ElectrostaticsElectrostaticsthe study of electrical

charges at rest

ElectrodynamicsElectrodynamicsthe study of electrical

charges in motion

Two oppositeopposite types of charge exist, namedpositivepositive and negativenegative by

Benjamin Franklin.

Charge is aproperty of

matter.

Charged particlesCharged particles exist in exist in atoms.atoms.

ElectronsElectrons are responsibleare responsiblefor for negativenegative charge; charge;

protonsprotons for for positivepositive charge; charge;neutronsneutrons have have nono charge. charge.

Small amountsSmall amounts of ordinary matter contain of ordinary matter containincredible amountsincredible amounts of subatomic particles! of subatomic particles!

ConductorConductormaterial that allows charges to move about easily

InsulatorInsulatormaterial through which charges will not easily move

Basic Law of ElectrostaticsBasic Law of ElectrostaticsLike charges repel; unlike charges attractLike charges repel; unlike charges attract

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Click here to view a simulation of thebehavior of pith ballsin the vicinity ofcharged rods.

Click Click here here to read about charging to read about charging objects by friction.objects by friction.

View a simulationView a simulationof charging aof charging aballoon by rubbing balloon by rubbing it on your hair it on your hair and then stickingand then stickingit to a neutralit to a neutralwall here.wall here.

charging a rodrod and electroscopeelectroscopepositivelypositively and negativelynegatively

by conductionconduction and inductioninduction

When charging by conductionconduction,the rod touchestouches the electroscope.

The electroscope gets the same chargesame charge as the rod.

When charging by inductioninduction, the rod does notdoes nottouchtouch the electroscope. The electroscope gets

the opposite chargeopposite charge of the rod.

Go Go here here to view simulations of charging to view simulations of charging an electroscope.an electroscope.

COULOMB’S LAWCOULOMB’S LAWThe force between two charged objects isThe force between two charged objects is

directly proportional to the product of theirdirectly proportional to the product of theircharges and inversely proportional to theircharges and inversely proportional to their

separation distance squared.separation distance squared.

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In equation form:In equation form:

FFF is the is the force of attractionforce of attraction, measured in , measured in newtonsnewtons,,between charges qbetween charges q11 and q and q22

k

kk is the is the Universal Electrostatic ConstantUniversal Electrostatic Constant, equal to , equal to

9.00 x 109.00 x 1099 N N mm22/C/C22

q1 q2

qq11 and and qq22 are the are the attracting chargesattracting charges, measured in, measured incoulombscoulombs

d2

dd is the is the distance between the chargesdistance between the charges, , and is measured in and is measured in metersmeters

The The SI unitSI unit of of chargechargeis the is the coulombcoulomb,,named in honor ofnamed in honor of

Charles Augustin Coulomb..

1 C = charge on 1 C = charge on 6.25 x 106.25 x 101818 electrons (or protons) electrons (or protons)

1 e1 e-- = = 1.60 x 101.60 x 101919 CC = elementary charge = elementary charge

Electric force is a Electric force is a vectorvector and must be treated as such. and must be treated as such.

Example Problem• What is the force on a +2.3 μC charge that lies 3.7 m

to the left of a -5.1 μC charge?• Given

• q1 = +2.3 μC = +2.3 x 10-6 C d = 3.7 m

• q2 = -5.1 μC = -5.1 x 10-6 C k = 9 x109 N*m2/C2

• F = ?

• B.E.• F = kq1q2/d2

• W.E.• F = (9 x109 N*m2/C2)*(+2.3 x 10-6 C)*(-5.1 x 10-6 C)

(3.7 m)2

• Answer• F = -0.0077 N • (“-” means attraction)

Example Problem• How far is a +4.5 mC from a -8.2 mC if there is a force

of 13 N between them?• Given

• q1 = +4.5 mC = +4.5 x 10-3 C d = ?

• q2 = -8.2 mC = -8.2 x 10-3 C k = 9 x109 N*m2/C2

• F = 13 N

• B.E.• F = kq1q2/d2

• W.E.• 13 N = (9 x109 N*m2/C2)*(+4.5 x 10-3 C)*(-8.2 x 10-3 C)

d2

• Multiply both sides by d2, then divide both sides by 13 N

• Answer• D = 160 m