Electrostatic Charge Prelab

Note: It will be helpful to read the appropriate chapter in the text to complete the prelab AND as a resource for the lab. In this lab, black PVC stripsreplace the black plastic rodsand clear acetate strips replacethe glass rods.

2. What is the sign of the charge on the surface of the Van de Graaf Machine? ______

Physics 126 Lab

Electrostatic Charge

Equipment includes some or all of the following: Black plasticstrips and clear acetate strips, pith balls on strings, wool, silk, fur, metal rod, Van de Graaf Generator with insulating floor mat, static genecon generator, variable capacitors and track, electroscope, electrical “banana plug” leads, metal spheres, pie plates, paper or Styrofoam plates.

Part 1: Van de Graaf Machine Demonstration

The Van de Graaf Machine uses friction between a belt, pulleys and copper brushes to produce a positive electrostatic charge when the motor is running.

1. Watch the demonstration with the “wig”. Why does the fur stand on end?

2. Watch the demonstration with the grounded metal sphere. What kind of energy was transformed into the light energy of the spark?

3. We are going to put 3 plates on top of the generator, first styrofoam/paper plates, then metal pie plates. Why do the plates fly off the dome? Explain why the metal plates fly off one at a time, while the paper plates fly off together (or not at all if they’re too heavy).

4. (Optional) Come on up, you hero, and see if you can make your hair stand on end (works better if you don’t use any hair products or conditioner)!

Part 2: Charging by friction and by contact

The plastic strips and the pith balls are insulators.

1. Without doing anything bring the black strip close to the pith ball. Repeat with the clear strip. Are either/both of these objects charged? _____

Explain how you know:

2. Touch the pith ball with your hand to remove any stray charges (this is called discharging the ball). Charge the black strip with the wool and bring it near (but not touching) the ball, which is still uncharged. If it touches the ball or if the ball touches the metal stand, touch it to discharge and repeat the experiment.

Was the ball attracted to the strip, repelled, or neither? Explain why the ball behaved as it did, considering that only the black strip was charged:

3. Recharge the black strip and use it to transfer some of the charge to the pith ball by touching it. Don’t let the ball touch the metal. Bring the charged strip back near the (now charged) pith ball. Observe what happens.

Now quickly charge the clear strip with the silk. Bring it near the pith ball, without touching it.

a. Was the charged ballattracted to the black strip, repelled, or neither?______

b. Was the charged ballattracted to the clear strip, repelled, or neither?______

c. Does the clear strip have the same charge as the black strip or the opposite charge? Explain how you obtained your answer:

4. Discharge the ball by touching it and repeat the above procedure, using a metal rod instead of one of the strips. Metals are conductors.

Can you charge the metal rod by friction? From what you know about conductors and insulators, explain why or why not:

Part 3: Using the Static Genecon and parallel-plate capacitors to determine the sign of charge:

Set up the capacitor plates so they are about 4-5 cm apart. Plug the red banana-type lead into the red jack, and the black banana-type lead into the black jack of the static genecon. Plug the other end of the each lead into either one of the capacitors.

Have the members of your group do the next steps quickly, as electrostatic charge tends to “leak” into the air.

  • Have one person rub the black strip with wool and transfer some of the charge to an uncharged pith ball.
  • Simultaneously crank the genecon machine, which will transfer charge to the capacitors (red positive charge, black negative charge).
  • Stop cranking, and carefully insert the pith ball into the space between capacitors without touching either plate.

Is the pith ballinitially attracted to the positive plate or the negative plate? ______

Discharge the pith ball, and repeat the procedure, this time using clear strip and silk. Is the ball initially attracted to the positive or negative plate? ______

Based on this experiment, answer the following questions (remember opposites attract!):

When rubbed with wool, the black strip acquires a ______charge.

When rubbed with silk, the clear strip acquires a ______charge.

Part 4: Charging by contact and induction,using the electroscope

Experiment 1: Note the construction of the electroscope. The pathway between the knob on top and the leaves is exclusively metal. Rub the black strip briskly with the wool and touch the metal knob.

1a. What happened to the metal foil leaves? Draw a picture of the electroscope, below, after touching the knob with the strip, using + and – to represent charges, and adding the foil leaves in the appropriate orientation. Write a short explanation of what happened:

1b. Uncharge the electroscope by touching the knob. Repeat using the clear strip and the silk (positive charge). Compare results with those of 1a.

Experiment 2: If charge can travel through the metal of the electroscope, is it reasonable to assume it can travel through the plastic strip? Rub one end of the black strip with the wool, but this time touch the knob with the opposite end of strip

2a Compare the response of the metal foil leaves with that of Experiment 1. If the response is different from that in Experiment 1, explain why:

2b. You have seen that both the metal of the electroscope and the plastic strips can be charged. What is the difference between the way charge behaves on these different materials?

Experiment 3: Discharge the electroscope. Charge the black strip once again and bring it very close to the knob of the electroscope (but not touching). Observe the foil leaves, then remove the strip from the vicinity of the metal knob.

3a. Did the electroscope as a whole acquire a net charge from the strip? If so, how? If not, explain the response of the foil leaves.

3b. What happens when you take away the strip?

Experiment 4: Repeat Experiment 3 only while the charged black strip is very close to the knob, touch the knob. Then remove the strip, as before. Explain why the foil leaves are still separated:

Part 5: Charged and neutral objects

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