2Objectives: After finishing this unit, you should be able to: Explain and demonstrate the first law of electro- statics and discuss charging by contact and by induction.Understand Coulomb’s Law and its symmetry with gravityDefine the electron and coulomb as units of electric charge.
3Electric ChargeWhen a rubber rod is rubbed against fur, electrons are removed from the fur and deposited on the rod.Electrons move from fur to the rubber rod.positivenegative--The rod is said to be negatively charged because of an excess of electrons. The fur is said to be positively charged because of a deficiency of electrons.
4Glass and SilkWhen a glass rod is rubbed against silk, electrons are removed from the glass and deposited on the silk.silkglassElectrons move from glass to the silk cloth.positivenegative+ +The glass is said to be positively charged because of a deficiency of electrons. The silk is said to be negatively charged because of a excess of electrons.
5The ElectroscopeLaboratory devices used to study the existence of two kinds of electric charge.Gold-leaf ElectroscopePith-ball Electroscope
6Two Negative Charges Repel 1. Charge the rubber rod by rubbing against fur.2. Transfer electrons from rod to each pith ball.The two negative charges repel each other.
7Two Positive Charges Repel 1. Charge the glass rod by rubbing against silk.2. Touch balls with rod. Free electrons on the balls move to fill vacancies on the cloth, leaving each of the balls with a deficiency. (Positively charged.)The two positive charges repel each other.
8The Two Types of ChargefurRubbersilkglassAttractionNote that the negatively charged (green) ball is attracted to the positively charged (red) ball.Opposite Charges Attract!
9The First Law of Electrostatics Like charges repel; unlike charges attract.PosNegPosNeg
10Charging by Contact 1. Take an uncharged electroscope as shown below. 2. Bring a negatively charged rod into contact with knob.--3. Electrons move down on leaf and shaft, causing them to separate. When the rod is removed, the scope remains negatively charged.
11Charging Electroscope Positively by Contact with a Glass Rod: Repeat procedures by using a positively charged glass rod. Electrons move from the ball to fill deficiency on glass, leaving the scope with a net positive charge when glass is removed.++
12Charging Spheres by Induction -Uncharged Spheres-+Electrons RepelledSeparation of Charge--++-Isolation of SpheresCharged by Induction
13Induction for a Single Sphere --+Uncharged SphereSeparation of Charge-+-+Electrons move to ground.Charged by Induction
14The Quantity of ChargeThe quantity of charge (q) can be defined in terms of the number of electrons, but the Coulomb (C) is a better unit for later work. A temporary definition might be as given below:The Coulomb: 1 C = 6.25 x 1018 electronsWhich means that the charge on a single electron is:1 electron: e- = -1.6 x C
15Coulomb’s LawThe force of attraction or repulsion between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them.Frqq’-+
16COULOMB’S LAW The force between two charged objects is directly proportional to the product of theircharges and inversely proportional to theirseparation distance squared.link1, link2, link3, link4, link4
17F k q1 q2 = d2 In equation form: F is the force of attraction, measured in NEWTONS,between charges q1 and q2k is the Universal Electrostatic Constant, equal to9.00 x 109 N m2/coul2q1 and q2 are the attracting charges, measured inCoulombsd is the distance between the charges,and is measured in METERS
18E = F/Q Electric Fields The magnitude of an electric field at any An electric field exists in a regionif space if a charge placed in thatregion experiences an electric force.The magnitude of an electric field at anygiven point is defined to be the ratio ofthe force on acharge at thatpoint to theamount of charge.E = F/QElectric field strength has unitsof Newtons/Coulomb (N/C).
19The field E exist independently of the charge q and is found from: Properties of SpaceAn electric field is a property of space allowing prediction of the force on a charge at that point.EElectric Field+Q.rThe field E exist independently of the charge q and is found from:E is a Vector
20The direction of the electric field at any point is defined to be the same direction as the direction of force on apositive test charge placed in the region at that point.Field lines point away from positiveand toward negative charges.