Electric Charge & Electric Field Chapter 16

Many previous forces are contact forces 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 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

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

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=1/4ϵo Where ϵo= 1/4k=8.85 x 10-12 C2/N.m2 Coulomb’s Law is then

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.