Electrostatics: Coulombs Law, Equilibrium, Electric Fields...the amount of electric charge lost by the other material. •Net charge remains unchanged. •Law the Conservation of Charge

Electrostatics: Coulombs Law, Equilibrium, Electric Fields

Atomic Structure

•Matter is made up of atoms.

Proton (positive charge)

neutron (neutral)

electron (negative charge)+

+

+

–

––

+

–

atom nucleus

Atomic Structure and Charge

•# electrons = # protons neutral

•# electrons > # protons negative charge(gain of e-)

•# electrons < # protons positive charge

(loss of e-)

Charge is Quantized: charges must be whole numbers and cannot exist in fraction form.

Triboelectric Series: Relative ranking of common materials based on their electron affinity

*electron donating materials (+, glass) * electron accepting materials (-, teflon)

• Glass

• Human Hair

• Nylon

• Silk

• Fur

• Aluminum

• Paper

• Cotton

• Rubber

• PVC

• Teflon

+ + + + +

+ + + +

+ + +

+ +

+

-

- -

- - - -

- - - - -

- - - - - -

Electric Charge is Conserved

•When two materials interact, the amount of electrons gained by one material must be equal to the amount of electric charge lost by the other material. • Net charge remains unchanged.

• Law the Conservation of Charge

Charge and Mass of Atomic Particles

Charge Mass

•Electron -1.60 x 10-19 C 9.109 x 10-31 kg

•Proton +1.60 x 10-19 C 1.673 x 10-27 kg

•Neutron 0 1.675 x 10-27 kg

Insulators

•Insulators: materials that do NOT allow electrons to flow through them easily.

• Insulators can be easily charged by friction as the extra electrons gained CANNOT easily escape.

• Some common insulators are glass, air, plastic, rubber, and wood.

•Conductors: materials that allow electrons to flow through them easily.

• Conductors CANNOT be easily charged by friction as the extra electrons gained can easily escape.

• common conductors are copper, aluminum, gold, and silver.

Conductors

Charging objects

Polarization: the process of separating opposite charges within an object, net charge of material remain the same

• positive charge becomes separated from the negative charge

3 Ways to Charge an Object

1. Friction: rubbing two different materials together creates polarization

• objects made of different materials will hold onto their electrons with different strengths.

• As objects rub past one another the electrons with weaker bonds are “ripped” off of one material and collected on the other material.

2. Induction - Bring a charged object (rod) close to a neutral one (ball) without contact

• electrons in the ball will be repelled by the rod leaving a positive side (polarization)

• The now positive side of the ball will be attracted to the negative rod

+ + + + +

+ + + +

- - -

- - -

- - -

3 Ways to Charge an Object

Induction is a temporary change if no grounding occurs

electrons are not transferred

Induction is a permanent change if grounding occurs

electrons are transferred

• charge induced is opposite

3 Ways to Charge an Object

3. Conduction is a permanent charge with contact between 2 objects

electrons are transferred

• Charge conducted is the same

• After conduction the balls will repel each other

Lost

Gained El Electrons

3 Ways to Charge an Object

What is grounding?

• Removing a static charge by producing a path to the ground

• Electrons move from a negatively charged objects to the ground until the object is neutral

• Electrons move from ground to neutralize positively charged objects

• The earth both accepts and gives electrons while remaining overall neutral

Electric Charge: Section 2

• Symbol: Q or q

• Unit: coulomb (C)

• 1 C = the charge on 6.24 x 1018 electrons

Extra info to help you with problems

• 1 electron = -1.60 x 10-19 C

• A coulomb is a huge charge so charge is usually stated in µC (1x10-6 C) or nC (1x10-9 C)

Coulomb’s LawCharged objects can exert a force on each other. This force can

be either attractive force or a repelling force.

3 factors affecting the magnitude of the force between two charged objects:

1) Charge on the objects (magnitude of charge)

2) Distance between objects

3) Material separating objects

F: electrical force

Q1: charge 1

Q2: charge 2

d: distance between charges

k: constant depending on materials separating objects

- For air, k = 8.99 x 109 N·m2/C2, this is the value we will use for calculations

Coulomb’s Law

How are charge and electrical force related?

How is electrical force related to distance between the charges?

When using this equation:

• A positive force (F) signifies repulsion• Both charges (Qs) must be positive or both negative

• A negative force (Fe) signifies attraction• One charge (Q1 or Q2) must be positive and the other

negative

Sample Problemsa. What is the electrostatic force between

two objects, +13 μC and -22 μC which are 0.055m apart

(μC = x 10-6 C)

b. Is it an attraction or a repulsion?

Attraction (Q1 and Q2 are opposite signs)

Problem Set 11) What is the electrostatic force between two objects of -14x 10-6 C and -22 x 10-6 C that are 0.86 m apart?

2) The electrostatic force between two objects is +6.4 N when the charges are -4.5 x 10-6C and -8.8 x 10-6C. How far apart are the objects?

3) A force of -1.1N is exerted between two charged objects when they are 43 cm apart. The charge on one object is -5.7 x 10 -6 C. What is the charge on the other object?

Finding Net Force with more that one charge:Superposition

• See beige box 17 B on page 649 in your text book

Final Electricity Test Includes

• Ohms Law

• Resistance Rules

• Equivalent (total) Resistance

• Complex Circuit Resolution

• Electrostatics Concepts

• Coulombs Law (force)