 Now we introduce the concept of power. So power is the rate of energy transfer. In calculus notation, this is the derivative of the energy with respect to time. Algebra-based, we could look at it as the average power is the change in energy in the change in time. So over some time span, how much energy was transferred or exactly how much energy is being transferred as a rate of time? If I come back to the concept of work that I've been studying here in general physics, what I see is that work is a transfer of energy. So then the rate of work done is the power. So I could think of it as the power is the rate that work is done. Let's take a look at the units. Start with our basic equation here. For power is the work per time. Recognize that work has units of joules and time has units of seconds. So power must have units of joules per second. I can break that down a little bit more because I remember that a joule is a Newton meter. So a Newton meter per second is also a unit for power. And if I break that Newton down even more, that Newton meter gives us a kilogram meter squared per second squared, the unit for energy, divided by a second. And simplifying this fraction gives us that the unit for power is a kilogram meter squared per second cubed. And that's in our fundamental units there. So what we do with power is we actually define a new unit. And that new unit is called a watt. And again, watt was a scientist who did a lot of early work on power. And we simplify that watt as a capital W. And so a watt is equal to a kilogram meter squared per second cubed, which is also equal to a Newton meter per second or a joule per second. Look at a quick example of this. If I have 25 joules of work and it's done in 10 seconds, the power associated with that work can be calculated. And I plug in my 25 joules in for work and my 10 seconds in for time. And that 25 joules in 10 seconds is the same as a rate of 2.5 joules per second. Or I could write that as 2.5 watts. So that's our introduction to power.