Nadiah Alenazi 1 Chapter 23 Electric Fields 23.1 Properties of Electric Charges 23.3 Coulomb ’ s Law 23.4 The Electric Field 23.6 Electric Field Lines 23.7 Motion of Charged Particles in a Uniform Electric Field

Nadiah Alenazi 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 C N is some integer

Nadiah Alenazi 3 From Coulomb ’ s 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 Coulomb ’ s Law

Nadiah Alenazi 4 Consider two electric charges: q 1 and q 2 The electric force F between these two charges separated by a distance r is given by Coulomb’s Law The constant k e is called Coulomb’s constant  0 is the permittivity of free space The smallest unit of charge e is the charge on an electron (-e) or a proton (+e) and has a magnitude e = 1.6 x C

Nadiah Alenazi 5 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 m. Find the magnitudes of the electric force.

Nadiah Alenazi 6 When dealing with Coulomb ’ s law, you must remember that force is a vector quantity The law expressed in vector form for the electric force exerted by a charge q1 on a second charge q2, written F12, is where r ˆ is a unit vector directed from q 1 toward q 2 The electric force exerted by q 2 on q 1 is equal in magnitude to the force exerted by q 1 on q 2 and in the opposite direction; that is, F 21 = -F 12.

Nadiah Alenazi 7 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

Nadiah Alenazi 8 Double one of the charges –force doubles Change sign of one of the charges –force changes direction Change sign of both charges –force stays the same Double the distance between charges –force four times weaker Double both charges –force four times stronger

Nadiah Alenazi 9 Example: Three point charges are aligned along the x axis as shown. Find the electric force at the charge 3nC.

Nadiah Alenazi 10 Example 23.2 Find the Resultant Force Consider three point charges located at the corners of a right triangle, where q 1 =q 3 = 5.0μC, q 2 = 2.0 μC, and a= 0.10 m. Find the resultant force exerted on q 3.

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