Chapter 1 Electric Field 2 1 Coulombs Law

Chapter 1 Electric Field 2. 1 Coulomb’s Law 2. 2 Electric Field 2. 3 Electric field lines Slide 1 Fig 23 CO, p. 707

Figure 23. 2 When a glass rod is rubbed with silk, electrons are transferred from the glass to the silk. Because of conservation of charge, each electron adds negative charge to the silk, and an equal positive charge is left behind on the rod. Also, because the charges are transferred in discrete bundles, the charges on the two objects are +/-e, or +/-2 e, or +/-3 e, and so on. Slide 3

(a) The charged object on the left induces a charge distribution on the surface of an insulator due to realignment of charges in the molecules. Slide 4

Slide 5 Fig 23 -5 b, p. 710

Two point charges separated by a distance r exert a force on each other that is given by Coulomb’s law. The force F 21 exerted by q 2 on q 1 is equal in magnitude and opposite in direction to the force F 12 exerted by q 1 on q 2. (a)When the charges are of the same sign, the force is repulsive. (b) When the charges are of opposite Slide 6 signs, the force is attractive. Fig 23 -7, p. 713

Coulomb’s experiments showed that the electric force between two stationary charged particles • 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 q 1 and q 2 on the two particles; • is attractive if the charges are of opposite sign and repulsive if the charges have the same sign. Slide 7 Charles Coulomb French physicist (1736– 1806)

The value of the Coulomb constant ke depends on the choice of units. The SI unit of charge is the coulomb (C). The Coulomb constant ke in SI units has the value ke = 8. 9875 x 109 N. m 2/C 2 Slide 8

Example : Tow protons in an atomic nucleus are typically separated by distance of 2 x 10 -15 m. The electric repulsion force F between the protons is (note that k=9 x 109 N. m 2/C 2, charge of proton q=1. 6 x 10 -19 C) Slide 9

Example: The electric force of two electrons separated by r = 20 x 10 -9 m (note that k=9 x 109 N. m 2/C 2, charge of electron q=1. 6 x 10 -19 C) q q r Slide 10

3. The electric force between an electron and a proton separated by 3 nm is Slide 11

* An electric field is said to exist in the region of space around a charged object. When another charged object enters this electric field, an electric force acts on it. The vector E has the SI units of newton per coulomb (N/C), Slide 12

Example : In the figure, the electric field of the charge q= 24 x 10 -6 C at a distance r = 2 m (Note K=9 x 109 N. m 2/C 2) r Slide 13 q

Example: An electric field force is acting on a charge of q=4 x 10 -9 C by F=16 N. What is the magnitude of the electric field Slide 14

Example: The electric filed at a point r=1 km far from a point charge of q=3 C is (Note K=9 x 109 N. m 2/C 2) Slide 15

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Example: The figure shows the electric field lines for two charges separated By a small distance. The ratio (q 2/q 1) equal n=4 a -q 2 +q 1 b -q 2 -q 1 c +2 q 1 Slide 18 -q 2

Electric charges have the following important properties: • Unlike charges attract one another, and like charges repel one another. • Charge is conserved. • Charge is quantized—that is, it exists in discrete packets that are some integral multiple of the electronic charge. Conductors are materials in which charges move freely. Insulators are materials in which charges do not move freely. Slide 19

where ˆr is a unit vector directed from the charge to the point in question. The electric field is directed radially outward from a positive charge and radially inward toward a negative charge. The electric field due to a group of point charges can be obtained by using the superposition principle. That is, the total electric field at some point equals the vector sum of the electric fields of all the charges: Slide 20

Electric field lines describe an electric field in any region of space. The number of lines per unit area through a surface perpendicular to the lines is proportional to the magnitude of E in that region. Slide 21

ﺍﻟﺸﺤﻨﺔ ﺍﻟﻜﻬﺮﺑﻴﺔ Ch 1 If the nucleus of carbon isotope has 6 protons, the total charge of the nucleus is where the charge of proton q = 1. 6 x 10 -19 c Q = ﺍﻟﺸﺤﻨﺔ ﺍﻟﻜﻠﻴﺔ n ﺍﻟﻌﺪﺩ x ﺷﺤﻨﺔ ﺍﻻﻟﻜﺘﺮﻭﻥ q The total electric charge Q of number of electrons n= 1. 6 x 1019 is where the charge of electron = 1. 6 x 10 -19 c Q=ne Slide 22

Two protons in an atom are separated by a distance of r = 2 x 10 -15 m , the electric repulsion force F between the protons is where q = 1. 6 x 10 -19 c Answer The capacitance C of two parallel plate capacitor with an area A and a separation d is Slide 23

The figure shows the electric filed lines for two charges separated by a small distance. The ratio (q 1/q 2)is Slide 24

A parallel plate capacitor of Area A= 4 x 10 -4 m 2 separated by thickness d= 2 x 10 -3 m of dielectric material (k=3. 2). What the capacitance C of the capacitor where 0= 9 x 10 -12 What is the electric field E at a point far a=2 x 10 -3 m from charge q = 4 x 10 -6 c a Slide 25 +q

If the electric field at a point is E= 15 x 105 N/C, the electric force F acting on a charge q = 3 x 10 -12 C Slide 26