M R . R O C K E N S I E SR E G E N T S P H Y S I C S

ELECTRICITY

ATOMIC STRUCTURE

WHAT IS THE STRUCTURE OF AN ATOM?

Li atom

3 protons3 electrons4 neutrons

ATOM IONNa - e Na+

Cl + e Cl-

N + 3e N-3

WHAT ARE THE THREE SUBATOMIC PARTICLES?

Particle Symbol Charge Mass

Proton p or H

1 positive elementary charge =

+1.6 x 10-19 C

1 amu = 1.67 x 10-27 kg

(on the reference table)

Neutron n 0 1 amu = 1.67 x 10-27 kg

Electron e

1 negative elementary charge =

-1.6 x 10-19 C

1/1836 amu = 9.11 x 10-31 kg

11

11

10

0-1

1 e = 1.6 x 10-19 C (Coulombs)

6.25 x 1018 e = 1 C

On the reference table

Examples:

1)+10 e (elementary charges) formed by

_______________________________2) -4 e formed by

_______________________________

An atom losing 10 electrons

An atom gaining 4 electrons

Examples:

1) What is the charge on 2 protons in elementary charges? Coulombs?

2) What is the charge on 4 electrons in elementary charges? Coulombs?

3) An object has a charge of -2Ca) Does the object contain more protons or electrons?b) How many more electrons than

protons does it contain?

STATIC ELECTRICITY

WHAT IS STATIC ELECTRICITY

1)The study of the transfer of electrons between objects and the electrostatic force that the charges exert upon each other

2)Rubbing or contact produces equal, but opposite charges.

HOW DO CHARGES BEHAVE?

1) Opposites attract:

2) Like charges repel:

3) Any charge attracts a neutral object:

+ -

+ + --

+ - + + - + - + - - - + - + + + + - + -

+ - -- - + - ++ + -- + + - +

LAW OF CONSERVATION OF CHARGE

WHAT IS THE LAW OF CONSERVATION OF CHARGE?

1.Charge is not created or destroyed. It can be moved from one object to another, but the total amount is constant (Like momentum and energy)

2.Charge occurs in specific quantities, integer multiples of the elementary chargeEx: q=ne

where e = 1.6x10-19C and n=0, 1, -1, 2, - 2, 3, -3…

PRACTICE PROBLEMS

+3 +5

If these two spheres come in to contact with each other and then are separated, what will the resulting charge be on each?

PRACTICE PROBLEMS

-2 +4

If these two spheres come in to contact with each other and then are separated, what will the resulting charge be on each?

PRACTICE PROBLEMS

If these three spheres come into contact with each other and then are separated, what will the charge on each be?

ELECTRIC FORCES

HOW EASILY DO ELECTRONS FLOW THROUGH MATERIALS?

Electron motion depends on how tightly the substance holds the electrons.

Conductors (weakly bound electrons) EX: Metals - “sea of electrons”Ionic solutions

Insulators (tightly bound electrons)Organics – (rubber, plastic, paper)Crystalline Solids (silicon, etc.)

All materials fall somewhere along the spectrum

Charge will move easily through a conductor.Charge will stay in place on an insulator.

WHAT ARE ELECTRIC FORCES?

Also called “Coulomb Forces”Like charges repel

Opposite charges attractEither charge will attract a neutral

HOW DO THESE FORCES INTERACT WITHIN AN OBJECT?

On a conductor, all charge rapidly moves to the outside surface due to repelling forces.

Charge will always move to a lesser charges or uncharged object, or “ground” (literally earth)

- This is the reason we experience a static shock

WHAT IS COULOMB’S LAW?

Fe = kq1q2

r2

r

q1 q2

Force between two small charges.

k = 8.99x109 N· m2/C2 (use 9.0x109)k = the electrostatic constant

PROPERTIES OF COULOMB’S LAW

Positive forces indicate repulsionNegative forces indicate attraction

Coulomb’s Law is an inverse-square law.

Fe

r

Ex: Doubling the distance yields one-fourth the force

PRACTICE PROBLEM

Force is a vector – multiple charges create multiple forces. Find the resultant

+q1 +q2

-q3

ResultantForce from q2

Force from q1