Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Charging by Induction

A charged object may induce a charge in a nearby neutral object without touching it. For example, if a positively charged object is brought near a neutral object, the electrons in the neutral object are attracted to the positive object. Some of these electrons flow to the side of the neutral object that is nearest to the positive object. This side of the neutral object accumulates electrons and becomes negatively charged. Because electrons leave the far side of the neutral object while its protons remain stationary, that side becomes positively charged.

Since the negatively charged side of the neutral object is closest to the positive object, the attraction between this side and the positive object is greater than the repulsion between the positively charged side and the positive object. The net effect is an attraction between the objects. Similarly, when a negatively charged object is brought near a neutral object, the negative object induces a positive charge on the near side of the neutral object and a negative charge on the far side. As before, the net effect is an attraction between the objects.

The induced charges described above are not permanent. As soon as the charged object is taken away, the electrons on the other object redistribute themselves evenly over it, so that it again becomes neutral.

An object can also be charged permanently by induction. If a negatively charged object, A, is brought near a neutral object, B, the electrons on B are repelled as far as possible from A and flow to the other side of B. If that side of B is then connected to the ground by a good conductor, such as a metal wire, the electrons flow out through the wire into the ground. The ground can receive almost any amount of charge because Earth, being neutral, has an enormous capacitance. Object B is said to be grounded by the wire connecting it to Earth.

If this wire is then removed, B has a positive charge, since it has lost electrons to Earth. Thus B has been permanently charged by induction. Even if A is subsequently removed, B still remains positive because the wire has been disconnected and B cannot regain electrons from Earth to neutralize its positive charge.