Static Electricity Review https://youtu.be/yc2-363MIQs

Whiteboard Review Questions

Whiteboard Practice An initially neutral balloon acquires negative charge after it is rubbed against a man’s hair (which was also initially neutral). What charge does the man’s hair have now: positive, negative, or neutral? Why?

Whiteboard Practice What is the difference between a material that is a good insulator and a material that is a good conductor, in terms of its atoms?

Whiteboard Practice A neutral conducting sphere is connected to the ground with a conducting wire. A negatively charged rod is brought near the sphere and then leaves without touching it. What charge does the sphere have on it now?

Whiteboard Practice A negatively-charged balloon is brought near a neutral electroscope, but does not touch it. Illustrate what happens to the charges inside the electroscope and how the leaves at the bottom will react.

Whiteboard Practice Illustrate what happens to an aluminum can when a negatively-charged balloon is brought next to it. Is it attracted or repelled?

Whiteboard Practice Illustrate what happens to an aluminum can when a positively-charged balloon is brought next to it. Is it attracted or repelled?

Whiteboard Practice Illustrate what happens to a piece of wood (an insulator) when a negatively-charged comb is brought next to it. Is it attracted or repelled?

Whiteboard Practice Two charged objects attract each other. If the amount of charge on one object doubles and the other triples, what would happen to the electrostatic force between them?

Whiteboard Practice Two charged objects repel each other. If the amount of charge on one object is reduced by a factor of 4 and the distance between the objects is reduced by a factor of 8, what would happen to the electrostatic force between them?

Whiteboard Practice Make your own problem. You may change either or both of the charges and/or the distance by any factor.

Coulomb’s Law Mathematics Electrostatic Force Mathematics

Charge is measured in “Coulombs” The unit for how much electrical charge is measured in Coulombs (C). A proton has a charge of 1.60 x 10-19 C. An electron has a charge of -1.60 x 10-19 C.

Charge is measured in “Coulombs” The unit for how much electrical charge is measured in Coulombs (C). qp = +1.60 x 10-19 C qe = -1.60 x 10-19 C

What is a “Coulomb?” -1.6 x 10-19 C / electron How many electrons does it take to make a charge of -1.0 C? -1.6 x 10-19 C / electron

What is a “Coulomb?” -1.0 C -1.6 x 10-19 C / electron How many electrons does it take to make a charge of -1.0 C? -1.0 C -1.6 x 10-19 C / electron

What is a “Coulomb?” -1.0 C = 6.25 x 10-18 electrons How many electrons does it take to make a charge of -1.0 C? -1.0 C = 6.25 x 10-18 electrons -1.6 x 10-19 C / electron

What is a “Coulomb?” -1.0 C = 6.25 x 10-18 electrons How many electrons does it take to make a charge of -1.0 C? -1.0 C = 6.25 x 10-18 electrons -1.6 x 10-19 C / electron Often, we represent charges in terms of “micro-Coulombs” (μC = 10-6 C) or “nano-Coulombs” (nC = 10-9 C)

Coulomb’s Law Math Examples

Example 1: What’s the force between an electron and a proton that are 1 x 10-10 m apart?

Example 2: Two charges, +6.0nC each, repel each other with a force of 1.0 N. How far apart are they?