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If you have managed to follow along this far, then you are ready for the three laws of thermodynamics:

1. Energy can change from one form to another, but it can never be created or destroyed.

2. In all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will be less than that of the initial state.

3. As the thermodynamic temperature of a system approaches absolute zero, its entropy approaches zero.

The British scientist and author C. P. Snow came up with a great way of remembering the three laws:

1. You cannot win (you cannot get something for nothing, because matter and energy are conserved).

2. You cannot break even (you cannot return to the same energy state, because there is always an increase in disorder).

3. You cannot get out of the game (because absolute zero is unattainable).

Moving swiftly on.

☞ THE LAWS OF CONSERVATION OF ENERGY AND MASS

The most common of these laws states that energy in a closed system cannot be created or destroyed (it’s similar to the first law of thermodynamics), and nor can mass. At a more advanced level, similar laws apply to electric charge, linear momentum, and angular momentum, but most people never get that far.

☞ NEWTON’S THREE LAWS OF MOTION

1. A body remains at rest or moves with constant velocity in a straight line unless acted upon by a force.

2. The acceleration (a) of a body is proportional to the force (f) causing it: f = ma, where m is the mass of the body in question.

3. The action of a force always produces a reaction in the body, which is of equal magnitude but opposite in direction to the action.

Newton also came up with a law of gravity, which states that the force between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The universal gravitational constant that makes this equation work is called G, and its value is 6.673 x 10-11 newton m2 per kg2.

However, Einstein’s general theory of relativity describes gravity more accurately.

☞ EINSTEIN’S THEORIES OF RELATIVITY

Before reviewing Einstein’s general theory of relativity, take a look at his special theory of relativity. Before Einstein—that is, until the start of the 20th century—it was believed that the speed of light relative to an observer could be calculated in the same way as the relative speed of any other two objects (such as two cars driving at different speeds). Einstein’s theory is based on the assumption that the speed of light in a vacuum is a constant (186,000 miles—or 2.998 x 108 m—per second), regardless if the observer is moving or at what speed. Furthermore, he suggested that as bodies increase in speed, they increase in mass and decrease in length (relative to the observer)—although this effect became noticeable only as objects neared the speed of light.

Relative to each observer, time moves at a slower rate. All this led him to the conclusion that mass and energy are two different aspects of the same thing, which led to the famous equation

E = mc2,

where E is energy, m is mass, and c is the velocity of light.

So, back to gravity. The special theory of relativity concerned motion in which there was no acceleration—that is, a constant speed. The general theory extended this to consider accelerated motion. According to this, gravity is a property of space and time that is “curved” by the presence of a mass. Einstein posited that the motion of the stars and planets was controlled by this curvature of space in the vicinity of matter, and that light was also bent by the gravitational field of a massive body. Subsequent experiments have shown him to be correct.

☞ ELECTRIC CURRENT

There are also a handful of laws to do with electricity. Here’s one of the more familiar:

Ohm’s law states that the current (I) flowing through an element in a circuit is directly proportional to the voltage drop or potential difference (V) across it: V = IR,