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Electrostatics - PowerPoint PPT Presentation

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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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Electrostatic materials
Electrostatic Materials


Materials that transfer charge easily.


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


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


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

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


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


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.