1. Electric Charge
Chapter 20.1

2. Main Idea
Like electric charges repel, and unlike electric charges attract
Have you ever touched a metal doorknob on a dry winter day and gotten a shock?
You got a shock because electrons were transferred from your hand to the doorknob.
How is this shock similar to lightning?

3. Essential Questions
How can you demonstrate that charged objects exert forces, both attractive and repulsive?
How do we know that charging is the separation, not the creation of electric charges?
What are the differences between conductors and insulators?

4. Evidence of charge
You might have rubbed your shoes on a carpet and then produced a spark when you touched someone.
In 1750, Benjamin Franklin, who made many contributions to the study of electricity, suggested that lightning would have a similar effect on a metal key attached to the string of a kite flown in a thunderstorm.

5. Evidence of charge
As electricity traveled down the string to the key, loose threads of the string would stand up and repel one another, just as your hair might after rubbing your shoes on a rub.
If you touched the key, you would generate a spark and experience a shock.
Electric effects produced in this way are called static electricity.

6. Evidence of charge
In this chapter, you will investigate electrostatics, the study of electric charges that can be collected and held in one place.
Electrostatics is the study of static electricity.
The effects of static electricity are observable over a vast range of scales, from huge displays of lightning to interactions between electrons on the atomic scale.
Later, we will look at electric current, which is the net movement of electric charge.

7. Electrostatic force
Have you ever noticed that on a dry day, your hair is attracted to your comb after you comb it?
Perhaps you have noticed that socks sometimes stick together when you take them out of a dryer.
You might also recognize what happens after you rub a balloon on hair (like pg 1273).

8. Electrostatic force
There must be a strong force pulling upward on the hair because it overcomes the gravitational force pulling downward on the hair.
The balloon produces an electrostatic force on the hair that is greater then the gravitational force from Earth.
The electrostatic force and the gravitational force are both forces, but they act differently.

9. Electrostatic force
One way they are different is that hair is attracted only after the balloon has been rubbed; if you wait a while, the attractive property of the balloon diminishes.
The gravitational force, on the other hand, does not require rubbing and does not decrease.
You will become aware of other differences as we continue.

10. Like charges You can investigate electric charge using strips of tape.
Fold over about 5mm of the end of a 10cm strip of tape to use as a handle.
Stick the tape to your table.
Repeat with a 2nd piece of tape right next to the 1st .
Quickly pull both strips off and bring the NON-sticky sides near one each other.
What happens?

11. Like Charges
Pulling the tape off the table gives them the ability to repel each other.
The strips repel because they have accumulated electric charge.
The strips were prepared in the same way, so they have the same charge.
Two objects with like charges always repel each other.

12. Unlike Charges
Now stick one strip of tape on the table and place the second strip on top of the 1st.
Us the handle of the bottom strip to slowly pull both strips of the table together.
Rub them with the fingers of your other hand until they are no longer attracted to you.
By doing this, you discharge, or remove the charge from, the strips.

13. Unlike charges
Now, with one hand hold the handle of one strip, and the other hand holding the handle of the second strip, quickly pull the two strips apart.
What happens?
Bring them close to each other.
Do they still repel each other?

14. Unlike charges No, the strips are now attracted to each other.
Because the strips attract each other, you know they do not have the same charge; like charges repel.
This suggests that there is a second type of charge and that unlike charges attract.

15. Investigating charge
You can learn more about charge by doing additional investigations with two pieces of tape.
For example, try to determine whether the strength of attraction and repulsion changes with distance.
You will find that if you wait a while after charging your tape, especially in humid weather, the charge goes away.
You can recharge it by sticking it to the table and pulling it off again.

16. Interactions between charged objects
Can you use your tape to explore the charge of other objects?
Rub a plastic comb on your clothing and place the comb on the table.
Now, prepare your tape strips as you did before: stick one strip to the table and the 2nd strip on top of the 1st.
Label the handle end of the bottom strip B and the top T
Pull the pair off together.
Discharge them, and then pull them apart.

17. Interactions between charged objects
Now, holding your two strips of tape by the sticky ends, one in each hand, bring first one and then the other close to the comb (picture on pg 1275)
You will find that one strip will be attracted to the comb and the other repelled.

18. Interactions between charged objects
You can attach your charged tape strips to a desk to explore how they interact with other charged objects.
To charge objects, simply rub them with different types of material.
To charge glass, rub it with silk, wool, or plastic strap.

19. Interactions between charged objects
To explore how silk or wool behaves, slip a plastic bag over your hand.
Rub the cloth and then take you hand out of the bag.
Bring the bag and the cloth near the charged tape.
Did the charges objects always attract one strip and repel the other?
Did you ever find an object that attracted or repelled both?

20. Types of charge
From your investigations with two strips of tape, you can make two lists.
You can label one list B for objects that have the same charge as the tape you labeled B (repelled).
And you can label the other list T for objects with the same charge as tape T.
There are only 2 lists because there are only two types of charge.
Benjamin Franklin called them positive and negative charge.

21. Types of charge
Material have varying degrees of ability to acquire charge.
Hard rubber and plastic have a tendency to become negatively charged.
Glass and wool have a tendency to become positively charged.

22. Types of charge
Just as you showed that an uncharged pair of tape strips can gain unlike charges, you probably were able to show that if you rubbed plastic with wool, the plastic gained one type of charge and the wool gained the other type of charge.
The two kinds of charges did not appear alone, but in pairs.

23. Types of charge
These investigations suggest that matter normally contains both types of charges, positive and negative.
Rubbing the materials together separates the two types of charge.
To explore this further, consider a microscopic view.

24. A microscopic view of charge
Electric charges exist within atoms.
In 1897 JJ Thomson discovered that all materials contain low-mass, negatively charged particles.
These particles are called electrons.

25. A microscopic view of charge
Between 1909 and 1911, Ernest Rutherford, who had earlier worked as Thomson’s assistant, discovered that the atom had a massive, positively charged nucleus surrounded by a cloud of orbiting electrons.
From their experiments and many others, scientists know that atoms are normally neutral.
For a neutral object, the amount of negative charge exactly balances the amount of positive charge.

26. Transfer of electrons
With the addition of energy, the outer electrons of an atom can be removed from the atom.
An atom that is missing electrons has a net positive charge.
The freed electrons can remain unattached or they can become attached to other atoms, resulting in atoms with net negative charge.
From a microscopic viewpoint, acquiring charge is a process of transferring electrons.

27. Separation of charge
If you rub two neutral objects together, each object can become charged.
For instance, when you rub rubber shoes on a wool rug, the energy from the rubbing removes outer electrons from atoms in the wool and they transfer to the rubber shoe (Fig 4, pg 1278)

28. Separation of charge
The extra electrons on the shoe result in a net negative charge on the shoe.
The electrons missing from the rug result in a net positive charge on the rug.
The combined total charge of the two objects remains the same.
Charge is conserved, which is one way of saying that individual charges are never created or destroyed.
A net + or – charge means that electrons have been transferred.

29. Separation of charge
Processes inside a thundercloud can cause the cloud bottom to become negatively charged and the cloud top to become positively charged.
Electric charge can be transferred from a road to the car traveling on it.
The rubbing of your MP3 player in your pocked can sometimes cause your earbuds to become charged and shock you.
All of these involve separation but not creation of charges.

30. Conductors and insulators
Hold a plastic rod or comb at its midpoint and rub one end on your clothing.
You will find that only the rubbed end becomes charged.
In other works, the electrons that transferred to the plastic object stayed where they were; they did not move along the object.

31. Conductors and insulators
A material through which a charge will not move easily is called an electric insulator.
The strips of tape that you charged earlier acted as insulators because the charge that accumulated on them did not move; it stayed localized.
Glass, dry wood, most plastics, cloth and dry air are all

32. Conductors and insulators
Suppose you support a metal rod on an insulator so that it is completely surrounded by the insulator.
When you touch a charged comb to one end of the metal rod, you will find that the charge spreads very quickly over the entire rod.
A material that allows charges to move about easily is called an electric conductor.
Electrons move and thus conduct electric charge through the metal

33. Metals
From past experience, we know that most metals are good conductors.
This is because at lease one electron of each atom of a metal can be removed easily.
These electrons no longer remain with any particular atom, but move through the metal as a whole.
Fig 5 contrasts how excess charges behave on a conductor and on an insulator.

34. Metals
Copper and aluminum are both excellent conductors and are used commercially to carry electric charge.
Some nonmetal materials are also good conductors.
These include plasma, which consists of negative electrons and positive ions, and graphite.

35. Air as a conductor We said that air is an insulator.
Under certain conditions, charges can move through air as if it were a conductor.
The spark that jumps between your finger an a doorknob after you have rubbed your feet on a carpet discharges you.
In other words, you have become neutral because the excess charges have left you.

36. Air as a conductor Lightning does the same thing to a thunder cloud.
In both of these cases, air becomes a conductor for a brief moment.

37. Air as a conductor
Remember that electrons are free to move in conductors.
For a spark or lightning to occur, freely moving charged particles must be formed in the normally neutral air.
In the case of lightning, excess charges in the cloud and on the ground exert enough force to temporarily remove electrons from the molecules in the air.
The electrons and positively charged ions form a plasma, which is a conductor.