My name is James Lincoln. I'm a physics teacher here for About.com. If you work in physics, you usually choose between theoretical and applied physics. But historically physics is divided into classical and modern physics. Classical physics is usually described as Newton's laws or systems that obey Newton's laws of mechanics. In fact, mechanics is itself an entire branch of physics. Another classical field is thermodynamics, which is the way heat moves. And it concerns itself with atoms or molecules in a gas or the heat transfer from one object to another or how hot an object gets while being heated. A third classical area is waves: sound, light, and even earthquake waves. Waves also have a lot of application in quantum mechanics. Electricity and magnetism are also a study in physics. Here we have since combined them into a single area. Here we study static charge and the properties of magnetic materials, focusing on the fields of forces around the objects as the charges are pushed or as the iron or whatever other magnetic material is attracted or repelled. Astronomy is sometimes considered its own area of physics but it really is its own subset of all the other laws of physics. The way it works is we use Newton's laws to describe the motion of planets and galaxies and we use quantum mechanics to describe the light that they emit. So, really, astronomy is a subset of all the other physics working together in one place. In modern physics, we describe new additions to physics, and by new I mean after 1900 or 1920. Here we're mostly describing atomic physics or molecular physics, things that don't really match with the classical models. Also included here are Einstein's ideas about special relativity and general relativity. There's also solid state physics, which concerns itself with the way particles move inside crystals or other solids and that is mostly an engineering perspective. But it is sold state physics, so it is a branch of physics. I'm James Lincoln for About.com. Thanks for watching.