Electrostatics Review EQ: What causes you to get an “electric shock” when you walk across the carpet on a cold winter’s day and reach for the door knob?

Electrostatics Clothes tumble in the dryer and cling together. You walk across the carpeting to exit a room and receive a door knob shock. You pull a wool sweater off at the end of the day and see sparks of electricity. During the dryness of winter, you step out of your car and receive a car door shock as you try to close the door. Sparks of electricity are seen as you pull a wool blanket off the sheets of your bed. You stroke your cat's fur and observe the fur standing up on its end. Bolts of lightning dash across the evening sky during a spring thunderstorm. And most tragic of all, you have a bad hair day. These are all static electricity events - events that can only be explained by an understanding of the physics of electrostatics.

Electrostatics Electrostatics: The study of electric charges that can be collected and held in one place. The study of static electricity, where static electricity is electricity that is confined to one area.

Charge & Mass

Charged Objects After Rubbing: Positively Charged Glass and wool Negatively Charged Hard rubber and plastics Charges are not created! Like ChargesRepel Opposite ChargesAttract

Conductors and Insulators Insulator A material through which a charge will not move easily. Conductor A material that allows charges to move about easily. Air can be a conductor Lightning Storms

Theorems of Electrostatics 1. All unbalanced charge flows to the outside surface of a conductor. 2. Charge density is higher near corners, points. 3. There is no unbalanced charge inside of conductors 4. Charge will flow from a point of higher density to a point of lower density until the charge densities at the two points are equal.

Electrostatic Force How strong is Electric Force? Compare it to Gravity! What do we Know? There are two kinds of electric charges: positive and negative. Charges exert forces on other charges at a distance. The force is stronger when the charges are closer together. Like charges repel; opposite charges attract

Charging Friction – charging a body by rubbing. Conduction – charging a neutral body by touching it with a charged body. Induction – charging an object without touching it. Grounding – the process of connecting a body to Earth to eliminate excess charge.

Coulomb’s Law The force that act between two or more charged objects. Force depends on distance Force depends on charge Charge denoted by “q” and the distance between charges denoted by “r”.

Coulomb’s Law The magnitude of the force between charge q A and charge q B, separated by a distance r is proportional to the magnitude of the charges and inversely proportional to the square of the distance between them. F is proportional to q A q B r 2

Coulomb’s Law The unit of charge is a Coulomb “q” symbol for charge 1 Coulomb = 6.24X10 18 electrons or protons Coulomb’s Constant K = 9 X10 9 Nm 2 C 2 F = q A q B r 2 The force between two charges is equal to Coulomb’s constant, times the product of the two charges, divided by the square of the distance between them. K

Electric Field A property associated with the space around a charged object. Its interaction with another charged object in that field is manifested in the electric or Coulomb Force. We represent the E-Field graphically with rays.

Electric Field The field flows outward, away from a positively - charged object. The field flows inward, towards a negatively -charged object. Strength is indicated by spacing of lines

Electric Field The strength of an electric field is equal to the force on a positive test charge divided by the strength of the test charge E = F q’ E is the electric field intensity generated by a field charge q q’ is the small test charge placed in the E-field to measure the strength of the field at some point q’<<<q F is the Coulomb or electric force generated by the field on the test charge q’ Units N/C

Electric Field A charge (q), known as the field charge generates the electric field (E). IN ORDER TO MEASURE THIS FIELD we introduce a second significantly smaller charge (q’), called the test charge, into the field and observe the resultant Coulomb Force (F) exerted on this test charge. Although we may be able to deduce things about it, we never actually know anything about the field charge itself. An alternative formula may be derived by substitution of the Coulomb’s Force equation into the electric field equation, resulting in E = K q r 2 Where the strength of the electric field at some distance (r) from a field charge (q) may be determined.

Comparison of F G and F Q F = Kq 1 q 2 F = G m 1 m 2 d 2 d 2