# 1 - Electric Charge and Field Chapter 16(Giancoli All sections except 16.10(Gauss's law Comparison between the Electric and the Gravitational

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1 Electric Charge and Field • Chapter 16 (Giancoli) • All sections except 16.10 (Gauss’s law) Comparison between the Electric and the Gravitational forces Both have long range, The electric charge of an object plays the same role in determining the electric force as does the object’s mass in the gravitational force Both are central forces (forces act along the line joining the objects) 2 1 r F • But! Mass is always a positive quantity the gravitational force is always an attractive force. Electric charge can be positive or negative i.e. the electric force can be attractive or repulsive A very useful rule for determining the direction of the force: “Like charges repel, unlike charges attract” Strength The electric forces exerted by one part of an atom on another part are much stronger than their gravitational attraction. i.e. For atomic/subatomic particles, the gravitational force is usually neglected . (We shall see an example later on) The law of conservation of charge Total amount of charge in an isolated system remains constant. “One cannot create or destroy a charge” Example : The decay of the neutron. An isolated neutron decays in about 15 minutes. 0 0 v e p n + + + (neutron) (proton) (electron) (neutrino) (Total charge is zero before and after the decay) The Electronic Charge The magnitude of the smallest (elementary) free charge is the charge on the electron or the proton. This charge is denoted by e . By convention: Charge on proton = + e Charge on electron = –e In the SI system, the unit of charge is the coulomb [C]. e = 1.6 × 10 -19 C © Z. Altounian

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2 Coulomb’s Law : Provides a description for the electric force. If two charges, Q 1 and Q 2 are separated by a distance r Q 1 Q 2 r the magnitude of the force acting on any charge is proportional to the product of the two charges. It is also proportional to the inverse square of the distance between the charges. ) 2 1 Q Q (F ) r 1 (F 2 Putting these together, This is Coulomb’s Law and gives the magnitude of the force between two charged particles. k, the proportionality constant, is given by
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## This note was uploaded on 04/07/2008 for the course PHI 102 taught by Professor Altonian during the Spring '08 term at McGill.

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1 - Electric Charge and Field Chapter 16(Giancoli All sections except 16.10(Gauss's law Comparison between the Electric and the Gravitational

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