Concept SummaryAdapted from:Batesville High School Physics

ElectrostaticsElectrostatics is the study of electric

charge at rest. (Or more or less at rest, as opposed to

current electricity.)

Electrical ChargesElectric charge is a fundamental

property of matter. Two types of electric charges

Positive charge - every proton has a single positive charge.

Negative charge - every electron has a single negative charge.

Electrical ForcesLike charges repel.Opposite charges attract.

Likes Repel, Opposites Attract

Elementary ChargesProtons carry the smallest positive

charge.The smallest negative charge is the

charge on the electron.The charges carried by the proton and

electron are equal in size.The mass of the proton is about 2000

times the mass of the electron.

Units of ChargeThe SI unit of charge is the coulomb.The elementary charge of a proton or

an electron is 1.60 x 10−19 C.

The proton is positively charged, while the electron is negatively charged.

Electrical ChargeAn object with an excess of electrons

is negatively charged.An object with too few electrons (too

many protons) is positively charged.An object with the same number of

electrons and protons is neutral.

StrokingIf something gets a positive electric charge, then it follows that something else:

a) becomes equally positively charged.b) becomes equally negatively charged.c) becomes negatively charges, but not necessarily equally

negatively charged.d) becomes magnetized.

StrokingIf something gets a positive electric charge, then it follows that something else:

a) becomes equally positively charged.b) becomes equally negatively charged.c) becomes negatively charges, but not necessarily equally

negatively charged.d) becomes magnetized.

StrokingIf something gets a positive electric charge, then it follows that something else:

a) becomes equally positively charged.b) becomes equally negatively charged.c) becomes negatively charges, but not necessarily equally

negatively charged.d) becomes magnetized.

Charge is ConservedElectric charge is conserved -

Electric charge moves from one place to another - no case of the net creation or destruction of electric charge has ever been observed.

In solids, only electrons can move. In liquids, gasses, and plasmas, both positive

and negative ions are free to move.

Conductors & InsulatorsMaterials in which charges are free to

move about are called conductors.Materials in which charges are not free

to move about are called insulators.

“Creating” an Electric ChargeWhen you “create” an electric charge

(by rubbing your feet on a carpet) you are actually separating existing charges – not creating charges.

One object ends up with an excess of electrons (− charge), and the other a deficit of electrons (+ charge).

Charging by Friction If one neutral material has more affinity

for electrons than another (neutral) material, it will attract electrons from the other.

One material becomes negatively charged, the other positively charged.

Material Relative charging with rubbingRabbit fur + + + + + +

Glass + + + + +Human hair + + + +Nylon + + +Silk + +Paper +Cotton -Wood - -Amber - - -Rubber - - - -PVC - - - - -Teflon - - - - - -

Charging by Contact If a charged object is brought in contact

with a neutral object, charges will be repelled from (or attracted to) the charged object.

The neutral object will gain a charge of the same sign as the charged object.

GroundingProviding a path from a charged object

to the Earth is called grounding it.Charges will be attracted from (or

repelled to) the Earth by the charged object.

Since the Earth is so large, both the charged object and the Earth are neutralized.

Charging by Induction1. Bring a charged object near (but not

touching) a neutral object.2. Ground the neutral object.3. Remove the ground.4. Remove the charged object5. The neutral object now has a charge

opposite to the charged object.

Induction

Under the InfluenceTwo uncharged metal balls, X and Y, stand on glass rods. A third ball, Z, carrying a positive charge, is brought near the first two. A conducting wire is then run between X and Y. The wire is then removed, and ball Z is finally removed. When this is all done it is found that:

a) balls X and Y are still unchargedb) balls X and Y are both charged positivelyc) balls X and Y are both charged negativelyd) ball X is + and ball Y is −e) ball X is − and ball Y is +

Under the InfluenceTwo uncharged metal balls, X and Y, stand on glass rods. A third ball, Z, carrying a positive charge, is brought near the first two. A conducting wire is then run between X and Y. The wire is then removed, and ball Z is finally removed. When this is all done it is found that:

a) balls X and Y are still unchargedb) balls X and Y are both charged positivelyc) balls X and Y are both charged negativelyd) ball X is + and ball Y is −e) ball X is − and ball Y is +

Under the InfluenceTwo uncharged metal balls, X and Y, stand on glass rods. A third ball, Z, carrying a positive charge, is brought near the first two. A conducting wire is then run between X and Y. The wire is then removed, and ball Z is finally removed. When this is all done it is found that:

a) balls X and Y are still unchargedb) balls X and Y are both charged positivelyc) balls X and Y are both charged negativelyd) ball X is + and ball Y is −e) ball X is − and ball Y is +

ElectroscopesMetal plate

Metal shaft

Metal leaf

Metal plate

Glass window

Insulated container

Insulator

PolarizationBringing a charged object near (but not

touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. Like charges in the neutral object are

repelled by the charged object. Unlike charges in the neutral object are

attracted by the neutral object.The neutral object returns to normal

when the charged object is removed.

Polarization

Electric DipolesAn object that is electrically neutral

overall, but permanently polarized, is called an electric dipole. Example: H20 molecule

Electrical ForcesThe electrical force between 2 charges

depends on: The size of each charge

More charge means more force. The distance between the charges

More distance means less force.

Electrical ForcesThe electrical force between 2 charges

is: Directly proportional to each charge. Inversely proportional to the square of

the distance between the charges.

Coulomb’s Law

The End