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Electrostatics

Electrostatics

Electrostatics is electricity at rest. It involves electric charges, the forces

between them and their behavior in materials.

Electrical Forces and Charges

Electrical Forces are acting on us all the time. There is both an attractiveforce and a repulsive force. These

forces arise from the particles that make up atoms.

A Bit of History

� Ancient Greeks

– Observed electric and magnetic phenomena as early as 700 BC

� Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers

� Magnetic forces were discovered by observing magnetite attracting iron

A Bit More History

� William Gilbert, M.D.

– 1600

– Found that electrification was not limited to amber

– Wrote De Magnete and distinguished electrical from magnetic phenomena.

� Charles Coulomb

– 1785

– Confirmed the inverse square relationship of electrical forces

History Final

� Hans Oersted– 1820

– Compass needle deflects when placed near an electrical current

� Michael Faraday– A wire moved near a magnet, an electric current

is observed in the wire

Properties of Electric Charges

� Two types of charges exist

– They are called positive and negative

– Named by Benjamin Franklin

Like charges repel and unlike

charges attract one another

More Properties of Charge

� Nature’s basic carrier of positive charge is

the proton

– Protons do not move from one material to another because they are held firmly in the nucleus

� Nature’s basic carrier of negative charge is

the electron

– Gaining or losing electrons is how an object becomes charged

Conservation of Charge

� The outer electrons of most atoms are not tightly bound to the nucleus. The amount of energy need to remove electrons varies for each element. For example, rubber keeps electrons, but fur gives them up.

� Electrons cannot be created or destroyed, only transferred.– For electricity we call this principle conservation

of charge.

� Electrons always are given and taken in whole units

Question #1

The charge on sphere 2 is three times the

charge on sphere 1. Which force diagram is

correct? (e) is none of the others.

Answer #1: (d)

The charge on sphere 2 is three times the

charge on sphere 1. Which force diagram is

correct? (e) is none of the others.

More Properties of Charge

� Electric charge is always conserved

– Charge is not created, only exchanged

– Objects become charged because

negative charge is transferred from one

object to another

Properties of Charge

� Charge is quantized

– All charge is a multiple of a fundamental unit of charge, symbolized by e

– Electrons have a charge of –e

– Protons have a charge of +e

– The SI unit of charge is the Coulomb (C)

� e = 1.602 x 10-19 C

Conductors

� Conductors are materials in which the

electric charges move freely

– Copper, aluminum and silver are good conductors

– When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material

Insulators

� Insulators are materials in which electric

charges do not move freely

– Glass and rubber are examples of insulators

– When insulators are charged by rubbing, only the rubbed area becomes charged

� There is no tendency for the charge to move into other regions of the material

Semiconductors

� The characteristics of semiconductors are

between those of insulators and conductors

� Silicon and germanium are examples of

semiconductors

Charging…

� Three ways

– Friction

– Conduction (or Contact)

– Induction

Charging by Friction

Charging by Conduction

� A charged object (the rod) is physically touches the other uncharged, object (the sphere)

� The same type of charge is CONDUCTED from the rod to the sphere

Charging by Induction


1. NO physical contact between between

charged & uncharged

object

2. OPPOSITE charged

is INDUCED

Charging by Polarization

o When induction occurs in an insulator, the

insulator molecule will change orientation.

� This is seen with a comb attracting an

uncharged piece of paper or a balloon

attaching itself to an uncharged wall.

� Molecules that have a more positive side and

a more negative side are said to be electric

dipoles.


••A charged object like a comb is brought near a neutral A charged object like a comb is brought near a neutral object like paper, which repels the like charges on the object like paper, which repels the like charges on the surface of the papersurface of the paper

Question #2

An alpha particle with two positive charges and

a less-massive electron with a single negative

charge are attracted to each other.

The force on the electron is:a) Greater than that on the alpha particleb) Less than that on the alpha particlec) Same as that on the alpha particled) I haven’t a clue…

Answer #2: (c) Same

The force on the electron the same as that on the alpha particle -Newton’s Third Law.

Question #3

An alpha particle with two positive charges and

a less-massive electron with a single negative

charge are attracted to each other.

The particle with the most acceleration is thea) Alpha particleb) Electronc) Neither - they have the same accelerationd) I haven’t a clue…

Answer #3: (b) Electron

The particle with the most acceleration is the ELECTRON. Newton’s Second Law (F=ma)

Question #4

An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. As the particles get closer to each other, each experiences an increase in:

a) forceb) speedc) accelerationd) All of thesee) None of these

Answer #4: (d) ALL

As the particles get closer, the FORCE

⇑ and thus the ACCELERATION ⇑ and also the SPEED ⇑

Coulomb’s Law

� Governs forces and charges,

� ke is called the Coulomb Constant

– ke = 8.99 x 109 N m2/C2

� Typical charges can be in the µC range

� Remember that force is a vector quantity

F = k e

q1 q2

r2

Question #5

If q1 = +20 µC and q2 = +10 µC and the two charges are 3 meters apart, what is the

MAGNITUDE of the force between them?

F = k e

q1 q2

r2

a) 0.2 Nb) 0.6 Nc) 22.22 N d) 2.0 x 10 11 Ne) I don’t have a clue

Answer #5: (a) 0.2 N

F = k e

q1 q2

r2

F =9x109( ) 20x10−6( )10x10−6( )

32

F = 0.2N

Vector Nature of Electric Forces

� Two point charges are separated by a distance r

� The like charges produce a repulsive force between them

� The force on q1 is equal in magnitude and opposite in direction to the force on q2

Vector Nature of Forces, cont.

� Two point charges are separated by a distance r

� The unlike charges produce a attractive force between them

� The force on q1 is equal in magnitude and opposite in direction to the force on q2

Question #6


DIRECTION of the force between them?

F = k e

q1 q2

r2

a) Away from each otherb) Towards each otherc) One chases the other d) Nothing - they don’t move at alle) I don’t have a clue

Answer #6: (a) Away


DIRECTION of the force between them?

Like charges repel

Electrical Fields

Electrical Field

� An electric field is said to exist in the region

of space around a charged object

– When another charged object enters this electric field, the field exerts a force on the second charged object

Electric Field, cont.

� A charged particle, with charge Q, produces an electric field in the region of space around it

� A small test charge, qo, placed in the field, will experience a force

Electric Field

� Mathematically,

� The electric field is a vector quantity

F = q0E

E pt chg = kq

r2

Question #8

� What is the magnitude of the electric field

0.50 meters away from a -3µC point charge?

F = q0E

E pt chg = kq

r2

a) 1.08 x 105 N/C

b) -1.08 x 105 N/C

c) 5.4 x 104 N/C

d) -5.4 x 104 N/C

e) I don’t have a clue…

Answer #8: (a) 1.08x105 N/C



F = q0E

E pt chg = kq

r2

a) 1.08 x 105 N/C

b) -1.08 x 105 N/C

c) 5.4 x 104 N/C

d) -5.4 x 104 N/C


Question #9



F = q0E

E pt chg = kq

r2

a) 1.08 x 105 N/C

b) -1.08 x 105 N/C

c) 5.4 x 104 N/C

d) -5.4 x 104 N/C


Direction of Electric Field

� The electric field produced by a negative charge is directed toward the charge

– A positive test charge would be attracted to the negative source charge

Direction of Electric Field, cont

� The electric field produced by a positive charge is directed away from the charge

– A positive test charge would be repelled from the positive source charge

Question #9

� What is the electrostatic force acting on a 2 nCcharge placed in a 335 N/C electric field?

a) 0 N

b) 6.7 x 10-4 N

c) 6.7 x 10-7 N

d) 6.7 N


F = q0E

E pt chg = kq

r2

Answer #9: (c) 6.7 x 10-7 N

� What is the electrostatic force acting on a 2 nCcharge placed in a 335 N/C electric field?

F = qE

F = 2x10−9C( ) 335

N

C

F = 6.7x10−7N

Electric Field Lines

� A convenient aid for visualizing electric field

patterns is to draw lines pointing in the

direction of the field vector at any point

� These are called electric field lines and were

introduced by Michael Faraday

Electric Field Lines, cont.

� The field lines are related to the field by

– The electric field vector, E, is tangent to the electric field lines at each point

– The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region

Electric Field Line Patterns

� Point charge

� The lines radiate equally in all directions

� For a positive source charge, the lines will radiate outward


� For a negative source charge, the lines will point inward


� An electric dipoleconsists of two equal and opposite charges

� The high density of lines between the charges indicates the strong electric field in this region


� Two equal but like point charges

� At a great distance from the charges, the field would be approximately that of a single charge of 2q

� The bulging out of the field lines between the charges indicates the repulsion between the charges

� The low field lines between the charges indicates a weak field in this region

Electric Field Patterns

� Unequal and unlike charges

� Note that two lines leave the +2q charge for each line that terminates on -q

Electric Field Lines

Electrostatics

� The End…

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