Concept Summary Adapted from: Batesville High School Physics

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Electrostatics. Concept Summary Adapted from: Batesville High School Physics. Electrostatics. Electrostatics is the study of electric charge at rest . (Or more or less at rest, as opposed to current electricity.). Electrical Charges.

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Electrostatics

Concept Summary

Batesville High School Physics

Electrostatics
• Electrostatics is the study of electric charge at rest.
• (Or more or less at rest, as opposed to current electricity.)
Electrical Charges
• Electric 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 Forces
• Like charges repel.
• Opposite charges attract.
Elementary Charges
• Protons 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 Charge
• The 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 Charge
• An 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.
Stroking

If 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.

Stroking

If 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.

Stroking

If 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 Conserved
• Electric 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 & Insulators
• Materials 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 Charge
• When 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.
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.
Grounding
• Providing 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 Induction
• Bring a charged object near (but not touching) a neutral object.
• Ground the neutral object.
• Remove the ground.
• Remove the charged object
• The neutral object now has a charge opposite to the charged object.
Under the Influence

Two 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 uncharged

b) balls X and Y are both charged positively

c) balls X and Y are both charged negatively

d) ball X is + and ball Y is −

e) ball X is − and ball Y is +

Under the Influence

Two 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 uncharged

b) balls X and Y are both charged positively

c) balls X and Y are both charged negatively

d) ball X is + and ball Y is −

e) ball X is − and ball Y is +

Under the Influence

Two 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 uncharged

b) balls X and Y are both charged positively

c) balls X and Y are both charged negatively

d) ball X is + and ball Y is −

e) ball X is − and ball Y is +

Electroscopes

Metal plate

Insulator

Metal shaft

Glass window

Metal plate

Insulated container

Metal leaf

Polarization
• Bringing 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.
Electric Dipoles
• An object that is electrically neutral overall, but permanently polarized, is called an electric dipole.
• Example: H20 molecule
Electrical Forces
• The 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 Forces
• The electrical force between 2 charges is:
• Directly proportional to each charge.
• Inversely proportional to the square of the distance between the charges.