Electrostatics
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Electrostatics. Electrostatic Materials. Conductor Materials that transfer charge easily. Insulator Materials that don’t transfer charge easily. Law of Charges. Like charges repel. Opposite charges attract. Conservation of Charge.

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Electrostatics

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Electrostatics

Electrostatics


Electrostatic materials

Electrostatic Materials

Conductor

Materials that transfer charge easily.

Insulator

Materials that don’t transfer charge easily.


Law of charges

Law of Charges

Like charges repel.

Opposite charges attract.


Conservation of charge

Conservation of Charge

The net charge on an object or system must remain constant.


Example 1

Example #1

Two charged conductors, +6Q and –4Q, touch one another and are then separated. What’s the final charge on each conductor?


Fundamental charge

Fundamental Charge

Electric charge is quantized.

All net charge is a multiple of the quantum of electric charge, e.

e = 1.6 X 10-19 C

Q = Ne

  • where Q is the net charge

  • N is the number of charges

  • e is the fundamental charge.


Using example 1

Using Example #1

Is there an excess or deficit of electrons on each of the conductors? How many?


Example 2

Example #2

A plastic rod is rubbed against a wool cloth and acquires a charge of –8 C. How many electrons are transferred from the wool cloth to the plastic rod?


Methods of charging

Methods of Charging

Conduction

The process of touching a charged object to a neutral object.

Induction

The process of charging a neutral object without touching.

Polarization / Separation of Charge

The separation or realignment of charge within an insulator.


Coulomb s law

Coulomb’s Law

FE is the electrostatic force

q1 and q2 are charges

r is the distance between the charges center-to-center.

k = coulomb’s constant = 9 X 109


Qualitative understanding

Qualitative Understanding


Example 3

Example #3

Calculate the gravitational force between two 1-kg masses 1 meter apart and the electrostatic force between two 1-C charges 1 meter apart.


Example 4

Example #4

Calculate the ratio FE/FG for above objects at a distance r apart.


Example 5

Example #5

Two unequal charges, q1 = 2q2, are separated by 3 meters. Find q1 and q2, if the force q1 exerts on q2 has a magnitude of 9 X 10-3 N.


Electric field

Electric Field

A region of space around a charged object in which a stationary charged object experiences an electric force because of its charge.


Electric field1

Electric Field

E is the electric field

FE is the electrostatic force

q is a positive test charge.


Electric field2

Electric Field

E is the electric field

FE is the electrostatic force

q is a positive test charge

Units - N/C


Electric field line rules

Electric Field Line Rules

  • The electric field lines must begin on positive charges and must terminate on negative charges.

  • Electric field lines may never cross.

  • The number of lines drawn is proportional to the magnitude of the charge.


Electric potential energy

Electric Potential Energy

Definition

Potential energy associated with an object due to its position relative to a source of electric force.

Electrical potential energy is a form of mechanical energy.

Unit: joule, J


Example 6

Example #6

A positively charged particle’s electric potential energy decrease by 7.5 X 10-19 J as it moves 1.6 cm in a uniform electric field having a magnitude of 324 N/C. What direction relative to the electric field did the particle move? What’s the magnitude of the charge?


Electric potential difference

Electric Potential Difference

Definition

The change in electric potential energy associated with a charged particle divided by the charge of the particle.

Unit: volt, V (joule per coulomb)


Example 7

Example 7

The electric field between two oppositely charged plates is 2.4X103 N/C. If the plates are 5 cm apart, what’s the electric potential difference between the plates?


Example 8

Example 8

An electron initially at rest is accelerated across a potential difference of 25 kV and crashes into a target coming to an abrupt stop. Calculate the kinetic energy of the electron at impact in joules and electron-volts.


Millikan oil drop experiment

Millikan Oil Drop Experiment


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