. And that generally happens when the gas expands. Now, putting these concepts together, we can understand the first law a little bit better. Both Q and W involve a transfer of energy into or out of the system. The net effect of transferring energy in via heat or work compared to how much energy was transported out via heat or work results in a change in internal energy of the system. If both Q and W are positive, the internal energy has to increase. If Q and W are both negative, the internal energy has to decrease. If Q and W are positive and negative, then we have to look at the balance. Was more energy added into the system? Or was more energy transferred out of the system? And that's going to tell us what happens to the overall internal energy of the system.