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Electrostatics

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Electrostatic Materials

Conductor

Materials that transfer charge easily.

Insulator

Materials that don’t transfer charge easily.

Conservation of Charge

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

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

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

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

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

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

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

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

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

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

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

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

- 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

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

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

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

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

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.