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Main » Electricity » Current & Potential Difference » Offline III.2.1

### Static Charge, Conductors and the Electroscope - Lab III.2.1

Purpose:

To use an electroscope to study electrostatic phenomena.

Materials: [click on the image for a larger view]
 » Electroscope » silk cloth » Lucite rod » fur

Background Info:

You can charge a neutral object such as the Lucite rod by rubbing it with the silk cloth. By rubbing the two items together you strip electrons off of the Lucite rod and place them on the silk cloth. When you transfer the charge from object to another the Lucite rod becomes electron deficient with a "net" positive charge and the excess electrons on the silk cloth causes the net charge to be negative.

An uncharged electroscope has a large number of positive and negative charges. When you bring the Lucite rod near (do not touch) the electroscope, negative charges within the electroscope are attracted. Free electrons within the electroscope move to the surface of the electroscope trying to get as close to the Lucite rod as possible. As these free electrons move to the surface of the electroscope the thin metal leaves at the opposite end become positively charged due to the lack of electrons. The leaves have the ability to move, so when they both become positively charged there is a force of repulsion and the leaves push each other away.

If the leaves of the electroscope stay apart after you remove the Lucite rod the electroscope is no longer neutral, it has a net charge. To remove this charge the electroscope can be grounded. Touch the top of the electroscope to metal electrical wall outlet cover. The metal cover is connected to conductors all the way to the earth. The earth due to its size and conducting nature can receive or give up large numbers of electrons without any effort. The potential of the earth is arbitrarily said to be zero. By giving up or gaining electrons the object grounded will become neutral. The symbol for the ground is:. An object connected directly to the ground can be described as being earthed.

Procedure:

1. Ground the electroscope so that it is neutral.
2. Charge the Lucite rod by rubbing it with silk to make it positively charged.
3. Draw pictures like the one above with the Lucite rod at various distances away from the electroscope.
4. Explain the pictures and their differences. Indicate which way the Lucite rod is moving (away or towards the electroscope) and the direction the leaves move (away or towards each other).
5. Charge the Lucite rod by rubbing it with fur to make it negatively charged.
6. Predict what the differences will be with using an oppositely charged rod. Use the rod to see if your predictions are correct.
• Make a detailed drawing of what is happening.

Charging by Induction:
1. Charge the Lucite rod so it has a positive net charge.
2. Bring the rod close to the electroscope so there is noticeable separation between the leaves.
3. Hold the rod in place and ground the electroscope. What happens?
4. Move the rod away from the electroscope, what happens?

Charging by Touch:
1. Charge the Lucite rod so it has a positive net charge.
2. Touch the rod to the electroscope, what happens?
3. Charge the Lucite rod so it has a negative net charge, bring it towards the electroscope, what happens?
• Make a detailed drawing of what is happening.
4. Charge the Lucite rod so it has a positive net charge, bring it towards the electroscope, what happens?
• Make a detailed drawing of what is happening.

Determine the Sign of Charge:
1. Use the process to separate the charge with the Lucite rod and various materials around the room.
2. Determine what the sign of the charge is on the material after the charge separation.

Data & Analysis:

1. Explain what is happening with the electroscope when charging it by induction.
2. Explain what is happening with the electroscope when charging it by touch.
3. Make a table listing the materials and the charge remaining on them after they have gone through the separation of charge with the Lucite rod.
4. What fundamental law is this lab based on?

Conclusion:

• Remember your conclusion must respond to the purpose.