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Electrical Interactions
PURPOSE & OBJECTIVES
The particle model of electricity is the current theory that we use, and explains many electrical
properties better than its predecessor, the fluid model. In the particle model, electric charge is seen as
having a certain amount of charge being the smallest charge possible. This amount of charge is
characterized in two fundamental particles: the proton (positive charge), and in the electron (negative
charge). Other charges exist (ie. positron, anit-proton), but the proton and electron are the most
common.
A charged object is an object that has more of one kind of charged particles than the other. A
charged object attracts objects of opposite charge (ie. positive attracts negative), and repels objects of
the same charge. When a charged object is brought near a neutral object, the charged object causes
the object to experience an effect called polarization: the positive charges become concentrated in
half of the object, and the negative charges are concentrated in the other half. So when a negatively
charged object is brought near a neutral object, the negative charges leave the side closest to the
negative object, because of the repulsion between the two negatives. This means that the side closest
to the negative object has an overall positive charge. This means that the two objects experience an
attractive force towards each other. There is also the repulsion between the two negative areas, but
since they are further apart than the positive and negative areas, this force is not as large. Therefore,
the overall force is towards each other.
When the two objects touch, however, an interesting effect can happen: the two objects are
then repelled. This is because when they come into contact, the charges from one object flow into
the other. This may result in both objects becoming the same charge, which causes them to repel.
The effects observed with the electrophorus are easily explained using the particle model as
well. When an insulator is rubbed, it builds up a negative charge on its surface (called static
electricity). Since the material is an insulator, the charge is not easily moved, except through outside
influence. This process occurs in Styrofoam, as it is an insulator. When the pie plate is placed on top
of the negatively charged Styrofoam, the electrons flow from the Styrofoam into the pie plate by the
polarization process described above. The electrons cannot leave the pie plate, since there are
insulators on all sides (air is an insulator). Since the human body is a conductor, when someone
touches the pie plate, there is a spark as the negative charges leave the plate and enter the person.
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11A- Electrical Interactions
A neon bulb can be made to glow, just by bringing it near the electrophorus. This happens
because the current (number of electrons per second) required to light the bulb is relatively low. Even
though air is an insulator, it still lets some electrons pass through it as long as the voltage (potential
difference between the two contacts of the bulb) is high enough. This is also a reasonably low
number; therefore, with the right adjustment to the setup, the bulb glows without using an outlet.
In this experiment, we will:
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Demonstrate evidence for the existence of two types of charge.
Investigate and explain electrostatic phenomena using the particle model of electricity.
Construct one of more electrostatic apparatus and explain how they function using the
particle model of electricity
MATERIALS
Styrofoam square
Piece of wool of fur
Neon bulb with exposed wires
Tape test strips
Aluminum pie pan (with Styrofoam cup insulating handle)
PROCEDURE
Part 1: Charging by Friction
1.
Charge your Electrophorus with the Styrofoam plate. Do this by rubbing the plate vigorously
over the flat area of Electrophorus.
2.
Determine whether the Electrophorus is charged. Touch one connector from the neon bulb to
the any point on the rim of the Electrophorus. When holding the neon bulb, try to keep your
fingers on the resistor.
Part 2: Charging by Conduction
3.
Charge an Electrophorus with the Styrofoam plate.
4.
Bring the charged Electrophorus in contact with an uncharged Electrophorus.
5.
Test to see if the uncharged Electrophorus is now charged. Bring one connector of the neon
bulb in contact any part of the uncharged Electrophorus.
Part 3: Polarization of Charge
6.
Charge an Electrophorus with the Styrofoam plate.
7.
Bring the charged Electrophorus very close to but not touching another Electrophorus as
shown below. Make sure that the Styrofoam cup is the only thing making contact with the
table.
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11A- Electrical Interactions
8.
Test to see if the uncharged Electrophorus is polarized. Bring one connector of the neon bulb
in contact with the uncharged Electrophorus on the side away from the charged Electrophorus.
Part 4: Charging by Induction I
9.
Charge an Electrophorus with the Styrofoam plate.
10. Place two uncharged Electrophorus in contact with each other as shown in step 1.
11. Bring the charged Electrophorus near but not in contact with one of the uncharged ones as
shown in step 2.
12. While still keeping the charged Electrophorus near, separate the two uncharged Electrophorus,
by touching the insulating region only as shown is step 3.
13. Once the two uncharged Electrophorus are separated remove the charged Electrophorus step
4.
14. Test to see if two uncharged Electrophorus are now charged.
Part 5: Charging by Induction II
15. Charge an Electrophorus with the Styrofoam plate.
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11A- Electrical Interactions
16. Place the charged Electrophorus next to an uncharged Electrophorus as shown in step 1.
17. While still keeping the charged Electrophorus near, touch the uncharged Electrophorus on the
other side than the charged Electrophorus, as shown in step 2.
18. Remove your hand, step 3, then remove the charged Electrophorus as shown in step 4.
19. Test to see if the uncharged Electrophorus is now charged.
ANALYSIS
Part 1: Charging by Friction
1. Did you feel any forces between the Electrophorus and the foam plate?
2. Was the Electrophorus charged? How did this happen? Discuss electron transfer in your
explanation.
3. The Styrofoam plate is an electron taker, which means it strips electrons from the
4. Electrophorus when they are rubbed together. After charging the Electrophorus, what are the
net charges on each?
Part 2: Charging by Conduction
1. Why is conduction called charging by contact?
2. Assuming that a charged Electrophorus has a net positive charge, sketch the charge pattern
that was produced when the two were in contact.
Part 3: Polarization of Charge
1. Was the Electrophorus polarized? How did this happen?
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11A- Electrical Interactions
2. Assuming that a charged Electrophorus has a net positive charge, sketch the charge pattern
that was produced in the diagram below.
Part 4: Charging by Induction I
1. Did the neon bulb light up for either of the uncharged Electrophorus?
2. Assuming that a charged Electrophorus has a net positive charge, sketch the charge pattern
that was produced in the diagram below.
3. What types of charge did you produce in each Electrophorus? How do they relate to each
other?
4. Which charge (positive or negative) seems to produce more light from the neon bulbs?
Part 5: Charging by Induction II
1. What was the purpose of touching the uncharged Electrophorus on the far side? Hint: Think
polarization of charge and what type of charge will to get away from the positively charged
Electrophorus.
2. Why would the finger touching the Electrophorus be considered “grounding”?
3. Assuming that a charged Electrophorus has a net positive charge, sketch the charge pattern
that was produced in the diagram below.
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11A- Electrical Interactions
4. What are the main differences between the two types of induction?
CONCLUSION
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11A- Electrical Interactions