Gain practice in building a device that detects the presence of electrical charge and observe the basic methods used to charge an object.
To gain practice in building a device that detects the presence of electrical charge and to observe the basic methods used to charge an object.
An electroscope is a device that detects the presence of electrical charge. When charged objects are brought near an electroscope, the foil strips inside will move, indicating the presence of electrical charge.
When a charged object is brought near the electroscope, it causes the electrons in the foil strips to move. If the strips develop the same charge, they repel each other and spread apart. This movement indicates the presence of electrical charge.
A Van de Graaff generator is a device that creates static electricity at a much higher voltage than simple friction methods. It uses a moving belt to accumulate electric charge on a hollow metal sphere. The generator can create charges strong enough to produce visible sparks and can cause hair to stand on end when a person touches it. It's an excellent tool for demonstrating the principles of electrostatics and for testing the sensitivity of an electroscope.
Using your hands and the pliers, straighten out and then bend the paperclip so that it ends up looking something like what is shown in Figure 11.11A.
Cut a thin slot in the plastic lid. Slide the loop of the twisted paper clip into the slot, then twist it 90 degrees so that the slot holds the loop in place. The loop should stand perpendicular to the lid. You may need to use some tape to hold it in this position (see Figure 11.11B).
Poke a hole near one end of each strip of foil and hang the foil strips on the tiny hooks that are at the bottom of the twisted paper clip.
Place the lid on top of the glass, so that the foil strips hang on the inside of the glass. You have just made an electroscope (in fact, truly sloppy). It should look something like what you see in Figure 11.11B.
What does an electroscope do? An electroscope detects the presence of electrical charge. To see this, inflate the balloon and tie it off so that it stays inflated.
Rub the balloon in your hair to charge it.
Slowly bring the balloon close to the loop of the twisted paper clip without actually touching it. The foil strips should start to move. If they do not, your balloon is probably not charged well. Rub it more vigorously in your hair or rub it in someone else's hair.
Note how the foil strips move as you bring the balloon closer to the loop. Don't actually touch the loop with the balloon! Record your observations.
Pull the balloon away from the loop and note how the foil strips move. Do steps 8 and 9 a couple of times so that you can describe the motion of the foil strips well. Record your observations.
Bring the balloon near the loop one more time. This time, however, allow the balloon to touch the paper clip. Record what happens to the foil strips.
Pull the balloon away. This time, the behavior of the foil strips should be noticeably different from what it was in step 9. Record the difference.
I want you to do one more thing. These next few steps might be a little tricky. Touch the loop with your finger. You should notice that the foil strips respond to your touch. Record what they do.
Take your finger away from the loop.
Bring the balloon close to the loop, but do not touch it with the balloon. When you see the foil strips move significantly, hold the balloon where it is and touch the paper clip with your finger. From your other hand, as soon as your finger touches the paper clip, the foil strips should move again. Keep your finger resting on the paper clip for a moment.
Pull your finger away so that it is no longer touching the paper clip.
Now pull the balloon away. The foil strips should move yet again and behave similarly to what you saw in step 11. This doesn't always work the first time, so try it a few times until it eventually works. Record your observations.
If you have access to a Van de Graaff generator, set it up on a non-conductive surface. Make sure it is unplugged before setup. Have an adult supervise this part of the experiment.
Before turning on the generator, ensure everyone is wearing eye protection. Keep all electronic devices at least 3 feet away from the generator. People with pacemakers or other electronic medical devices should maintain a safe distance.
Have an adult turn on the Van de Graaff generator. Allow it to run for about 30 seconds to build up a charge on the metal dome.
Carefully bring your electroscope near the Van de Graaff generator (about 1-2 feet away). Observe what happens to the foil strips. Move the electroscope closer and further away from the generator and note how the foil strips respond. Do not touch the generator with the electroscope.
Hold a small metal object (like a paper clip) using an insulated handle or plastic ruler. Bring it near the Van de Graaff generator dome without touching it. Then touch the metal object to the electroscope's paper clip loop. Observe what happens to the foil strips.
Compare the response of the electroscope when charged by the balloon versus when charged indirectly by the Van de Graaff generator. Note any differences in how far the foil strips spread apart and how long they remain charged.
Turn off and unplug the Van de Graaff generator. Clean up and put everything away in its proper place.
| Step | Action | Foil Strip Behavior |
|---|---|---|
| 7-8 | Charged balloon near loop (no contact) | |
| 10 | Balloon touches paper clip | |
| 11 | Balloon pulled away after contact | |
| 12 | Finger touches loop | |
| 14-16 | Induction charging process | |
| 20 | Electroscope near Van de Graaff generator | |
| 21 | Metal object transfer from generator to electroscope | |
| 22 | Comparing balloon vs. Van de Graaff charging |