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Electrostatics

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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– 1600– Found that electrification was not limited to amber

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

Question #2

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 #2: (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

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

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, final

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

Self-explanatory… (demo)

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

Charging by Induction

1. NO physical contact between between charged & uncharged object

2. OPPOSITE charged is INDUCED

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 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 #3: (c) Same

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

Question #4

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 #4: (b) Electron

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

Question #5

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 #5: (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 #6

If q1 = +20 C and q2 = +10 C 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 #6: (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 #7

If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the 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 #7: (a) Away

If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the DIRECTION of the force between them?

Like charges repel

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 #9

What is the magnitude of the electric field 0.50 meters away from a -3C point charge?

F q0E

E pt chg kq

r2

a) 1.08 x 105 N/Cb) -1.08 x 105 N/Cc) 5.4 x 104 N/Cd) -5.4 x 104 N/Ce) I don’t have a clue…

Answer #9: (a) 1.08x105 N/C


F q0E

E pt chg kq

r2


Question #9


F q0E

E pt chg kq

r2


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 #10

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

a) 0 Nb) 6.7 x 10-4 Nc) 6.7 x 10-7 Nd) 6.7 Ne) I don’t have a clue…

F q0E

E pt chg kq

r2

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

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

F qE

F 2x10 9C 335N

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 dipole consists 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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