Chapter 16: Electric Charge

Lecture #1

when two dissimilar materials are rubbed together, then charges can

transfer from one material to the other so that the material with an excess of charges

The transfer occurs because

the two materials have a different affinity for charges

like-signed charges

repels each other

opposite-signed charges

attract each other

the magnitude of the force (repelling and subtraction) depends on the

separation: strong when near each other, weak when they are far apart

electric charge is a

fundamental property of matter

electric charge comes in two types

positive and negative

electric charge is

conserved, no net electric charge can be created or destroyed

In nay interaction, the total charge after the interaction must be

the same as the total charge before the interaction

when a glass rod is rubbed with a cloth, the glass acquires

a positive charge and the cloth acquires an equal amount of negative charge

electrical charges originate

in the atom

atoms in the nucleus

protons (postive) and neutrons

electric charges in the atom

electrons (not in the nucleus)

the elementary charge is the magnitude of the charge

of the electron or proton

the charge of a proton is the name in magnitude but opposite

sign to that of an electron

the SI unit of charge is the

coulomb, (C)

in a neutral atoms, there is an equal number of

electrons and protons and its net charge is zero

in a neutral atom, the overall net charge

of the material composed of neutral atom is zero as well

nucleus is tiny but

massive : do not move

the protons are tightly bound within the nucleus (due to the strong nuclear force) and are

very hard to remove/change

electron is

light (mass)

the electrons are mostly bound to the nucleus (due to the electric force)

and are relatively easy to remove or even add

the charging of a solid object by rubbing can be explained by the

transfer of electrons from one object to the other

when a glass rod becomes positively charged by rubbing with a cloth, electrons are transferred from the glass rod to the cloth,

leaving the glass rod with a positive charge equal in magnitude to the negative charge acquired by the cloth

we will usually use ___ to indicate an amount of charge

q or Q

materials can be electrically classified by how well charges move through the materials

conductors, insulators, and semiconductors

electrical conductors

have some very loose bound electrons —> electron are free to move

electrical insulators

electrons are tightly bound to the nuclei —> electron are held in place

semiconductors

between conductors and insulators

in conductors, charges are

free to rearrange themselves

if a negatively charged rod is Brough near by, the charges in the conductor are rearranged

the rod initially pushes electrons to the opposite side of the sphere, making one side positively charged and the other side negatively charged

nonconductors won’t become charged by

conduction or induction, but will experience charge separation

electrically neutral atoms

have equal number of electrons and protons

ions

are atoms that have lost or gained electrons and therefore are have a net charge

a positive ion has

lost electron

a negative ion has

gained electrons

some neutral molecules are polar

meaning that different ends have opposite charges (charge separation or polarization)

normally when objects are charged by rubbing, they hold their charge

only for a limited time and eventually return to the neutral state

usually the excess charge

“leaks off” onto water molecules in the air

water molecules are polar - that is,

even though they are neutral, their charge is not distributed uniformly

thus, the extra electrons on a charged plastic ruler can “leak off” into

the air because they are attracted to the positive end of water molecules

a positively charged object, on the other hand, can be neutralized by transfer of loosely

held electrons from water molecules in the air

on dry days, static electricity is much more noticeable since

the air contains fewer water molecules to allow leakage of charge

distribution of charges in an insulator

charges do not move and not not distribute across shape

distribution of charges in conductors

charges move uniformly to distribute themselves

what happens to the charge of a neutral object after conduction

two metals end up with the same sign of charges

what happens to the charge of a neutral object after induction

the object ends up with an opposite sign of charges compared to the rod

nonconductors won’t become charged by conduction or induction but will

experience charge separation