 Now, we can look at the ideal gas law. Specifically, we're going to look at the equations and the variables involved in this equation. See in the textbook, you'll see PV equals nRT. This is the way we express it in physics. Taking each of those values individually, we're going to focus first on R. R is called the universal gas constant. In physics, we use a value of 8.314 joules per mole Kelvin. We call this a universal gas constant because no matter which ideal gas we have or exactly which process we take it through, this value of 8.314 is always used, no matter whether we're talking about hydrogen or helium, something that happens at constant pressure or constant volume. No matter what the starting conditions are, this value for R is always the same. Now, it's always the same 8.314 if I'm using the units of joules per mole Kelvin. In some books, particularly in chemistry books, you might see this written out as a slightly different number because they're using slightly different units. They may have calories per mole Kelvin or something similar to that, in which case you might have a different number in there. But as long as you're using those units, you would use their value. Since in physics, we're using this joule per mole Kelvin, it also helps us to understand the other units we're going to be using for the other variables. So for example, pressure. P in this equation stands for pressure. And if we break that down into its physics concepts, pressure is really a force spread out over a particular area. In physics, force is described as Newtons, an area is meters squared. And so a Newton per meter squared gives us a new unit called Pascal's. And that's described as just a PA. Now, in a practical problem, you may be given pressures in a different unit called atmospheres. And there's a conversion factor between atmospheres and Pascal's, which is 1.013 times 10 to the fifth. Then we get to volume. V in the equation stands for volume. And in physics, volume has a standard unit of meters cubed. Again, in real-word problems, you may be given volumes in units of liters. And a liter can be converted into meters cubed by 1 times 10 to the minus 3. Then we get to n. And n in our equation is the number of moles of gas. Now, mole is kind of like a dozen. It's a set number. A dozen is 12. A mole happens to be this very large number, 6.02 times 10 to the 23rd particles. And this particular number is sometimes called Avogadro's number. That leaves us with T, which stands for the temperature in this equation. And this must be in units of Kelvin. Some of our thermodynamics equations, we can use Celsius or Kelvin. This one must be in Kelvin. Part of the reason that must be in Kelvin is to come back and really look at what we're talking about here. The nRT, the moles times a joule per mole Kelvin times a Kelvin, gives us a unit of joules. So this whole nRT side of the equation is related to something about the energy. And T in Kelvin is the only temperature scale where the temperature directly relates to the amount of energy involved. Now similarly, the p and the v, if I multiply those together, the Pascal times the meters cubed. Well remember, Pascal was a Newton per meter squared. So this is really a Newton meter, which is also equivalent to joules. So both sides of these equations express something about the amount of energy involved in the gas. Taking this all together, we can read this ideal gas law not just as pv equals nRT, but realizing that the pressure times the volume of the gas equals the number of moles times the universal gas constant times the temperature.