1. Electric Charge

2. Goal of the class
To understand the basic properties of electric charge and differentiate between conductors and insulators.
Question of the Day: What did Millikan's oil drop experiment discover?
Previous answer: monochromatic light where d sin θ = ±mλ for a bright fringe
Previous question: What are the conditions required for interference to occur?

3. Question What happens when we rub a balloon on our hair?
Why do you think that is?
In the top picture, the girl has rubbed the balloon on her hair, and now there is a force of attraction between them. Normally, a balloon and hair would not attract each other.
What happened to each to produce this force?
In the lower picture, the two balloons are repelling each other.
How was this force of repulsion produced?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
Students may say the cause is “static electricity.” Ask them what that means. If they say “opposites attract,” ask them what is “opposite” about the balloon and the hair, and how that might have occurred. Try to gather as many ideas as possible about the nature of static electricity.
Next stick the balloon to the ceiling and ask why it sticks.
This is a little harder as the wall isn’t charged.
Why does it fall after a long time?

4. Electric Charge This is called the law of charges
There are two types of charge, positive and negative.
Like charges repel.
Positive and positive
Negative and negative
The two balloons
Opposite charges attract.
Positive and negative
The balloon and the hair.
This is called the law of charges
Mono means 1 and chrome means colour

5. Origin of Electric Charge
Atoms are made of subatomic particles called protons, electrons and neutrons.
Electrons can be easily transferred from one atom to another.
Stress the following:
Atoms are neutral because they have equal numbers of protons (+) and electrons (-).
Only electrons are free to move.
Positive charge occurs because electrons are lost and the positive charges remain behind.
The amount of + charge always equals the amount of - charge because protons and electrons have equal charges (just opposite signs).
Some materials attract electrons more strongly than others, so when contact occurs, the electrons are transferred. In this case, the balloon attracts electrons more strongly than the hair.
Draw balloon –ve and hair +ve

6. Millikan's Oil Drop Experiment
Millikan sprayed oil drops between charged metal plates.
The oil drops were negatively charged by friction.
By adjusting the voltage on the plates, he could make the drops rise and fall.
In 1909, physicist Robert Millikan determined the charge of the electron with his famous oil drop experiment.
I did this lab at uni and people had extreme eye pain after it.

7. Millikan’s Results
Millikan found that the amount of charge on objects was always a multiple of some fundamental charge (e).
In other words, charge is quantized.
e turned out to be the amount of charge on an electron, where e =  coulombs.
When doing the Millikan oil drop experiment, every oil drop has a charge that is some multiple of -1.6 x C. The drops might have a charge of -3.2 x C or -9.6 x C or -4.8 x C, but there is NEVER an oil drop with a charge of x C or -7.0 x C. This suggests that charge can only come in certain values (is quantized) and those values are all multiples of the basic charge of x C.

8. Conductors and Insulators
What is meant by the term electrical conductor?
Provide a few examples.
What is meant by the term electrical insulator?
Why do conductors and insulators behave differently?
Conductors allow electrons to flow freely through them.
Silver, copper, aluminum, and other metals
Conductors allow electrons to flow freely through them.
Silver, copper, aluminum, and other metals
Electrons do not flow freely though insulators.
Plastic, rubber, glass
Outer electrons in metals are loosely bound to the nucleus and relatively free to move.
Electrons do not flow freely though insulators.
Plastic, rubber, glass

9. Metallic Bonding
The valence electrons in metals are delocalised and are free to roam.
These electrons are what carries electrical current in wires
Talk about electron microscopes

10. Charging by Contact
Both insulators and conductors can be charged by contact.
Rubbing two materials together results in a transfer of electrons.
When charging metal, the charge may move through your body into the ground.
The metal and your body are conductors, so the charge moves through them.
You must hold the conductor with an insulating material, such as rubber gloves, to keep the charge on the metal.
This is why people get electrocuted.
It’s also why wires are coated with plastic so they don’t conduct.

11. Charging by Induction
A charged rod is held near a metal sphere. Why do the charges in the metal arrange themselves as shown?
The metal sphere is connected to the ground with a conductor. Why did some of the electrons move off the sphere?
The negative charges (electrons) are repelled and the positive charges (protons) are attracted. The negatives move because it is a metal and the electrons are loosely bound.
The metal sphere is grounded (touching it with your finger will do nicely). Because they are repelled by the charged rubber rod and free to move, some of the electrons move off the sphere into the ground. (Because Earth is so large, it can absorb a large number of electrons without becoming locally charged to any degree.)

12. Charging by Induction
The conductor connecting the sphere to ground is removed. What type of net charge does the sphere now possess?
The negatively charged rod is removed. Why do the charges move into the positions shown?
The sphere is now positively charged because it has excess protons (due to the loss of electrons).
The electrons repel each other and are attracted to the protons, so they move back into stable positions.
Emphasize that charging by induction leaves the object with an opposite charge, and no contact occurs between the rod and the object being charged. This can only occur with conductors because insulators do not allow the free movement of electrons to the other side or to the ground.
Additional questions to ask:
Would the sphere be charged if the rubber rod was removed before the ground was removed? Why or why not? (No, because the electrons would be attracted by the extra + charges and as a result they would move back into the sphere.)
Have students sketch the four diagrams over with just one change: make the rod a positively-charged glass rod. Students should show some electrons moving to the left, extra electrons entering the sphere because they are attracted to the positive charges, the ground removed, and the sphere left with extra electrons spread around evenly. The sphere is now negatively charged.

13. Surface Charges Why does a charged balloon stick to the wall?
A positive surface charge is induced on the wall by the negatively-charged balloon.
Electrons shift within atoms due to attraction or repulsion.
The insulator does not have a net charge.
The diagram shows the opposite case.
Why can a charged balloon pick up little pieces of paper?
Be sure students understand that the diagram shows a positively-charged object inducing a surface charge on a neutral object. This is opposite the behavior of the balloon and the wall because the balloon is negatively charged.
Note that the positively-charged object shown in the diagram will be attracted to the negatives in the wall and repelled by the positives in the wall. Ask the students why the net effect is one of attraction. (Because the negative charges in the wall are closer, the attractive force is stronger.)
The comb picks up paper pieces by the same principle. The charged comb induces charges on the surfaces of the paper pieces, and the charged pieces are then attracted to the comb.
The PhET web site has two interesting simulations:
Choose “Go to Simulations,” then “Electricity and Magnets,” and then choose either “Balloons and Static Electricity” or “John Travoltage.”
The “Balloons and Static Electricity” simulation allows you to rub the balloon on a sweater and pick up static charge. The balloon can then be released and it will go toward the sweater (opposites attract) or toward the wall (an induced charge on wall’s surface will attract the balloon as it gets close). Once the balloon sticks to the sweater or wall you can pull it away and release it again.
The “John Travoltage” simulation simply shows the build up of charge from rubbing your feet against the carpet and then the discharge into a door handle.

14. Homework
Please complete questions on page 571
Q 1, 2, 3, 5, 7