Charge
Measuring charge
You can't see an electric charge. Until it's too late. Let's rewind that. Look at that little blue spark. That's a bunch of electrons jumping from the door knob to Philip's hand.
Ouch! Let's rewind a bit more. At this point, Philip's body has a positive electric charge. But in real life, we can't see that, can we? Unless we have an ELECTROSCOPE!
Let's build one. Here's what you need: A glass jar. A piece of cardboard. A pair of scissors. A metal paper clip.
Some aluminium foil. And insulation tape. Use the cardboard to make a lid for the jar. Straighten the paperclip, then form a hook, like the letter J. Cut out two pear-shaped leaves of aluminium foil.
Carefully make holes in the leaves, and hang them on the paper clip hook. Yes, just like that. Insert the hook from below, through the cardboard lid, and place the lid on the jar. Fix the lid and the hook with tape. Now crumple up the last bit of aluminium foil to a ball, and place it around the top of the paper clip.
That's it, you're done. You have an electroscope! Let's see if it works. To do that, we need something that is electrically charged. Like a comb.
Comb you hair, Philip! When you comb your hair, electrons from your hair jump over to the comb. The hair is now positively charged, and the comb is negatively charged. Hold the comb close to the aluminium ball, without touching it. Now, look inside the jar, at the aluminium leaves!
They move! Remove the comb, and they return to hanging down again. Here's what happens: The comb has an excess of electrons. Electrons are negatively charged. And negative charges want to be far away from other negative charges.
Like charges repel each other. When the electrons in the aluminum ball sense the presence of the negative charges in the comb, they rush away from it. To get as far away from the comb as they can, the electrons move downward, to the aluminium leaves. Now there is an excess of electrons in the leaves. And what happens with charges of the same type?
They repel each other. In fact, the electrons repel each other with such force, that the entire aluminium leaves move away from each other. So, while it's true that you can't see an electrical charge, you can detect it with the help of an electroscope.