Chapter 20 & 21: Electrostatics

There are four forces that exist in nature: ______, that is, 1. ______they only act over very small distances. 2. ______3. ______and can act over very large distances. 4. ______

Rules of Electrostatics: 1. There are two kinds of charge that exist in nature (______charge and ______charge) a. ______one another b. ______one another. 2. The ______between charges varies as the inverse square of the ______, and directly with the ______. (______) 3. ______4. ______(quantized means small discrete packets that can not be further subdivided. For example you can have one electron or 2 electrons, but never half an electron) Charge:  The basic unit of ______charge is the ______. (Although protons are ultimately made up of quarks)  The basic unit of ______charge is the ______. It is almost always electrons that are moving when charge “______”  The SI unit of charge is the ______(C).  Charge of 1e- = 1 proton = 1.6x10-19 Coulombs

Conductors and Insulators:  ______are materials in which ______are free to ______. Example: ______ ______are materials in which ______. Examples: ______How to Charge Objects Consider the “pith” ball (pith is a material kind of like cork). Why is the neutral pith ball attracted to the negatively charged rod? Why does it bounce away?

Consider the electroscope (an electroscope measures charge):  Charge by ______- a physical transfer of charge from a charged object. How does this work?

 Charge by ______- A transfer of charge, but only two neutral objects touch. How does this work?

Coulomb’s Law Where k = 9.0x109 N m2/C2 (Coulomb’s Constant) d = Distance between charges (m) Q = Charges (C) Coulomb’s Law describes the forces that: 1. Binds ______to the nucleus. 2. Binds ______to form molecules. 3. Binds ______to form solids and liquids.

Example: Two charges, ______and ______are 5.0 m apart. Calculate the force between them.

Example: Find the net force on the 5.0 nC charge. (Draw Picture) Example: Two point charges are 4 cm apart. They are moved to a new separation of 2 cm. By what factor does the resulting mutual force between them change?

Example: If the size of the charge value is tripled for both of two point charges maintained at a constant separation, the mutual force between them will be changed by what factor?

The Electric Field: • A field is an area or volume that has a ______. • The number can be ______or ______. • Any ______around it. • Any other ______in that electric field will experience an ______& will either be attracted or repelled by the original charge. • Electric field is defined as the force per ______charge. Masses only attract so all gravitational fields point towards the mass creating them.

The Test Charge • To determine the direction of the electric field, a test charge is used. • A test charge is a pretend ______point charge that is infinitely small and has a very very small positive charge so it will not disrupt the field it is in at all. • The test charge is always ______, never negative.

Drawing Electric Field Lines Draw the electric field for A positive Charge A negative Charge: Rules for Drawing Electric Fields:  The lines must ______on ______charges and ______on ______charges, or at infinity.  The ______drawn leaving a positive charge or approaching a negative charge is ______to the amount of charge.  Field lines may not cross or touch each other.  Field lines must meet conductors or charges perpendicular to the surface of the conductor or charge.  Example 2 Charges: Example Charge & Plate:

The electric field produced by a point or spherical charge is given by k = Coulomb’s Constant (9.0x109 N m2/C2) Q = The charge producing the field. Given in Coulombs d = The distance to the point in question The ______of the ______is based on the ______of ______for a positive charge.

Another way to calculate the electric FORCE: F = Electric Force (N) E = Electric Field (N/C) Q = Charge placed in the field (C) Example: What is the electric field 20.00 m away from a (__) ______point charge?

Example: Two charges are along the x-axis. Q1 is 3.0 m from the origin and has a charge of ______. Q2 is 4.5 m from the origin and has a charge of ______. (all charges are along the positive x-axis) a) Calculate the electric field 8.0 m from the origin. (DRAW THE PICTURE) b) b) What force will a ______charge experience if it is placed 8.0 m from the origin?

Example: Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that ___ field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to field lines?

Measuring the Charge on an Electron – Millikan’s Oil Drop Experiment  In 1909 Robert Millikan measured the charge of an electron using an oil drop experiment  In 1923 he received the Nobel Prize for his work.  Millikan shot X-rays at a spray of oil drops, giving the oil drops a negative charge. The charged oil drops then went into an electric field controlled by Millikan.  The oil drops were then attracted to the positively charged plate which created the field.  Millikan varied the electric field in between the plates, until the electric force on the “target” oil drop balanced the force of gravity and the oil drop stayed suspended between the plates. Example: A 3.2x10-8 kg oil drop is suspended in an electric field of strength ______N/C. (a) What is the charge on the oil drop? (b) how many extra electrons does the oil drop have?

Cool things electrostatics explains: 1. Shocking fingers & lightning rods  On a ______shaped object, charges are ______spread.  On an irregularly shaped object, charge tends to accumulate at areas of ______curvature/smallest radii.  In other words, charge accumulates at ______. 2. Faraday Cage  The Electric Field inside a conducting surface is ______o Conducting cup on a stand: (draw the pictures, there are 3) Explanation: The negative charges from the polarized inside get neutralized as the positive ball comes in contact with them. The charge from the positive ball is now left on the outside of the cup. o Negative rod with a conducting sphere: (draw the pictures, there are 3)

 A faraday Cage is a ______enclosure in which charge will always flow to the outside, thus leaving the inside ______.  Not only do Faraday cages block ______, they more importantly block ______.  Applications of a faraday cage: