Basics of Electrostatics

By Quantum Yuri Lubrica

based on the PPP of

Ma’am Bhazel Pelicano

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What if there were a universal force…?

varies inversely as r2

Billion billion x stronger than Fg

If it were attractive,

Tight ball

If it were repulsive,

Gaseous cloud

Suppose…

the universe consisted of 2 kinds of

particles

Like repel

Unlike kinds attract

There are equal numbers of the 2

kinds

--the force is equally balanced

… we’d have THIS universe!

Electrostatics

• Electric Charges

• Electric Force

• Electric Field

• Behavior in materials

Image courtesy of Hewitt’s Conceptual Physics

+ + - - - +

Electric Charge

• A scalar quantity no direction

• SI unit coulomb (C)

• Properties

–Dichotomy

–Conservation

–Quantization

Electric Charge

• Conservation

− The algebraic sum of all electric

charges in a closed system is

constant.

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Electric Charge

• Quantization

− The magnitude of the charge of an

electron or proton is a natural unit of

charge.

− Natural unit of charge: e = 1.602×10-19 C

− Of proton: +e = +1.602×10-19 C

− Of electron: -e = -1.602×10-19 C

Types of materials

• Conductors permits easy movements of electrons through them (e.g. most metals)

• Insulators do NOT permit easy movements of electrons through them (e.gceramic, wood, rubber)

Types of materials

• Superconductors with zero resistance against the motion of electrons through them

• Semiconductors with properties between conductors and insulators

• Germanium, Silicon (impurity), Selenium (light)

A little bit of history

• 600 B.C. amber

rubbed with wool,

amber attracts other

objects!

• Elektron from the

Greek word for amber

• Benjamin Franklin

gave the electron a

negative charge.

Why? (assignment)

Laser printers

Car paint

-- negatively

charged paint

adheres to

positively

charged metal

Fingerprinting

-- Fine mist of

negatively charged

gold particles

adhere to

negatively charged

protein on

fingerprint

Ways of charging a material

• Charging by friction

– Charge of material changes

– There is electron transfer

A little experiment

Ways of charging a material

• Charging by conduction

– By contact

– By lightning

Ways of charging a material

• Charging by induction – without grounding

Ways of charging a material

• Charging by induction – with grounding

Polarization – redistribution of charges

Example

Two identical conducting spheres, one with

initial charge Q, the other initially uncharged,

are brought into contact.

• What is the new charge on each sphere?

• While the spheres are in contact, a

negatively charged rod is moved close to

one sphere, causing it to have a charge of

2Q. What is the charge on the other sphere?

Q/2, Q/2

-Q

Exercise

Consider a small metallic bead suspended

from a thread. When a positively

charged rod is brought near but not in

contact with the bead, the bead is

attracted towards the rod. What could

possibly the charge of the bead?

Negatively charged, or

neutrally charged

Exercise

Two uncharged conducting spheres, A and

B, are suspended from insulating threads

so that they touch each other. While a

negatively charged rod is held near, but

not touching sphere A, the two spheres

are separated. HOW will the spheres be

charged, if at all?A – positive

B – negative

|QA| = |QB|

Exercise

A charged conductor is brought near an uncharged

insulator. Which of the following statements is TRUE?

A. Both objects will repel each other.

B. Both objects will attract each other.

C. Neither object exerts an electrical force on each

other.

D. The objects will repel each other only if the

conductor has a negative charge.

E. The objects will attract each other only if the

conductor has a positive charge.

Exercise

Two identical conducting spheres are charged by

induction and then separated. Sphere 1 has

charge Q and sphere 2 has charge –Q. A third

identical sphere is initially uncharged. If sphere 3 is

touched to sphere 1 and separated, then touched

to sphere 2 and separated, what is the final charge

on each of the three spheres?

Sphere 1: Q/2

Sphere 2: -Q/4

Sphere 3: -Q/4

Electrostatic Force

k = proportionality constant

= 8.99 × 109 N·m2/C2

= 1/4o

o = permittivity of free space

Charles-Augustin de Coulomb

• Born June 14, 1736 at

Angouleme, France

• The definition of

electrostatic force of

attraction and repulsion

• Died August 23, 1806 at

Paris, France

Electrostatic Force

• The magnitude of force on charge q1 due to charge q2 is

equal in magnitude to the force on charge q2 due to charge

q1. (recall Newton’s third law of motion)

• It is an inverse square law (see Newton’s Law of

Gravitation)

• For atomic particles, the electric force is greater than the

gravitational force between them.

Assignment, by twos 1 whole sheet of

paper

1. Explain how Franklyn assigned negative to the electron.

2. Why don’t protons in the nucleus of an atom mutually repel

and fly apart?

For 1 & 2, cite your references; re-word and simplify

3. Calculate the electric and gravitational force between the

proton and the electron of a hydrogen atom if they are

separated on average by a distance of 5.3 x 10-11 m? Find

the ratio between the two forces. You are only allowed to

search for the masses of the particles.