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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, UiTM2 18.1 The Origin of Electricity Cutnell & Johnson 7E The electrical nature of matter is inherent in atomic structure. coulombs

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Copyright DR JJ,FSG, UiTM3 Electric Forces-Coulomb’s 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 earth’s gravitational field. F is inversely proportional to square of separation between masses. r F me

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Copyright DR JJ,FSG, UiTM4 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, UiTM5 Electric Charges-Coulomb’s 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, UiTM6 18.5 Coulomb’s Law Cutnell & Johnson 7E COULOMB’S 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, UiTM7 18.5 Coulomb’s 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, UiTM8 18.5 Coulomb’s Law Cutnell & Johnson 7E

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Copyright DR JJ,FSG, UiTM9 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, UiTM10 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, UiTM11 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, UiTM12 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, UiTM13 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, UiTM14 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, UiTM15 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, UiTM16 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 up down Then

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Copyright DR JJ,FSG, UiTM17 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, UiTM18 Electric Charges 6.Describe and explain gravitational and electric field. 7.Draw and explain electric field lines for point charges, dipoles and other charge configurations. 8.Derive mathematical relations for electric field of point charges, line charges and surface charges and the use of Gaussian surface. Objectives:

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Copyright DR JJ,FSG, UiTM19 Electric Field Gravitational Field Gravitational Field: an area where any object will experience a gravitational force Electric Field Electric Field: an area where any charged object will experience an electric force. So. At q o

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Copyright DR JJ,FSG, UiTM20 Electric Field Electric Field Electric Field: an area where any charged object will experience an electric force Positive test charge placed at point 1 thru 8 will experience a repulsive force in the direction shown. The electric field lines around a positive point charge E Field

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Copyright DR JJ,FSG, UiTM21 Electric Field Electric Field Electric Field: an area where any charged object will experience an electric force The electric field lines around a negative point charge The electric field lines around a pair of point charges (dipole). Originates from + & terminates at – charge. E Field

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Copyright DR JJ,FSG, UiTM22 Electric Point Charges Electric field Electric field: Determine E-field at point P + P 2r q 1 =2e - + q 3 =2e q 0 = - e + q 4 =2e 2r E3E3 E4E4 r E0E0 r3r3 E1E1

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Copyright DR JJ,FSG, UiTM23 Electric Point Charges Electric field Electric field: Determine E-field at P + 2r q 1 =2e - r + q 3 =2e q 0 = - e + q 4 =2e 2r E3E3 E4E4 E0E0 E1E1 P r3r3

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Copyright DR JJ,FSG, UiTM24 Electric Point Charges Electric forces Electric forces: Determine E-field at P + q 2 =2e r 2r E3E3 E4E4 E0E0 E1E1 E 2y E 2x E2E2

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Copyright DR JJ,FSG, UiTM25 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge at P E 2y E 2x E 2, a + A positively charged particle placed at point 2 will feel a force (push) and will accelerate in the direction of E A negatively charged particle placed at point 2 will feel a force (pull) and will accelerate in the direction opposite to E E 2y E 2x E2E2 - a

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Copyright DR JJ,FSG, UiTM26 Elrctric field & forces Assignment: Due Aug 3 rd 2006. Chap 18 Exercises from the back of chapter. 1,3,5,9, 13, 17,70, 71, 75 Submit all

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