Electrostatics Or Opposites Attract 1m* Students know how to solve problems involving the forces between two electrical charges at a distance(Coulomb’

Electric Charge Like charges repel, opposite charges attract Charge due to relative numbers of protons and electrons in object; measured in coulombs. Charge is a scalar, conserved quantity Most things are neutral in charge Question: What is an electrical charge? What are the two types of charge and how do the number of protons and electrons compare for each? What do we mean by saying that electric charge is a conserved quantity? Activities: Demo: Show two types of charge by rubbing hard rubber rod with Easter Bunny fur and repelling similar charged rod suspended horizontally on string. Show second type of charge using fur to attract suspended rod. Comment that we currently attribute what we see here to excess and deficiencies in electrons. State that positive charge is believed to arise when the number of protons exceed the number of electrons and negative charge when the opposite is true. Discuss how charge is a conserved quantity meaning we start and end up with the same number and type of charges. It is there location that may change. Define a coulomb as the charge of 1.6 x 10 19 electrons or protons and note that charge is a scalar quantity without direction.

Check Question Why do we not see more evidence of the forces between electric charges in our everyday lives? Most things are neutral in electric charge Most things are positive in charge Most things are negative in charge The force between charges only acts at very small distances

Electric Force Electrical Force increases with charge Electrical force decreases as the square of distance Action at a distance force Stronger than gravity Felectricity= K Qsource Qtest / d2 B Question: Is the electrical force stronger or weaker than gravity? Is the electrical force between the following charged objects attractive, repulsive or is there no force between the objects: a) positive and negative b) negative and negative c) positive and positive d) neutral and positive e) neutral and negative Activities: Demonstrate that electric force is stronger than gravity by picking up small pieces of paper with rubber rod. Demonstrate that force is repulsive for like charges and attractive for opposite charges using horizontal rod set-up and tape. Demonstrate that force weakens with distance and increases with charge. Show slide Put charges on picture and figure out direction of force on each charge.

Check Question A positive and negative point charge attract one another with a force of 8 N. What will be the force between the charges if: One charge doubles Both charges double The distance between the charges is halved The distance between the charges is doubled One charge and the distance between the charges is doubled?

Conductors and Insulators Charge moves easily on surface Weak bonds between outer electrons and atomic nuclei Low resistance and high conductivity Examples include most metals Charge stays stationary once placed on surface Strong bonds between outer electrons and atomic nuclei High resistance and low conductivity Examples include plastics, rubber, air, glass, and most nonmetals Question: Are charges allowed to move freely over a conductor? How about an insulator? Compare conductors and insulators in terms of the bond strength between outer electrons and the nuclei of their atoms. List some examples of conductors? List some examples of insulators. Activities: Demonstrate different resistances to flow for different materials using an Ohm meter and the document camera. Define resistance as a resistance to the flow of charge over the surface. Discuss how some materials allow charge to move easily over their surface. These materials are called conductors and they have a low resistance and high conductivity. It is believed that they have this property because they have weak bonds between the outer electrons of there atoms and their atomic nuclei. The opposite is true for insulators. Show slide

Check Question Which of the spheres pictured is a conductor and which is an insulator? Both spheres are insulators Both spheres are conductors The upper sphere is a conductor, the lower is an insulator The upper sphere is an insulator, the lower is a conductor +++ + + +

Methods of Charging Conduction: charging by touching to charged object Induction: Charging without touching to charged object Charging by grounding Two conducting spheres example Polarization: Charging by rearranging polar molecules in substance. Question: What type of charging requires contact with a charged object? How about charging an object without touching it to a charged object? How about charging a polar material by rearranging its molecules using an outside charge? Demo charging by conduction using the charged insulating rod and the electroscope. Demonstrate charging by induction using the fun fly stick with first a can and then the mylar fliers. Go through the two transparencies showing charging by induction. Mak sure to point out that charge moving on two connected conductors does not stop at junction. Demonstrate Charge polarization using balloon and white board. Yes I know its cheating. Draw picture of polar molecules before and after balloon is introduced. Mention that water will be attracted to charge since it is polar which is used in some Las Vegas fountain displays.

Lightning Occurs due to charge build up from air and water molecules rubbing against one another (hypothesis needs work) Induces opposite charge on ground Discharge occurs when electric force between opposite charges overcomes resistance of air Lightning Rod Gives lightning direct path to ground without going through building Charge leakage off point may prevent some lightning strikes if rod contacts cloud Question: What is the cause of lightning? How does a lightning rod prevent lightning? What other service does a lightning rod provide? Activities: Show Hewitt video on Electrostatics starting with electrophorus and finishingwith discussion of lightning rod on churches. Show slide and point out that lightning rod also provides a safe path to ground for the charge.