# Chapter 17 Electric Forces and Fields 17-1 Electric Charge Properties of Electric Charge  Ancient Greeks discovered static charge when they observed amber.

## Presentation on theme: "Chapter 17 Electric Forces and Fields 17-1 Electric Charge Properties of Electric Charge  Ancient Greeks discovered static charge when they observed amber."— Presentation transcript:

Chapter 17 Electric Forces and Fields 17-1 Electric Charge Properties of Electric Charge  Ancient Greeks discovered static charge when they observed amber rubbed with wool  ‘Electricity’ comes from the Greek and Latin words for amber.

Benjamin Franklin (1752) Credited with being the first to discover that lightning and thunder are the result of electrical charges.

2 Kinds of Electric Charge  Both were named by Ben Franklin 1. Positive charge (+) 2. Negative Charge (-)  Like charges repel  Opposite charges attract 2 like charges repel

Electrostatics  Involves electric charges, forces between them, and their behavior in materials

Electric charge is conserved  Electrons are transferred from one object to another when they come into contact  One object becomes negatively charged  One object becomes positively charged *** No charge is created or destroyed  The overall charge remains the same  The overall charge remains the same

Rubbing a balloon – transfer of charge  Charge transferred by unlike materials  Positive charge on hair is equal to negative charge on balloon – charge is conserved. Chitin – organic compound (+) found crabs, lobsters, used in cosmetics

Van de Graaff Generators use a moving belt to accumulate a large amount of electric charge on a round metal sphere. Can generate up to 100,000 Volts.

Van de Graff Generator

Electrostatic painting – paint is positively charged and wraps around negatively charged metals

Objects can be charged by rubbing (through touch) or Induction (holding a charged object near another object) Inducing a charge

Quantized Charge

Millikan’s oil droplet experiment determined that charged is “quantized” Millikan's Oil Drop Experiment Charge and mass of an electron

 Millikan’s Experiment – showed charge and mass of electrons  Electric charge is measured in Coulombs (C)  SI unit of charge and is symbolized by “e”  Quantized = occurs in discrete amounts +e, +2e, +3e, etc. +e, +2e, +3e, etc. * Electrons have a charge of – e * Protons have a charge of + e 1e = -1.60 x 10 -19 C 1 p = +1.60 x 10 -19 C 1 p = +1.60 x 10 -19 C

Question: If the charge of an electron is -1.60 x 10 -19 C, How many excess electrons are in lightning bolt that has a total charge of 15 C? 15C = 1 electron -1.60 x 10 -19 C = 9.4 x 10 19 electrons

17-2 Electric Force  Electric Force – two charged object exert a force on the other object. Much stronger than gravitational force  Either an attractive or repulsive force (unlike gravitational force) Coulomb’s law:  Electric Force is proportional to the product of the two charges and inversely proportional to the square of the distance between the two charges. K c = Coulomb’s Constant = 8.99 x 10 9 N x m 2 / C 2

Example: Consider the Forces F 1 = F and F 2 = -F acting on two charged particles separated by a distance, d. Explain the change in the forces exerted on each particle under the following conditions. a)The distance between the two particles doubles? b)The charge on one particle doubles? c)The charge on each particle doubles? d)The charge on each particle AND the distance between them doubles?

Example: Consider the Forces F 1 = F and F 2 = -F acting on two charged particles separated by a distance, d. Explain the change in the forces exerted on each particle under the following conditions. a) The distance between the two particles doubles? F 1 = ¼ and F 2 = - ¼ b) The charge on one particle doubles? F 1 = 2F and F 2 = -2F c) The charge on each particle doubles? F 1 = 4F and F 2 = -4F a)The charge on each particle AND the distance between them doubles? F 1 = F and F 2 = -F

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