PPT Induction

Developer Notes

  1. Should we show charge movement by conduction in this activity or another one?

Version / Date / Who / Revisions
01 / 2005/02/07 / dk / ·  Taken from 11.4 charge 03dk.doc, broken in two
·  11.4b shows charge movement by induction
· 
02 / 2005/03/17 / dk / ·  Re-numbered and saved
03 / 2005/04/08 / dk / ·  Incorporating Arny’s comments
04 / 2005/04/18 / dk / ·  added equipment info on wand

Goals

  1. Students should understand that charge can move around on objects.
  2. Students should know that movement of charge without contact is called induction.
  3. Students should understand that charged objects attract uncharged objects by forcing the electrons on them to redistribute.
  4. Students should know that movement of charge by contact is called conduction.
  5. Students should know that moving charge is called current.

Concepts & Skills Introduced

Area / Concept
Physics / Charge induction

Standards Addressed

Time Required

Warm-up Question

Presentation

The most basic goals in this activity are to get the students to understand that (in solids)

  1. Electrons move around (but protons generally don’t).
  2. Charge can be induced.

Based on the previous activities, students should understand that charge (generally electrons) can be transferred to or from objects, thereby creating a net charge on the objects. They should also understand that opposite charges attract and likes repel. From the magnets activity, they should have a feel for induction of magnetic charge. In this activity, they see that electric charge can be induced. By induction we mean forced movement of charge without contact.

During the activity, students should be visualizing how the electrons are moving. If the students can get a picture of electrons moving around on materials, but always spacing themselves evenly due to repulsion, that will be a big help in understanding current, electromagnetism, light, and chemistry. Get some magnets out and remind the students of the magnets activity and how the magnets repelled each other when they were pushed around. Birds on a wire tend to keep one pecking distance away from each other, children spread out in the back seat of a car.

Metals are good conductors because they have a lot of electrons that are loosely bonded, so they can move easily – the metal is neutral on the triboelectric list. The electrons can travel either on the surface of the metal or through it, although the surface is easier.

Insulators are not good conductors because their electrons are tightly bonded and don’t move easily. Even electrons on the surface tend to stay there because they are strongly attracted to the material.

In the activity, we try to show movement of electrons in several ways. Assume we are using a material, like polystyrene, that attracts electrons. At first, the leaves of the electroscope are neutral (have a zero charge), and they hang straight down next to each other.

Bring a negatively charged “wand” near the leaves and they will separate. Bring the wand from the top and the electrons are forced to the ends of the leaves and repel each other. Bring the wand from the bottom and the electrons are forced to the top of the leaves; positive charge is left on the bottom and the leaves repel each other. Notice how the leaves are drawn to the wand when it is near the bottom of the leaves – a positive charge has been induced in the ends of the leaves, and it is attracted to the negative charge on the wand.

Sometimes charge will arc from the wand to the electroscope, and the leaves will repel each other even if the wand is moved away. If you have good hearing, you can hear the snap of the arc. Touch the paper clip to equalize the charge and the leaves will hang straight again.

Bring the wand near the leaves from the top. Electrons are pushed onto the ends of the leaves and the body of the electroscope. Touch the end of the paper clip momentarily and the electrons are conducted into your body, leaving a positive charge on the electroscope. Even with the positive charge, the leaves don’t repel each other because they have a balanced charge – they have extra protons because electrons have been conducted away, but they also have extra electrons pushed onto them by the wand. Now remove the wand and the leaves will repel each other. The electrons re-distribute themselves evenly, leaving a net positive charge on the leaves. Now bring the wand near the leaves from the top again. The leaves will move back together because electrons are forced onto them, neutralizing the positive charge.

A lot of confusing things can happen with the wand and electroscope, depending on how they are brought together, whether they touch, whether there is an arc, etc. Try to keep it simple.

After the activity, reinforce the concepts by having the students model the movement of electrons. Ensure that the students understand charge comes from an uneven number of electrons and protons, that opposites attract and likes repel, and that electrons move around. Divide the class into protons and electrons and make them move around. Use boundaries to restrict their movement, uneven numbers of protons and electrons, bring a group of electrons near a group of electrons and protons, etc.

Assessment

Observe: Rub a balloon in your hair and stick it to a wall.

Explain: electron, proton, induction

Writing Prompts

Relevance

Answers to Exercises

  1. If your hair takes a static charge, why does it get dirty faster? [Hair with a charge attracts small pieces of dirt, just like a charged wand attracts bits of paper.]
  2. Explain how an object with a positive charge and an object with a neutral charge can attract each other. [The positive charge on the first object attracts the negative charges on the second object. As a result, the negative charges move closer, leaving the region farther away positively charged. Since the negative charges are closer, the attractive force between the two objects overcomes the repulsive force.]
  3. In a dark room, pull off a synthetic sweater and watch the sparks. What are the sparks, and where do they come from? [The sparks are discharges of energy due to differences in charge. The difference in charge comes from the friction between the sweater and your body or another piece of clothing. Electrons are transferred from one object to the other.]
  4. Why are anti-static sheets used in dryers? [Dryers cause a lot of friction. Friction leads to exchange of electrons and static build-up. Anti-static sheets allow the charge to move around from one piece of clothing to another so that no charge accumulates in any one place and clothes don’t cling.]
  5. In flammable chemical storage areas, there is a pipe connected to the ground. All of the containers are connected to the pipe, as are any people who work in the area. This helps prevent a spark from igniting the chemicals. Why does it work? [The common ground point ensures that no charge can build up on any item relative to another item. If there’s no difference, there’s no energy to create a spark.]
  6. Electronics technicians assemble delicate parts. They have a conductive mat that is grounded, and they have wrist straps connected to the mat. Why? [Electronic parts are susceptible to static discharge. Connecting the mat and the person to ground leaves at the same potential so that no static charge can build up.]
  7. Why would vehicles that work around flammable chemicals need tires that conduct electricity? [Tires create friction, therefore charge. A large charge can build up on the vehicle. When it discharges, it creates a spark that can ignite flammable gases.]

Answers to Challenge/ extension

Equipment
See the previous (Charge) activity for instructions on an electroscope.
For the wand, just use a small plastic strip. PS (type 6) or PETE (type 1) work well, and you can cut them from cups or soft drink bottles. About 5 x 10 cm is good.
Background
You know that there are electrons (negative charge) and protons (positive charge) and that opposites attract and likes repel. You also know that electrons are freer to move than protons. Think about charge as you go through this activity.
Problem
Learn more about static electricity.
Materials
1 plastic strip (the “wand”)
1 piece of wool
1 electroscope

Procedure
Place one leaf of the electroscope on the table.
  1. Charge the wand by rubbing it briskly with the wool. Move the wand slowly toward the leaf on the table.
  2. What happens to the leaf? Is it attracted to the wand?
  3. Try to explain what happens based on what you know about charge.
  4. Does the charge on the wand have energy? How do you know?

Put the second leaf on the hook. The leaves should be able to swing freely.

  1. Touch the paper clip with your finger (ground it). Charge the wand. Move the wand slowly up toward the bottom of the leaves, then away.
  2. What happens to the leaves?
  3. Try to explain it.
  4. Ground the paper clip and charge the wand. Move the wand slowly down toward the top of the leaves, then away.
  5. What happens to the leaves?
  6. Again, try to explain it.
  7. Move the wand slowly toward the leaves from the side.
  8. Are the leaves drawn toward the wand or pushed away from it?
  9. Try to explain it based on what you know about charge.
  10. Ground and charge. Move the wand slowly toward the side of the cup farthest from the leaves, then move it away.
  11. What happens to the leaves?
  12. Try to explain it.
  13. Ground and charge. Move the wand slowly down toward the top of the leaves until it is near the paper clip. The leaves should separate. Momentarily touch the paper clip at the end, away from the leaves, then stop touching it.
  14. What happens to the leaves?
  15. Try to explain it.

Move the wand up.

  1. What happens to the leaves?
  2. Try to explain it.

Move the wand back down.

  1. What happens to the leaves?
  2. Try to explain it.
  3. Move the wand slowly toward the leaves from the side.
  4. Are the leaves drawn toward or pushed away from the wand?
  5. Does this help to explain what is happening? Try to explain the whole sequence again.

Summary

  1. Draw pictures of the charge on the wand and the leaves for the situations above. Use +’s and –‘s.


Reading

You have seen that charged objects attract uncharged objects, and you have seen that opposite charges attract. How does that work? Even neutral objects experience a force when placed near a charged object because the electrons on them redistribute due to the electric field of the charged object. The wand has a negative charge. The paper has an equal mix of positive and negative charges. Hold the wand near the paper and the negative charges on the wand force the negative charges on the paper away, leaving positive charges nearby – opposites attract, so the paper is drawn to the wand. Movement of charge without contact is called induction.

Movement of charge is called current. When you grounded the electroscope by touching it, charge flowed between your body and the electroscope. The charge moved due to electric force, which is caused by uneven distribution of charge.

In the previous activity, and perhaps this one, you saw charge arc through the air. That is conduction of charge through air. It is the same process as lightning.

Exercises

  1. If your hair takes a static charge, why does it get dirty faster?
  2. Explain how an object with a positive charge and an object with a neutral charge can attract each other.
  3. In a dark room, pull off a synthetic sweater and watch the sparks. What are the sparks, and where do they come from?
  4. Why are anti-static sheets used in dryers?
  5. In flammable chemical storage areas, there is a pipe connected to the ground. All of the containers are connected to the pipe, as are any people who work in the area. This helps prevent a spark from igniting the chemicals. Why does it work?
  6. Electronics technicians assemble delicate parts. They have a conductive mat that is grounded, and they have wrist straps connected to the mat. Why?
  7. Why would vehicles that work around flammable chemicals need tires that conduct electricity?

Challenge/ extension

Glossary

·  Induction – movement of electric charge without contact

·  Current – movement of electric charge

·  Conduction – movement of electric charge by contact

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