The Physics Philes, lesson 131: Popcorn Physics
After what seems like a million year hiatus, I’m back with another edition of The Physics Philes! Time for your physics fix.
If I remember correctly, we have finished our discussion of the thermal properties of matter. Now we’re going to take that knowledge – everything we’ve learned about temperature, pressure, and heat – and combine it with everything we know about energy transfer through mechanical work so we can understand one of the most fundamental and useful laws of physics.
Allow me to introduce the first law of thermodynamics!
You’ve probably heard of the first law of thermodynamics. It’s kind of a big deal. The first law of thermodynamics is really just an extension of the principle of conservation of energy; it says that, in an isolated system, the total energy remains constant.
OK, that’s great and all, but what does it mean. Be patient. We’ll get to it. But first, we need to know some preliminaries, like what we mean when we say a system. Because none of this will make any sense if we don’t know what a system is.
A thermodynamic system is any collection of objects that is convenient to regard as a unit and that has the potential to exchange energy with its surroundings.
That’s not very helpful, though, is it? Think about popping popcorn. Whether you’re popping it in the microwave or on the stove, you’re adding heat to the kernels. As the popcorn pops and expands, it does work. It exerts a force on bag or the lid of the pot on the stove top. The state of the popcorn changes in this process because the volume, temperature, and pressure of the popcorn changes. In this example, the popcorn kernels are a system, and the process they undergo is called a thermodynamic process.
The first law of thermodynamics is a relationship between heat and work. We describe thermodynamic processes in terms of the quantity of heat Q added to the system and the work W done by the system. These quantities can be negative, positive, or zero. A positive heat quantity represents an energy flow into a system. Negative Q is energy flow out of the system.
So far, so good. But here is where it gets a little tricky. In mechanics, when we talk about work we talk about work done by forces acting on a body. That means that work is positive when something moves. However, our convention is different when we talk about thermodynamic systems. A positive work value represents the work done by the system against its surroundings, which corresponds to energy leaving the system. A negative work value is work done on the system by its surroundings, which means that work is entering the system. Don’t worry if this is a little confusing right now. As we move deeper into the material, it will start to make more sense. I promise.
That’s a good start for now. Next time we’ll dig deeper into the first law by looking at a situation in which the volume of a system changes.
Featured image credit: Rosana Prada via Flickr