electrostatics. electrostatics electricity at rest

Electrostatics

ELECTROSTATICS

Electricity at rest

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– positive and negative– Named by Benjamin Franklin

+

+ -

--

+

•Like charges repel •Opposite charges attract

- +

Charges continued

The natural order is balanced charges Net charge of zero

-

-

-

-

-

-

+

+

++

+

+

-

-

-

-

-

-

+

+

++

+

+

+

+

++

+

+

-

-

-

-

-

-

•Unbalanced charges are possible

Question #2

•The charge on sphere 2 is three times the charge on sphere 1

• Which force diagram is correct?• A,B,C,D, or E (none of them)

More Properties of Charge

Positive charge carrier is the proton– Protons do not move from one material to another

Held in Nucleus

Negative charge carrier is the electron (e-)– An object becomes charged (+ or -) by gaining or

losing electrons

More Properties of Charge

Electric charge is always conserved– Charge is not created, only exchanged– Charging occurs through the exchange of

electronsLose an electron

– Gain a positive charge

Gain an electron– Gain a negative charge

Properties of Charge, final

The SI unit of charge is the Coulomb (C) Charge is quantized

– All charges are a multiple of the fundamental unit of charge, symbolized by (e)

– Electrons have a charge of e-

e- = -1.602 x 10-19 C– Charge of -2 = 2* e- = 2* (-1.602 x 10-19 C)

– Protons have a charge of e+

e+ = 1.602 x 10-19 C– Charge of +2 = 2* e+ = 2* (1.602 x 10-19 C)

Conductors

Conductors: 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 : 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 : characteristics between those of insulators and conductors– Silicon and germanium are examples

Charging…

Three ways– Friction

Mechanical motion (rubbing)

– Conduction (or Contact)Direct contact (no rubbing)

– InductionCharge alteration without any contact

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 Conduction

--- -- --

-- - -

---

- ---- -- --

-- - -

---

- -

-

-- - --

-

--

--

-

-- -

-

Charging by Induction

• Induced charge - NO physical contact between charged & uncharged object

• OPPOSITE charge is INDUCED

Temporary charge Induction

--- -- --

-- - -

---

- --

-

-

-

-

-

+

+

++

+

+

--- -- --

-- - -

---

- --

-

-

-

-

-

+

+++

+

+

ELECTRICALLY POLARIZED

Permanent charge Induction

-

-

-

-

-

-

+

+

++

+

+

--- -- --

-- - -

---

- -

--- -- --

-- - -

---

- -

++

++++

GROUNDING

Another way to Induce a charge

-

--

-

-++

++

+

+

--

- -

-

+

+ +

+

- Net Charge of Zero

+

+ -

-

-

--

+

+ - -+

+

+

+ -

-

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

Electrical Field

Gravitational Field - A force field that exists around any object with mass– Interacts with mass

Electric field - A force field that exists around a charged object– Interacts with charges– How do we know it exists?

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

direction of movement determines charge of the field

Visualizing an Electric Field

Michael Faraday developed the concept of drawing Electric Field Lines– Vector quantity– Proximity of field lines indicates field strength– Arrows indicate direction of field

Direction indicates the charge– Out of positive– Into negative

Electric Field Lines

Point Charge– Field lines radiate

equally in all directions Radiate out on positive

– Proximity to each other indicates field strength

+

Negative Point Charge– Lines point inward

Towards the charge

-

Electric Field Line Patterns

+ -

Electric dipole - consists of two equal and opposite charges

– Add field lines Connected field lines

indicates opposite charge

– Matching numbers of field lines indicates similar charge values

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

Electric Field Lines

Electric Field Line Patterns

Two equal but like point charges

Zoomed out (far away)– the field would be appear

to be one charge Zoom in (close-up) No connections indicate

like charges; (repulsion) Low density of field lines

between the charges indicates a weak field in region “C”

Electric Field Lines

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, cont.

Electric Field, cont.

How do we know they are there?– Interact with charges

How do we know what charge they are?– Experimenting (testing)– test charge, placed in the field, will experience a force

Electric Field Testing, cont.

+++

++

+++

++

+++

-

Electric Field Testing, cont.

+

-

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

Electric Field

Mathematically,

The electric field is a vector quantity

Question #9

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

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

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

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…

Question #9

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

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…

Electrostatic Forces

• If like charges repel and opposites attract…• That means there is motion• If there is motion there must be a force (F = ma)

• Newton’s Second Law

• There must be a way to calculate the electrostatic force!

Coulomb’s Law

F = electrostatic force or electrical forceke = electrostatic force constant

aka – proportionality constantaka – Coulomb’s Constant= 9.0x109 Nm2/C2

q1 = charge for particle 1q2 = charge for particle 2r = radius (distance between charges)

Question #4

Two charges (+20 C and -10 C) are 3 m apart. What is the magnitude of the force between them?

a) 0.2 Nb) 0.6 Nc) 22.22 N d) 2.0 x 10 11 N

F = ?q1 = +20uCq2 = -10uC r = 3mke = 9x109 Nm2/C2

F

m

CxCxCNmxF

2

66229

)3(

10101020/109

0.2 N

Balloon on a Ceiling

Which is stronger the force of gravity, or electrical force?

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 Electric Forces

FORCE

OPPOSITE

EQUAL

Two point charges are separated by a distance r

The unlike 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 Electric Forces

FORCE

OPPOSITE

EQUAL

r

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?

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

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?

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

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…

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?

Electrical Shielding

Electrical charges spread over the surface of a conductor in such a way that the net charge INSIDE (at the center) of the conductor is zero

Electrical Potential Energy

Energy possessed by a charge by virtue of its location

A function of the charge sizes involved and their proximity to one another

++++

++++ +

++

++

Electrical PE >> KE

Electric Potential

ech

nergypotentialeelectricaltentialelectricpo

arg

coulomb

jouleVolt 11

Electrostatics

The End…