Chapter 32 - ELECTROSTATICS

 

I.              Demonstrations:

1.    Rub fur on rubber rod. Charge pith ball. Bring rod to pith ball.

2.    Rub silk on glass rod. Bring near pith ball.

3.    Plastic and metal plate

 

II.           Like charge repel. Unlike charge attract.

 

III.         There are four ways to charge things.

1.    Rubbing: This is where electrons are stripped away from the object.

n    Where the electrons are gone= positive charge.

n    Where the electrons are extra= negative charge.

 

2. Touching : When an object with missing or extra electrons touches a neutral object, electrons are transferred.

á    if object has extra electrons (negative) – neutral object gains electrons and becomes negative

á     if object is missing electrons (positive) – neutral object loses electrons and becomes positive

á     object becomes the same charge as the charging object

 

2.    induction—when a charged object is brought near another neutral object, the object must be grounded.

 

                                              i.     grounding means the object is touching the ground so that electrons can go from the object, or to the object, from the ground. The earth is an infinite supply of electrons, one more or less makes no difference to its charge

 

                                            ii.     if the charging object has extra electrons it repels the electrons in the neutral object into the ground= missing electrons ˆ positive

 

                                          iii.     if the charging object is missing electrons, it attracts the electrons in the neutral object so that electrons from the ground come up into the neutral object= extra electronsˆ negative

 

                                          iv.     induction causes object to acquire the opposite charge of the charging object

 

3.    Master card, Visa, American Express!!

 

 

IV.         Electrostatic series: anything above will lose electrons to anything below due to affinity for electrons

 

1.    glass

2.    wool

3.    cat fur

4.    calcium, magnesium, lead

5.    silk

6.    aluminum, zinc

7.    cotton

8.    paraffin

9.    ebonite

10.carbon, copper, nickel

11.rubber

12.sulphur, platinum, gold

 

V.           Conductors vs. insulators

 

1.    Conductors hold onto their electrons loosely so that the electrons are able to move easily, like metals.

2.    Insulators hold onto their electrons tightly so that the electrons are harder to remove, like nonmetals.

3.    What would you consider water?? DEMONSTRATION:

Light bulb and water and magic powder!

 

VI.         Demonstrations:

1. Balloon rubbing and wall

     wall is polarized = charges are separated

2. Water is permanently polarized because the hydrogen atoms have a slight positive charge and the oxygen atoms have a slight negative charge.

Charge rod and place next to running water to show attraction to either side.

 

VII.      Electricity is a much GREATER force than gravity. The formulas for the forces are the same

 

Fg= Gm1m2/R2

Fe = kqaqb/R2

 

n    butÉ G = 6.67 x 10-11  Nm2/kg2  and

n            k = 9x109 Nm2/c2

 

Where

Q= charge on the object a or b in coulombs (C)

R= separation between the centers of the charge objects

C= Coulomb = measure of charge, (charge on 6.25 x 108 electrons), a rubbed comb has a net charged of one millionth of a C; lightning DISTRIBUTES about 1 coulomb  between ground and cloud

 

VIII.    Sample problem

A positive charge of 6 x 10-6 C is .3m from a second positive charge of 3x 10-6 C. What is the force between the charges?

 

G: qa = 6 x 10-6C, qb = 6 x 10-6C, R = .3m,

K = 9 x 109   Nm2/C2

F: Fe= ?

S,R; Fe= Kqaqb/R2

C: Fe= ( 9 x 109 Nm2/C2) (6 x 10-6C) (6x 10-6C)

                                (.3m)2

Fe = 1.8 N

 

 

 

Chapter 33 - ELECTRIC FIELDS AND POTENTIAL

 

1)   E- Fields-

 

a)    You have all heard of fields before, ex:

i)     Football field- where the game is played

ii)   International field of influence- places a nation had influence

 

b)   In physics we will also deal with fields:

i)     Electric fields (E- fields) – an area over which an electric charge has influence.

ii)   Gravitational field ( G- field)- area over which the affects of gravity can be felt.

iii) Magnetic field ( B- field) – area over which the affects of magnetism can be felt.

 

c)    Today we will deal primarily with E- fields.

 

2)   E- Fields Vs G- fields

 

a)    Similarities :

i)     surround an agent

ii)   must have mass

iii) obey the inverse- square law

 

b)   Difference – (only 1) gravity only attracts while electricity can attract and repel.

 

3)   Electric Field Lines-

 

a)    Because of the attraction and repulsion of electricity, the force it creates is a vector quantity with direction.

b)   A field is thought of as how a positive test charge would react if it was placed in the field, that is which way would it move

 

4)    Check Question: If a tiny test charge were dropped in the field I just drew, in which direction would it move?

 

5)   Shielding-

 

a)    A conductor will shield an electric field.

 

b)   This is why the safest place to be during lightning is inside a  car. The car is made of metal which will shield the electric field.

 

c)    The center of any shape conductor will always have a net electric force of 0 because the forces within it must reach equilibrium so as not to move. This occurs instantaneously.

 

6)   Electric Potential Energy and Electric Potential

 

a)    Fields  are called force fields because forces are exerted on the bodies within them. They could properly be called energy fields, because energy is stored in a field.

 

b)   Electric potential energy is analogous to gravitational potential energy.

 

c)    Electric potential energy = total energy provided by charged particles force field.

 

E= Fe/q

 

            Where : E= electric potential energy ( N/C) – total energy

            Fe= electric force experience by test charge  ( newtons)

                  q= charge on particle in field (coulombs)

 

d)   Electric Potential difference= potential energy difference between two points. This is what makes charge move. Negative electrons move away from the highest negative electric field potential to the highest positive electric field potential.

 

EPD = V= Ed

  

            Where:

            EPD= electric potential difference= V = voltage (volts)

            E= electric potential energy (N/C)

            d= distance between two charge objects creating E- Field (m)

 

e)    Van de Graaf demonstrations:

i)     pie plates stacked

(1) E- field is being generated by each pie tin and the repel.

ii)   fluorescent tube

(1) place one end near Van de Graaf

(2) electrons only move if there is an electric potential difference.

 

f)    Sample problems

 

i)     What is the electric potential energy of a charge particle if it exerts a 10 N force on a 2 C test charge?

 

         G: Fe = 10N, q= 2C

            F: E=?

            S, R: E= Fe/q

                 C: E= 10 N/2C = 5N/C

 

ii)   If two charged plates are 6m apart and have an electrical potential difference of 12 volts, what is the electric potential energy stored between the plates?

 

            G: V= 12V, d= 6m

            F: E= ?

            R: V= Ed

            S: E= V/d

            C: E= 12 V/6m= 2V/m= 2N/C