We normally think that the world contains matter-stuff-and energy (the ability to get the stuff to do something). And we often measure how much stuff we have by its mass. (Weight is the mass multiplied by the acceleration of gravity.) A real physicist would remind you, however, that mass and energy are different aspects of something more fundamental.
Einstein’s famous formula says that the energy content of something, E, is equivalent to its rest mass, m, multiplied by the square of the speed of light in a vacuum, c2. Because c is so large, a reaction that converts a little bit of mass can produce a lot of energy that is radiated away, as in an atomic bomb, for example.
The numbers are really wonderfully large. If you could somehow make an Einstein reactor to convert the matter in the food you eat directly to energy, just 1 gram (one-fifth of a teaspoon of water) would be enough to supply your 2000-calories-per-day diet for 30,000 years!
Laws of Thermodynamics
Suppose you don’t have an “Einstein reactor”, so you’re working in the ordinary world where any changes between rest mass and energy involve too little mass to be measured. Then, as described in the main text, energy is neither created nor destroyed, but it is changed from one form to another. This is often called the First Law of Thermodynamics, and also can be written that the change of the energy in a system is the amount of heat added to it minus the amount of work it does on its surroundings.
The first law of thermodynamics, by itself, might leave you thinking that after you burn the gasoline to move your car to drive to Grandma’s house, heating the surroundings, you could just collect the heat and the carbon dioxide and the water from your tailpipe, put them all back together again, put that gas back in your tank, and drive home. The Second Law of Thermodynamics says that you will fail; it is possible to use the heat to recombine things to make more gasoline, but you’ll never get as much energy back into the gasoline as you started with. “Disorder”, or “entropy”, increases, and the concentrated energy that is useful to us becomes spread out and no longer useful.
Physicists often discuss a zeroth law of thermodynamics, which says that if two things are in thermal equilibrium with each other (not having a net flow of heat from one to the other), they are in equilibrium with a third. This leads to a definition of temperature, and other useful things. And, there is a third law of thermodynamics which says that you can’t actually cool something to absolute zero, the point at which a perfect crystal would have zero entropy. These can be approximated as (this is often attributed to the British thinker C.P. Snow): You must play the game, but you can’t win, you could break even on a really cold day, but it never gets that cold.