Lesson Plan - Electric Charge

BLA Physics, 2004-2005
Mr. Z.
1/18/05

1. Questions to Answer and Skills to Practice

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2. Content Development

If I rub a baloon against my head, both the balloon and my hair start behaving weirdly. My hair seems to be pulled toward the balloon, and even when the balloon is nowhere around, my hair still sticks up. Furthermore, either my hair or the balloon will stick to any other object I bring near it: the balloon will stick to the wall, for example, and my hair will stick to my hand if I hold it over my head.

If I rub my hair with styrofoam instead, I notice the same thing happening: my hair stands up straight when left to itself, and sticks very much to the styrofoam. At this point, I might think that the styrofoam and the balloon would stick together really well, having a sort of double helping of stickiness; instead, the balloon, if touched to the styrofoam, will jump quickly away from it.

What we are dealing with here is, of course, electric charge. There are only two types of electric charge: positive and negative. The balloon in this example happens to be negative, as is the styrofoam; my hair has become positive. The rules for how charged objects behave around each other is that opposite charges attract, like charges repel.

What you might not know is that every object, not just those for which we observe static electricity, has a huge number of electrically charged bits. Every atom has an equal number of negatively charged electrons, and positively charged protons. There are trillions of each even in something as small as a speck of dust. But normally, we don't notice this, because when an object has equal amounts of negative and positive charge, they cancel each other out.

When an object is electrically charged, therefore, it means that this balance has been upset. For example, if I walk over a rug in rubber shoes, some electrons from the rug tend to stick to the rubber soles of my shoes, making the rubber have more negative charge and the rug have more positive. If I then touch a metal doorknob, the extra electrons from my shoes might jump off into it, giving me an electric shock.

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3. Activity or Experiment

Triboelectrification Lab

The lab today deals with how objects become electrified when rubbed together. When two things rub together, they end up fighting over who owns the electrons floating around at the boundary between them. Whichever object has a stronger grab on electrons will win this battle, and come out negatively charged. The other, having had its electrons stolen, has a positive charge; some of its protons are left without electrons.

We could build up a chart ranking how strongly different materials grab electrons. We know already that balloons are stronger than hair. If I wanted to find the rank of some new material, I could set up a "tournament" in which that material is rubbed against various already-ranked materials, and then the winner of each matchup is determined and the new material moves up or down in the standings, taking on a new contender, until it reaches its proper place, between a stronger material and a weaker one.

It is sometimes tricky to determine exactly who won a battle. For example, if I find that a PVC pipe rubbed with paper sticks to my (positive) hair, then the pipe might by negative. But, it might also simply be neutral. The only positive proof that something has negative charge is if it repels something else known to have negative charge. To make this easier for you, I've cut up a lot of small pieces of styrofoam that you can easly charge negative by running them through your hair; you can then toss them against both combatants and see which one repels. If neither repels, then either the objects have trouble holding on to the charge, or they are so close in ranking that neither steals from the other.

So, for example, I might want to determine where wool ranks relative to balloons and hair. I rub the wool against the balloon, and then try placing a test piece of styrofoam against both: it jumps off of the balloon but sticks to the wool. Therefore, the ballon is negative, and the wool positive. So, wool is weaker than balloon; it has allowed the balloon to steal its electrons.

Next, I try wool against my hair. After rubbing for a while, I do notice sparks, indicating that something is becoming charged. I test both my hair and the wool with styrofoam, and find that the wool is now negative and the hair positive. So, wool ranks between hair and balloons, weaker than hair but stronger than balloons.

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4. Debriefing



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5. Homework

Electric Charge
Scientist Question:

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6. Notes, Comments, and Reflections



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7. State Frameworks

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