LAB 1
Electric Charge & Charge Transfer
OBJECTIVES
1. Identify the two kinds of electric charge.
2. Discover some of the effects of static electricity.
3. Investigate charging by friction, charging by contact, and charging by induction.
EQUIPMENT
Plastic rods, wool, pith balls, electric pompoms, water stream, balloons, pie plates,
conducting spheres, conducting wand, and Van de Graff generator.
PROCEDURE
This is an “Exploratorium” type lab. There are numerous stations set up around the lab.
Experiment with all the equipment at each station and make diagrams that show what
you observed. Check at each station to see that your observations are consistent with
the concepts developed in class.
Part 1: Charged Rods and Pith Balls
a. Charge the gray plastic rod negatively by rubbing it with or wool or a paper towel.
Bring the rod close to but not touching a hanging pith ball. Explain what you observe.
Draw a sketch showing the charges in the pith ball and the rod.
b. Now let the rod touch the pith ball. What happens? Can you come up with an
explanation for this behavior?
c. We also have clear plastic rods. When these are charged, do they end up with the
same charge or a different charge than the gray rods? How can you tell? (Hint:
charge the ball with one type of rod and bring the other type of rod up to the ball.)
Part 2: Charged Rods & Electric Pom-Pom
a. Charge the gray plastic rod negatively by rubbing it with paper towel or wool. Bring
the rod close to but not touching the electric pom-pom. Explain what you observe in
terms of induced charge.
b. Now drag the charged along the top of the electric pom-pom. Notice that the strings
repel each other. Why?
c. Touch the top of the electric pom-pom with your finger while it is charged. What
happens and why?
d. Try the same things with the clear plastic rod. Repeat part 1c but with the pom-pom
instead of the pith ball.
Part 3: Charged Balloon
a. Rub a balloon on your hair or on a piece of clothing. Now touch the balloon to a wall.
Explain what you observe in terms of charging by friction and induced charge. Draw
a sketch showing the charges in the balloon and the wall when the balloon is against
the wall.
b. Rub a balloon on your hair or on a piece of clothing again, but this time put the
opposite side (i.e. not the side that you rubbed) of the balloon against a neutral wall.
What do you observe? What does this tell you about the balloon?
c. Look at the little red balls in the plastic case. When you shake the case the balls rub
against the case and it’s very similar to rubbing a balloon on your head. Is there a
way to tell whether the balls are negatively or positively charged? When you walk
around you sometimes get a static charge and you get shocked when you touch
something like a doorknob. Why doesn’t this static charge cause you to levitate like
these red balls?
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Part 4: Water Stream
Make a prediction: What will happen to a small neutral stream of water when a charged
gray rod is brought near? Why? Rub the rod with wool to charge it and observe its effect
on a water stream. Draw and describe what you see.
What do you think will happen if you use a clear plastic rod, which has the opposite
charge as the gray rod? After predicting, try it out.
Part 5: Van de Graff Generator
a. Turn on the generator while holding a second metal sphere connected to the bottom
of it. What do you observe when the two spheres are brought close together?
b. Place the aluminum pie plates stacked together on top of the Van de Graff generator
and turn it on. Why do the pie plates fly off one at a time?
c. Turn on the generator and bring an electric pom-pom near the charged sphere.
Explain what you observed in terms of electric field and induced charge.
d. Tape the electric pom-pom to the top of the Van de Graff generator. Turn on the
generator and record what you observed. Turn off the generator and observe the
strings of the electric pom-pom for a few minutes. Eventually, the electric pom-pom
would completely discharge. How? Charge the electric pom-pom again and then turn
off the generator. Bring your hand close to but not touching the generator. What did
you observe?
e. Hold a small piece of animal fur near the Van de Graff generator and then let it go.
Explain the behavior of the fur. See if you can get to animal fur to bounce back and
forth between your hand and the generator – it’s pretty cool!
f. Challenge: as the Van de Graff generator becomes charged, you should be able to
hear the motor slow down. Explain why the motor slows down
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