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The drawings below illustrate what determines the charge, if any, on an object. By nature, the number of protons (+) in each cup remains constant. (Protons don’t move, remember? They can’t enter or leave the cup.) If the zillions of protons and electrons are represented as shown, notice that it is the relative number of electrons that makes each cup positive, negative, or neutral. Your students should have this concept firmly fixed in their minds.


Positive


Negative


Neutral

Electrons are added or taken away through friction, such as when clothes tumble in a dryer, feet scuff a carpet, or a comb moves through hair. Not all objects or materials can be charged, though. For example, materials that conduct electricity well, like metals, are hard to charge because the extra electrons generated by friction will flow out of that material if it happens to be grounded or is touched by another conductor (which includes you).

Whenever objects become charged, the charges must obey the following two rules:

Opposite charges attract each other. Like (same) charges repel each other.

Neutral objects can behave like they are charged when they are near a charged object. (Don’t worry. Think electrons.) If a negatively charged (-) comb is brought near a neutral object, some of the electrons on the neutral object are repelled to the side away from the negatively charged comb. (Why? because same charges repel.) That leaves mostly protons (+), which don’t move, on the side facing the negatively charged comb. Since opposite charges attract, the neutral object will be attracted to the comb even though the object’s overall balance of electrons and protons hasn’t changed. Technically, it’s still neutral.

The same thing would happen if the comb were positively charged, only now some of the electrons would be drawn toward the side of the neutral object facing the comb (instead of being repelled to the opposite side, as in the preceding paragraph). So the neutral object also would be attracted to the positively charged comb. This process, called induction, ALWAYS results in a neutral object being attracted to a charged object. The kind of charge on the charged object doesn’t matter. The neutral object will always behave like it has the opposite charge.

TEACHING TIPS

Science vocabulary seems an insurmountable hurdle for many students. This unit has been intentionally written to minimize reliance on special terms. Therefore, place the emphasis on understanding the process. Demand explanations instead of memorization of terms.