Electric Charge & Electric Field

Chapter 16

• According to atomic theory, electric forces between atoms and molecules hold them together to form the liquids and solids.

• Many previous forces are contact forces• Friction• Normal• Elastic forces

16-1 Static Electricity

• The word electricity comes from the Greek word elektron which means amber

• Petrified tree resin and if rubbed gained an electric charge

• The word static means at rest• Static electricity means charges at rest• A build up of charge

• If an object is charged it possess a net electric charge. An unbalance of charge.

• Two types of charges ones that repel a charged rod and ones that attract

• These charges are referred to as positive or negative

• Like charges repel and unlike charges attract

• Benjamin Franklin choose the plastic rod (amber) to be negative and the glass to be positive (arbitrary)

• This convention is still followed today

• Positive charges and negative charges are treated algebraically

The law of conservation of electric charge

• The net amount of electric charge produced in any process is zero.

• No net electric charge can be created or destroyed

• If one area is charged positively the a nearby area is charged negatively

16-2 Electric Charge in the Atom

• The understanding of electricity begins with the atom itself.

• Positive charges are found in the nucleus• Negative charges are found in the outer

energy levels

• Neutral- equal numbers of positive and negative charges

• Ions are atoms that gains or loses electrons• In liquids and solids nuclei or ions can

move as well as electrons to transfer charge

16-3 Insulators & Conductors

• Conductors allow charges to transfer easily

• Metals• Insulators resist the transfer of the

charge• Wood, air, plastics

Semiconductors

• Most materials fall into conductors or insulators

• A few materials are in-between• Silicon and germanium• Used in electronics

16-4 Induced Charge

Electroscope

• A device that can be used for detecting a charge.

16-5 Coulomb’s Law

• Does a force exist between charges? How can you tell?

• How much force?• French physicist Charles Coulomb

investigated the electric forces.• Coulomb didn’t have a precise measuring

device, but developed a relationship with the ratio of charge

Coulomb’s Law as an Equation

221

r

QQkF

• F is force in Newtons, N• k is a proportionality constant,

8.988 x 109 N.m2/C2

• Q is charge in Coulombs, C• r is distance between charges in m

Compare to Universal Gravitation Law

221

r

mmGF

• Major difference between the two laws is the force in gravitation is always attraction and electric force can be attraction or repulsion

Elementary Charge

• The charge on one electron has been determined to be about

• e = 1.602 x 10-19 C

Permittivity of free space

• Constant, ϵo

• Related to k by k=1/4ϵo

• Where ϵo= 1/4k=8.85 x 10-12 C2/N.m2

• Coulomb’s Law is then

221

4

1

r

QQF

o

Principle of Superposition

• The net force on any one charge will be the vector sum of the forces on that charge due to each of the others

16-6 Coulomb’s Law & Vectors

• Must find the net force, force is a vector and if the forces are not along a line then vector addition must be used.

• Problem 13.