Lesson #15 Topic: Electrostatics Objectives: (After this class I will be able to) 1. Observe an example of electrostatics 2. Define and state the units of charge 3. Describe electric force 10/19/06 Warm Up: What makes the atoms of the desk stay together? Why doesn’t gravity make all of the atoms fall to the earth? Assignment: Hewitt Ch32 RQ # 2,3,4,6,9,13,17,21,24; T&E # 1;

Exploring 1. Why does the balloon stick to the wall? Form a hypothesis and be able to explain your reasoning. 2. Create a demonstration that shows that like charges repel one another. 3. The mass of a proton = 1.673x kg The mass of a neutron= 1.675x kg The mass of an electron = 9.11x kg There are approximately equal numbers of each particle in your body. Find the number of protons in your body.

Electrostatics Electrostatics means “electricity at rest” Electrostatics means “electricity at rest” In chemistry we learned that all atoms are made of protons, neutrons, and electrons. In chemistry we learned that all atoms are made of protons, neutrons, and electrons. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. So an atom with an equal number of protons and electrons has no charge. So an atom with an equal number of protons and electrons has no charge. Atoms have charge when they have a deficiency or surplus of electrons. Atoms have charge when they have a deficiency or surplus of electrons.

Electrostatics Example: Cl -1 = a chlorine atom with an extra electron. Example: Cl -1 = a chlorine atom with an extra electron. Example: Na +1 = a sodium atom missing an electron. Example: Na +1 = a sodium atom missing an electron. These “ions” attract one another and create molecules. These “ions” attract one another and create molecules. Sounds like a bunch of icky chemistry to me… Sounds like a bunch of icky chemistry to me… In physics we deal not with charged atoms, but the overall charge of a regular size object. In physics we deal not with charged atoms, but the overall charge of a regular size object. That means millions and millions of charged atoms. That means millions and millions of charged atoms.

Electrostatics So a Cl - atom will attract and bond with a Na + atom… What makes them move together? So a Cl - atom will attract and bond with a Na + atom… What makes them move together? Newton’s laws hold true not only for galaxies, planets, and people, but for atoms as well. Newton’s laws hold true not only for galaxies, planets, and people, but for atoms as well. For an atom at rest to move to another atom, some force must act on it. For an atom at rest to move to another atom, some force must act on it. The force that makes these two objects bond together is called the electromagnetic force. The force that makes these two objects bond together is called the electromagnetic force. This is our second fundamental force of nature. This is our second fundamental force of nature. We will later learn why it is called electromagnetic, but for now we will treat it as purely electric. We will later learn why it is called electromagnetic, but for now we will treat it as purely electric.

Electric Force The electric force is what bonds and holds all molecules together. The electric force is what bonds and holds all molecules together. This force is tremendously stronger than the force of gravity, but has a very similar relationship. This force is tremendously stronger than the force of gravity, but has a very similar relationship. This relationship was found in the late 18 th century by French physicist Charles Coulomb. This relationship was found in the late 18 th century by French physicist Charles Coulomb. The force of attraction (or repulsion) between any two charged objects is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. The force of attraction (or repulsion) between any two charged objects is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

Electric Force q = charge of an object q = charge of an object r = distance between charges r = distance between charges k = Boltzman constant k = Boltzman constant The constant k was found with a very similar apparatus as the one used to determine the gravitational constant G. It just used charged metal balls instead of heavy ones. The constant k was found with a very similar apparatus as the one used to determine the gravitational constant G. It just used charged metal balls instead of heavy ones. k = 8.99 x 10 9 Nm 2 /C 2 k = 8.99 x 10 9 Nm 2 /C 2 Looks a lot like the law of Universal Gravitation, but what the heck is q? Looks a lot like the law of Universal Gravitation, but what the heck is q?

Charge How do we know how much charge an object holds? How do we know how much charge an object holds? By determining how much excess or lack of electrons it has. By determining how much excess or lack of electrons it has. 1 Coulomb of charge is determined to be x10 18 excess electrons or lack of electrons (protons). 1 Coulomb of charge is determined to be x10 18 excess electrons or lack of electrons (protons). So the charge of an electron = x C So the charge of an electron = x C And the charge of a proton = x C And the charge of a proton = x C Ok… so how does one go about getting “charged”? Ok… so how does one go about getting “charged”?

Lesson #16 Topic: Net Charge and Electric Force Objectives: (After this class I will be able to) 1. Explain net charge and the conservation of charge 2. Describe polarization and charging by contact and induction 3. Solve electric force problems 10/22/06 Warm Up: What causes certain objects to become charged? Assignment: “ Electric Force”

Net Charge All objects have a tremendous amount of charge within them, however most of the positive charge is canceled by the presence of an equal amount of negative charge. All objects have a tremendous amount of charge within them, however most of the positive charge is canceled by the presence of an equal amount of negative charge. A net charge occurs when there is an imbalance of positive and negative charge within an object. A net charge occurs when there is an imbalance of positive and negative charge within an object. What would be your net charge if 10% of all the electrons in your body suddenly disappeared? What would be your net charge if 10% of all the electrons in your body suddenly disappeared? A net charge can be formed by contact, friction, or induction. (Not by disappearing electrons) A net charge can be formed by contact, friction, or induction. (Not by disappearing electrons)

Charging by Friction An insulator is made of material that has bound electrons that cannot move. An insulator is made of material that has bound electrons that cannot move. Examples: Rubber, plastic, glass. Examples: Rubber, plastic, glass. Any objects made of this material can be rubbed against something to build up charge. Any objects made of this material can be rubbed against something to build up charge. Electrons cannot easily move throughout the material and electrons pile up on top of one another. Electrons cannot easily move throughout the material and electrons pile up on top of one another. Demo: Rub a plastic rod with fur and notice the effects. Demo: Rub a plastic rod with fur and notice the effects.

Charging by Contact Conductors and semi-conductors are objects made of a material that allows electrons to flow freely throughout the material. Conductors and semi-conductors are objects made of a material that allows electrons to flow freely throughout the material. Examples: Metal, carbon, some forms of rock. Examples: Metal, carbon, some forms of rock. Any objects made of this material can be touched to another object of this material, and electrons will flow from one object to another. Any objects made of this material can be touched to another object of this material, and electrons will flow from one object to another. This will distribute the charge evenly throughout both objects. This will distribute the charge evenly throughout both objects. Demo: Van De Graph generator Demo: Van De Graph generator

Charging by Induction Conductors can charge other conductors with an opposite charge through polarization and grounding. Conductors can charge other conductors with an opposite charge through polarization and grounding. Polarization is when electrons flee to one side of an object or the other side of an object depending on where they are being forced by another charged object. Polarization is when electrons flee to one side of an object or the other side of an object depending on where they are being forced by another charged object. Grounding is when the conductor is attached to the ground such that the electrons can escape into the earth. Grounding is when the conductor is attached to the ground such that the electrons can escape into the earth. The earth’s charge is always neutral. The earth’s charge is always neutral.

Contact vs. Induction when charging a conductor Contact Induction

Lesson #17 Topic: Lab: Charging an Electroscope Objectives: (After this class I will be able to) 1. Experiment with an electroscope 2. Write a report on the different ways to charge an object. 3. Describe how the electric force is both attractive and repulsive 10/24/06 Task: Charge an electroscope by contact and then by induction. Question: How can you tell when the electroscope is charged? Assignment: Lab report due tomorrow

Electroscope lab On top of the usual lab report, be sure to include the following… On top of the usual lab report, be sure to include the following… How do you know when the electroscope is charged? How do you know when the electroscope is charged? How did you charge it by contact? By induction? How did you charge it by contact? By induction? Explain what is happening on an atomic level. Explain what is happening on an atomic level. Explain why the electroscope reacts as it does. Explain why the electroscope reacts as it does. When does it have a positive charge? When does it have a positive charge? When does it have a negative charge? When does it have a negative charge? How did you discharge it? How did you discharge it? What did you learn from these experiments? What did you learn from these experiments?