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Transcript
Chapter 2 Electric Field 2.1 Coulomb’s Law 2.2 Electric Field 2.3 Electric field lines Slide 1 Fig 23CO, p.707 INTRODUCTION Electric Chargeالشحنة الكهربية – تتكون المادة من جزيئات ،التي تتكون من ذرات ،تحتوي كل ذرة على جسيمات ذات شحنة كهربية موجبة – البروتونات -داخل النواه وجسيمات ذات شحنة كهربية سالبة -االلكترونات -حول النواه. Slide 2 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 +/-2e, or +/-3e, and so on. Slide 3 Fig 23-2, p.708 (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 Fig 23-5a, p.710 Slide 5 Fig 23-5b, 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 F21 exerted by q2 on q1 is equal in magnitude and opposite in direction to the force F12 exerted by q1 on q2. (a)When the charges are of the same sign, the force is repulsive. (b) (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 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. Slide 7 Charles Coulomb French physicist (1736–1806) Fq1 q 2 F 1 r2 F ke q1 q 2 r 2 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 x109 N.m2/C2 Slide 8 Example 23.2 Consider three charges located at the corners of a right triangle as shown in figure where q1=q3= 5 C, q2= -2 C and a = 0.1 m. Find the resultant force exerted on q3 q2 exert q3 by force F23 on negative x-axis F23 6 6 q2 q3 ( 2 x 10 )( 5 x 10 ) 9 ke 9N 2 (8.99 x10 ) 2 a (0.1) q1 exert q3 by force F13 on negative x-axis 6 6 ( 5 x 10 )( 5 x 10 ) q1q3 9 (8.99 x10 ) 11N F13 ke 2 2 2(0.1) ( 2a ) F13 divided into x-axis = F13x= 11 cos45= 7.9 N F3x = F13x+F23=7.9 - 9 = -1.1 N F3y= F13y= 7.9 Slide 9 F3= -1.1 i+7.9j and F13y= 7.9N Slide 10 Example : Tow protons in an atomic nucleus are typically separated by distance of 2x10-15 m. The electric repulsion force F between the protons is (note that k=9x109 N.m2/C2, charge of proton q=1.6x10-19 C) q q r Slide 11 Example: The electric force of two electrons separated by r = 20x10-9 m (note that k=9x109 N.m2/C2, charge of electron q=1.6x10-19 C) q q r Slide 12 * 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 13 Slide 14 Slide 15 Example : In the figure, the electric field of the charge q= 24 x10-6 C at a distance r = 2 m (Note K=9x109 N.m2/C2) q r Slide 16 Example: An electric field force is acting on a charge of q=4nC by F=16N. What is the magnitude of the electric field Slide 17 Example: The electric filed at a point r=1km far from a point charge of q=3 C is (Note K=9x109 N.m2/C2) Slide 18 Slide 19 q1 N1 q2 N2 Slide 20 Example: The figure shows the electric field lines for two charges separated By a small distance. The ratio (q2/q1) equal a -q2 +q1 b -q2 -q1 c +2q1 Slide 21 -q2 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 22 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 23 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 24 23-7; Three point charges are located at the corners of an equilateral triangle. Calculate the net electric force on the 7.00 uC charge. F1 60 60 F2 Slide 25 23-8: Two small beads having positive charges 3q and q are fixed at the opposite ends of a horizontal insulating rod extending from the origin to the point x =d. a third small charged bead is free to slide on the rod. At what position is the third bead in equilibrium? Can it be in stable equilibrium? Slide 26 3.The electric force between an electron and a proton separated by 3nm is q1q2 F k 2 r Slide 27