Chapter 20 Static Electricity

20.1 Electrical Charge Charged objects –Like charges –Opposite charges –Experimenting with charge –Types of charge

Microscopic View of Charge –Exist within atoms –Separation of charge –Charge is never created or destroyed Charges are only separated due to movement of electrons.

Conductors and Insulators –Insulators Materials in which charges will not move easily Glass, dry wood, most plastics, cloth, and dry air are all good insulators. –Conductors Material which allow the free movement of electrons. Electrons carry, or conduct, electric charge through the conductor. Metals are good conductors because at least one electron on each atom can be removed easily.

When air becomes a conductor –Under certain conditions air can become a conductor. –Lightning is one example. –Your finger on a doorknob after walking across a carpet is another. –Plasma forms as the air becomes charged.

20.2 Electrical Force Forces on charged bodies –There are two kinds of charges, positive and negative. –Charges can exert force on other charges over a distance. –The force is stronger when the charges are close together. –Like charges repel; opposites attract.

Charging by conduction –Charging a neutral body by touching it with a charged body. Separation of charge on a neutral object –Recall the electroscope leaves moving as a charged object is brought near. Charging by induction –Charge separation can be used to charge an object without touching it with the charged object.

Coulomb’s Law Coulomb found how the force between two charged spheres depended on the distance between them. –The symbol for charge is q –Coulomb showed that the force F varied with the inverse square of the distance between the spheres. –He also found that the force varied directly with the charge of the bodies.

One coulomb is the charge of 6.25 X electrons or protons. The charge on an individual electron is 1.60 X According to Coulomb’s law, the magnitude of the force on a charge q A caused by charge q B a distance d away can be written as follows The unit of charge: The coulomb

Coulomb’s Law –When the charges are measured in coulombs, the distance is measured in meters, and the force is measured in newtons, the constant, K, is 9.0 X 10 9 Nm 2 /C 2 F = K q A q B F = K q A q B d 2 d 2

PSS Sketch the system showing all distances and angles Diagram the vectors of the system; include derived vectors using dashed lines Use Coulomb’s law to find the magnitude of the force

Use your diagram along with the trigonometric relations to find the direction of the force. Perform all algebraic operations as well as the numbers. Make sure the units match the variable in question. Consider the magnitude of the answer. Is it reasonable?