Chapter 23
Electric Fields

23.1 Properties of Electric Charges

23.3 Coulombs Law

23.4 The Electric Field

23.6 Electric Field Lines

23.7 Motion of Charged Particles in a Uniform Electric Field

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23.1 Properties of Electric Charges

There are two kinds of electric charges in nature:Positive Negative Like charges repel one another and Unlike charges attract one another.Electric charge is conserved.Charge is quantized q=Ne

e = 1.6 x 10-19 C

N is some integer

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23.3 Coulombs Law

From Coulombs experiments, we can generalize the following properties of the electric force between two stationary charged particles.

The electric force

is inversely proportional to the square of the separation r between the particles and directed along the line joining them. is proportional to the product of the charges q1 and q2 on the two particles.is attractive if the charges are of opposite sign and repulsive if the charges have the same sign.

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Consider two electric charges: q1 and q2The electric force F between these two charges separated by a distance r is given by Coulombs Law The constant ke is called Coulombs constant0 is the permittivity of free spaceThe smallest unit of charge e is the charge on an electron (-e) or a proton (+e) and has a magnitude e = 1.6 x 10-19 C

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Example 23.1 The Hydrogen Atom

The electron and proton of a hydrogen atom are separated (on the average) by a distance of approximately 5.3 x 10-11 m. Find the magnitudes of the electric force.

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When dealing with Coulombs law, you must remember that force is a vector quantityThe law expressed in vector form for the electric force exerted by a charge q1 on a second charge q2, written F12, iswhere r is a unit vector directed from q1 toward q2The electric force exerted by q2 on q1 is equal in magnitude to the force exerted by q1 on q2 and in the opposite direction; that is, F21= -F12.

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When more than two charges are present, the force between any pair of them is given by Equation Therefore, the resultant force on any one of them equals the vector sum of the forces exerted by the various individual charges. For example, if four charges are present, then the resultant force exerted by particles 2, 3, and 4 on particle 1 is

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Double one of the chargesforce doublesChange sign of one of the chargesforce changes directionChange sign of both chargesforce stays the sameDouble the distance between chargesforce four times weakerDouble both chargesforce four times stronger

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Example:

Three point charges are aligned along the x axis as shown. Find the electric force at the charge 3nC.

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Example 23.2 Find the Resultant Force

Consider three point charges located at the corners of a right triangle, where q1=q3= 5.0C, q2= 2.0 C, and a= 0.10 m. Find the resultant force exerted on q3.

Nadiah Alenazi

Nadiah Alenazi

Nadiah Alenazi

2

2

12

0

0

Nm

C

10

85

.

8

where

4

1

-

=

=

e

e

p

k