Summary:

In this activity we will construct an electroscope using gold foil and an evacuated flask. We will use our electroscope to more systematically observe interactions between two charged objects and between a charged object and a neutral object. Weather permitting, we will attempt to get quantitative attraction/repulsion data to qualitatively model Coulomb’s law.

Learning Goals / Objectives:

1.) Students will learn that there are two types of charge: negative and positive.
2.) Students will learn that:

• a neutral object is charge balanced. It has – the same number of electrons (negative charge carrier) as protons (positive charge carrier)
• a negatively object has extra electrons
• a positively charged object has some electrons removed

3.) Students will learn that an objects like a glass rod or plastic rod, can be charged by rubbing it with materials that either remove or add electrons
4.) Students will see that charge can be transferred from one object to another object if the two objects are in direct contact with each other
5.) Students will observe how charged objects interact and see that:

•  objects with like charges repel
• objects with unlike charges attract
• neutral objects are attracted to both positively and negatively charged objects

6.) Students gain a qualitative understanding of Coulombs Law

Background

This activity, like all electrostatic activities that rely on building up and maintaining a charge, can be a bit temperamental. It will work best on a dry day with very low humidity. We have found that using partially evacuated flasks to house the electroscope helps offset high humidity.

Safety
This activity has little to no risk associated with it. Take care when inserting the metal rod through the rubber stopper. You can seriously hurt your hand if your hand slips while you are pushing the rod through. Use common sense.

Setup:

Teacher’s Discretion

Anticipatory Set:

Teacher’s Discretion

Instructions / Activities:

Part A: Build your Electroscope *
Make sure you have access to the following:

• Erlenmeyer flask with sidearm
• Hand pump
• Gold foil
• Rubber stopper with hole
• Metal rod (that fits through the hole in the rubber stopper)
• Scotch tape
• Razor or Scissors
• Ruler
• Foil

1.) Place metal rod through hole in rubber stopper. Leave ~6 cm of the rod sticking out of the top of the stopper.
2.) Cut two gold foil leaves ~1 cm by ~5 cm. The foil is VERY FRAGILE. You may want to use a razor and straight edge. You also may want to use a piece of paper to weigh down the foil while you cut. It may take a few attempts to get this right.
3.) Cut two strips of tape ~3 mm x ~8 mm (This is very small).
4.) Fix a strip of tape to the end of one of the foil leaves and affix the leaf to the bottom of the metal rod (see above). Repeat for the second foil leaf. Be sure the foil is in direct contact with the metal rod.
5.) Carefully lower the foil leaf end of the rod and stopper into the flask. The stopper should fit snuggly.
6.) Attach hand pump to sidearm of the flask. Pump down the flask.
7.) Attach a foil ball to the top of the electroscope (not shown).

*If electroscopes are already built jump to Part B.

Part B: Charge!!
Make sure you have access to the following: Charging equipment (balloons, plastic rods, glass rods, pelts, silk, wool or similar) (see Triboelectric series on page 34-35 of Stop Faking it!)
1.) Neutral electroscope; Your electroscope should start in a neutral state. If you think it is charged you can ground it by touching the electroscope rod with your hand
2.) Negatively charge the electroscope;
a. Start with a neutral electroscope
b. Use either a balloon and your hair or plastic rod and wool/pelt.
c. Vigorously rub the balloon or rod.
d. Rub the charged balloon or rod on the metal rod of the electroscope.
i. What happens to the gold leaves? Explain your observations.
ii. If the leaves do not stay separated, you may have to trouble shoot. ..
1. Try adding more charge from your balloon or rod.
2. Re-pump down the flask and add more charge.
3. Ground the electroscope and recharge.
e. To make sure you can reproducibly negatively charge the electroscope, ground the electroscope and recharge the electroscope (steps b and c).
3.) Positively charge the electroscope;
a. Start with a neutral electroscope.
b. Use either a balloon and your hair or plastic rod and wooVpelt.
c. Vigorously rub the balloon or rod.
d. Bring the charged balloon or rod close to the electroscope rod but do not touch (record what happens to the gold leaves. Can you explain this observation?).
e. With the balloon or rod near the electroscope rod (but not touching), ground the electroscope by touching it with your hand. Remove your hand first and then remove the balloon or rod.
i. What happens to the gold leaves? Explain your observations.
ii. Trouble shooting is very similar to the above. It may take a few attempts to get the leaves to persistently stay separate.
f. To make sure you can reproducibly positive charge the electroscope, ground the electroscope and recharge the electroscope (steps b and c).

Part C: Get Rigorous.
In parts I – 3 we are going to fill in the following data matrix. Record whether the leaves of the electroscope: Open, Close, Stay Constant.
1.) Start with a negatively charged electroscope.
a. Bring a negatively charged object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?
b. Bring a positively charged object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?
c. Bring a neutral object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference? (Note: if you are not sure if you have a neutral object, you can use what you learned in the “Charged Picker Upper” activity to be sure.)
2.) Start with a positively charged electroscope.
a. Bring a negatively charged object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?
b. Bring a positively charged object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?
c. Bring a neutral object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?
3.) Start with a neutral electroscope.
a. Bring a negatively charged object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?
b. Bring a positively charged object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?
c. Bring a neutral object near the electroscope rod. What happens to the leaves? Record your observations. Can you explain your observations? Does the distance between the charged object and the electroscope rod make any difference?

Assessment:

Teacher’s Discretion

Wrap-up / Closure:

Follow up questions:
1. Sum up your finding into a general rule.
2. Outline the steps you could take to determine if an unknown object has a charge. And, if the
object has a charge, what its charge is (positive or negative).

Attachments: https://ccsplibrary.com/wp-content/uploads/gravity_forms/1-98399c595b52310e23a081ee652e5a53/2013/09/gold-plated-charge-detector.pdf

These materials are contained in a kit in our Library!