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Electricity Lecture Series email: jjnita@salam.uitm.edu.my; drjjlanita@hotmail.com http://www3.uitm.edu.my/staff/drjj/ jjnita@salam.uitm.edu.mydrjjlanita@hotmail.comjjnita@salam.uitm.edu.mydrjjlanita@hotmail.com Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA Charges Electric Fields, Potential and Capacitors

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Copyright DR JJ,FSG, UiTM; Jan082 Electric Charges 1.Explain the gravitational forces acting on any object. 2.Mathematically represent the gravitational force and describe its impact on physical events. 3.Describe existence of electrical charges in matter and its quantization property. 4.Sketch and explain the charging by induction and charging by contact 5.Mathematically represent forces acting between electrical charges (Coulombs Law) Objectives:

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Copyright DR JJ,FSG, UiTM; Jan083 Electric Charges 6.Represent forces acting between charges both pictorially and vectorially. 7.Describe and explain gravitational and electric field. 8.Derive mathematical relations for electric field of point charges, line charges and surface charges and the use of Gaussian surface. 9.Describe and explain capacitors and its purpose. Objectives:

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Copyright DR JJ,FSG, UiTM; Jan084 Electric Charges 10.Write mathematical relations for parallel plate capacitors related to capacitance, potential difference, its geometrical dependence, and electric field it can store. 11.Obtain energies stored by capacitors. 12.Explain and pictorially and graphically represent the charging and discharging of capacitors. Objectives:

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Copyright DR JJ,FSG, UiTM; Jan085 Electric Charges 13.Describe, draw and obtain resultant capacitances for capacitors connected in parallel and in series respectively. 14.Solve conceptual and numerical problems associated with capacitors. Objectives:

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Copyright DR JJ,FSG, UiTM; Jan086 Electric Forces-Coulombs Law Gravitational forces (Law of Gravitation) Gravitational forces (Law of Gravitation): Earth pulls on the moon and vice-versa. Any 2 objects will always exert and feel attractive gravitational forces. They exert with same magnitude. Moon feels attraction because it is in the earths gravitational field. F is inversely proportional to square of separation between masses. r F me

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Copyright DR JJ,FSG, UiTM; Jan087 Electric Charges-Forces Gravitational forces Gravitational forces: F is inversely proportional to square of separation between masses. r, mr2,m2r2,m2 F, NRatios of F 10100F 1 =Gm 1 m 2 /10 0 F 1 /F 2 =4 20400F 2 =Gm 1 m 2 /40 0 F 1 /F 3 =9 30900F 3 =Gm 1 m 2 /90 0 F 2 /F 3 =9/4 r F 12

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Copyright DR JJ,FSG, UiTM; Jan088 18.2 Charged Objects and the Electric Force Cutnell & Johnson 7E Like charges repel and unlike charges attract each other.

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Copyright DR JJ,FSG, UiTM; Jan089 18.5 Coulombs Law Cutnell & Johnson 7E

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Copyright DR JJ,FSG, UiTM; Jan0810 Electric Charges-Coulombs Law Electric forces Electric forces: Charged objects exert electric forces on each other. Can be attractive or repulsive. Like charges repel. Unlike charges attract. + + r F 21 q 1 =e q 2 =2e 0 = permittivity of free space 0 = 8.85 x 10 -12 C 2 /Nm 2

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Copyright DR JJ,FSG, UiTM; Jan0811 18.5 Coulombs Law Cutnell & Johnson 7E COULOMBS LAW The magnitude of the electrostatic force exerted by one point charge on another point charge is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance between them.

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Copyright DR JJ,FSG, UiTM; Jan0812 18.5 Coulombs Law Cutnell & Johnson 7E Example 4 Three Charges on a Line Determine the magnitude and direction of the net force on q 1.

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Copyright DR JJ,FSG, UiTM; Jan0813 18.5 Coulombs Law Cutnell & Johnson 7E

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Copyright DR JJ,FSG, UiTM; Jan0814 Electric Point Charges Electric forces Electric forces: repulsive forces since both are positively charged. + + r F q 1 =e q 2 =2e F set1 F set2 < + + r/2 F q 1 =e q 2 =2e SET 2 Is acc set1 bigger or smaller than acc set2 ?? down

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Copyright DR JJ,FSG, UiTM; Jan0815 Electric Point Charges Electric forces Electric forces: attractive forces since opposite charges - + r F q 1 =2e q 2 =2e F set1 F set2 < - + r F q 1 =4e q 2 =2e Is acc set1 bigger or smaller than acc set2 ?? up

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Copyright DR JJ,FSG, UiTM; Jan0816 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 - + r F q 1 =2e q 3 =2e What is the magnitude and direction of net force acting on q 2 ? + q 2 =2e r up What is the magnitude and direction of acceleration of q 2 ?

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Copyright DR JJ,FSG, UiTM; Jan0817 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 - + 2r F q 1 =2e q 3 =2e What is the magnitude and direction of acceleration of q 2 ? + q 2 =2e r up

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Copyright DR JJ,FSG, UiTM; Jan0818 18.5 Coulombs Law Cutnell & Johnson 7E Example 3 A Model of the Hydrogen Atom In the Bohr model of the hydrogen atom, the electron is in orbit about the nuclear proton at a radius of 5.29x10 -11 m. Determine the speed of the electron, assuming the orbit to be circular.

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Copyright DR JJ,FSG, UiTM; Jan0819 18.5 Coulombs Law Cutnell & Johnson 7E For circular motion & using Newtons 2 nd law of motion

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Copyright DR JJ,FSG, UiTM; Jan0820 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 + + 2r F 12 q 1 =2e q 3 =2e What is the magnitude and direction of acceleration of q 2 ? + q 2 =2e r down up F 21

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Copyright DR JJ,FSG, UiTM; Jan0821 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 + + 2r q 1 =2e q 3 =2e - r + q 4 =2e q 2 =e + q 5 =2e 2r F 21 F 23 F 24 F 25 F 24 F 25 F 25 sin 25 F 24 sin 24 F 24 cos 24 F 25 cos 25

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Copyright DR JJ,FSG, UiTM; Jan0822 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 2r Pythagoras theorem: Charge 2 & 4 is separated by the distance: up down Charge 2 & 5 is separated by the distance: 2e r 2r 2e -e r 24 = ?? r 25 = ??

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Copyright DR JJ,FSG, UiTM; Jan0823 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 up down Then

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Copyright DR JJ,FSG, UiTM; Jan0824 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 F FxFx FyFy Take the arctan of theta to determine the force direction

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Copyright DR JJ,FSG, UiTM; Jan0825 18.5 Coulombs Law Cutnell & Johnson 7E

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Copyright DR JJ,FSG, UiTM; Jan0826 Electric field & forces Assignment: Due Feb 1 st 2008. Chap 18 Exercises from the back of chapter. 1,3,5,9, 13, 17,70, 71, 75 Submit all

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