Electricity Lecture Series Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA Charges Electric Fields, Potential and Capacitors

Copyright DR JJ,FSG, UiTM2 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 (Coulomb’s Law) Objectives:

Copyright DR JJ,FSG, UiTM3 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:

Copyright DR JJ,FSG, UiTM4 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:

Copyright DR JJ,FSG, UiTM5 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:

Copyright DR JJ,FSG, UiTM6 Electric Charges Matter Matter: made up of atoms and molecules Charged object Charged object: imbalance number of electrons & protons Positively charged Negatively charged Atom Atom: made up of nucleus, protons and electrons Conductors Conductors: charges can move freely Insulators Insulators: charges cannot move freely

Copyright DR JJ,FSG, UiTM The Origin of Electricity Cutnell & Johnson 7E The electrical nature of matter is inherent in atomic structure. coulombs

Copyright DR JJ,FSG, UiTM8 Electric Charges Charge quantization Charge quantization: charges exist in multiples of an elementary charge, the charge of an electron where N are the number of electrons & the elementary charge e is e =1.6 x C Number of charges in 1 C?? N=q/e =1 C/1.6 x C N = 6.25 x 10 18

Copyright DR JJ,FSG, UiTM Charged Objects and the Electric Force Cutnell & Johnson 7E LAW OF CONSERVATION OF ELECTRIC CHARGE During any process, the net electric charge of an isolated system remains constant (is conserved). Total number of negative charges (electrons) and positive charges (protons) must be equal

Copyright DR JJ,FSG, UiTM Conductors and Insulators Cutnell & Johnson 7E Not only can electric charge exist on an object, but it can also move through and object. Substances that readily conduct electric charge are called electrical Conductors – copper, iron, gold, water, human body Materials that conduct electric charge poorly are called electrical Insulators – plastic, wood, rubber

Copyright DR JJ,FSG, UiTM Charging by Contact Cutnell & Johnson 7E Charging by contact.

Copyright DR JJ,FSG, UiTM Charging by Induction Cutnell & Johnson 7E Charging by induction.

Copyright DR JJ,FSG, UiTM Charging by Contact and by Induction Cutnell & Johnson 7E The negatively charged rod induces a slight positive surface charge on the plastic.

Copyright DR JJ,FSG, UiTM14 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

Copyright DR JJ,FSG, UiTM15 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 = F 2 =Gm 1 m 2 /40 0 F 1 /F 3 = F 3 =Gm 1 m 2 /90 0 F 2 /F 3 =9/4 r F 12

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

Copyright DR JJ,FSG, UiTM Coulomb’s Law Cutnell & Johnson 7E

Copyright DR JJ,FSG, UiTM18 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 C 2 /Nm 2

Copyright DR JJ,FSG, UiTM 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.

Copyright DR JJ,FSG, UiTM 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.

Copyright DR JJ,FSG, UiTM Coulomb’s Law Cutnell & Johnson 7E

Copyright DR JJ,FSG, UiTM22 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

Copyright DR JJ,FSG, UiTM23 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

Copyright DR JJ,FSG, UiTM24 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 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 ?

Copyright DR JJ,FSG, UiTM25 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge r F q 1 =2e q 3 =2e What is the magnitude and direction of acceleration of q 2 ? + q 2 =2e r up

Copyright DR JJ,FSG, UiTM Coulomb’s 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 5.29x m. Determine the speed of the electron, assuming the orbit to be circular.

Copyright DR JJ,FSG, UiTM Coulomb’s Law Cutnell & Johnson 7E For circular motion & using Newton’s 2 nd law of motion

Copyright DR JJ,FSG, UiTM28 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge r 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

Copyright DR JJ,FSG, UiTM29 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge r 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

Copyright DR JJ,FSG, UiTM30 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 = ??

Copyright DR JJ,FSG, UiTM31 Electric Point Charges Electric forces Electric forces: Determine forces acting on charge 2 up down Then

Copyright DR JJ,FSG, UiTM32 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

Copyright DR JJ,FSG, UiTM Coulomb’s Law Cutnell & Johnson 7E

Copyright DR JJ,FSG, UiTM34 Elrctric field & forces Assignment: Due Aug 3 rd Chap 18 Exercises from the back of chapter. 1,3,5,9, 13, 17,70, 71, 75 Submit all