The Laws of Conversation

Whoops, lo siento mucho - I mean conservation

In physics there are many conservation laws. But what does that actually mean?
It is best illustrated by an example, and the one most people feel comfortable with is The Conservation of Energy.

This "Conservation of Energy" means that energy can neither be created nor destroyed, just converted from one form to another. It is an expression of the first Law of Thermodynamics.

In any physical process, the total amount of energy IN A CLOSED SYSTEM is constant and never changes. Closed systems mean that in the action studied, no energy is let in or out through some predetermined boundaries.

For some "closed systems" it is easy to understand this concept. Take for example, making a cup of tea. Into a closed room, take a recently boiled kettle, a cup, a teaspoon of sugar and a jug of milk. From outside the room, via robotic arms, you put some sugar and milk into the cup, with a teabag. Then you pour the near-boiling water into the cup and stir it.

Tea up! What has just happened? Well, the heat (energy) from the water has heated the cup, milk and tea. The hot water brings the flavor from the tea, dissolves the sugar, and the temperature of the cup of tea is now less than the initial water.

If a calculation is made (it's pretty simple) of all the energy before pouring, and after the tea has been brewed, the temperature of the milk has gone up, and the temperature of the water that made the tea, gone down, a similar measurement is made, there will have been no change. If the amount (mass) of each is taken into account (not to mention the energy released by the melted sugar), the amount of energy available before, is the same (exactly) as after.

If you think this was complicated, Consider the energy changes if the tea is made on a one meter table, then the full cup dropped to the floor to shatter. There are at least eight energy changes. Can you name them? In order?

Not only energy is conserved, either.

Momentum is mass x velocity, that is conserved also.
Electric charge is conserved.
Angular momentum ( r x p, when p is momentum, and r is the distance from the rotation point. However, the "x" is not a multiplication sign, but the "cross product" of two vectors)is also conserved.
Color charge is also conserved.

..and the somewhat esoterically sounding, "weak isospin" is also conserved.

There are other quantities conserved, but only under certain conditions.