13. How To Measure Disorder   Previous PageNext Page

If we let a ball loose on a slope, it will roll spontaneously downhill (right).

If we give one object a positive charge, it will be attracted spontaneously toward a second object with a negative charge.

If we bring the north pole of a bar magnet near a compass needle, the needle will rotate to point its south pole toward the magnet.

All three of these spontaneous processes are in the direction of lower energy - lower gravitational energy for the ball on a slope, lower electrostatic energy for the two charged objects, and lower magnetic energy for the compass and magnet.

Common sense seems to tell us that spontaneous processes are those that lead to a decrease in some form of energy. We would be surprised indeed to see boulders roll up a mountainside by themselves. There is a duck hunter's joke about a hardy breed of bird that always flies past the blind upside down, so that when they are hit, they fall up. We find this ridiculous because common sense tells us that things always happen spontaneously in the direction of lower, not higher, energy. But in predicting chemical reactions, common sense often is wrong.


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