Electrostatics

Electrostatics

• Electrostatics is the study of ; i.e.,

charged objects that are stationary or in a fixed position.

• Neutral Object: An object that has

. • # of =

• Charged Object: An object with an ` or of electrons.

-

+

++

Charged and Uncharged Objects

--

Elementary Charge (e)• What is the charge of an electron?What is the charge of an electron?

• (( ))

• What is the charge of a proton?What is the charge of a proton?• (( ))

• The magnitude of the charge of The magnitude of the charge of the the is equal and is equal and opposite that of the opposite that of the ..• Note: In physics, the charge of an Note: In physics, the charge of an

electron is NOT electron is NOT or or !!

Example #1

• An object has three excess electrons.– What is its “fundamental charge”?

– What is its charge in coulombs?

-

Note: - “ charge” = charge- “ ” is the variable letter used for

Example #2

• An object has 75 protons and 65 electrons– What is its “fundamental charge”?

– What is its charge in coulombs?

Note: You have fractions of the charge, e.g. of e or

Law of Conservation of Charge

• The total amount of charge in a closed system remains – charge is not or

, it only moves from one object to another

• Charge “moves” as a result of movement

ONLY!!!

Rubber and Wool/Glass and Silk

• Rubbing a with a piece of : The will pull the

electrons off the wool, so that the rubber rod will end up with a net charge and the will have a net charge.

• Rubbing a with a piece of : The will pull the

electrons off the , so that the rod will end up with a net

charge and the will have a net charge.

Conductors and Insulators• Conductors: Materials that allow for the

of electrons.– The best conductors come from the

of the periodic table.– The characteristics of the

because at least electron is not held tightly by the nucleus.

• Insulators: Materials that do allow electrons to .

Forces of Attraction and Repulsion

• In nature there are three principle forces – ,

and (strong & weak).– Electromagnetic:

• Attractive Forces: charges attract one another. A charged object will be attracted to a charged object and vice versa.

• Repulsive Forces: charges repel. Two charged objects or two charged objects in

proximity to one another will experience a repulsive force.

How do Objects Become Charged?

• : Through direct contact.• : Separation of Charge.• : Without direct contact.

Charge by Conduction

• Electrons flow from a charged object to an uncharged object through .

Charge

• Conductors: will the space between them, which means that they will reside on the surface of the object with a distribution.

• Insulators: will be located largely where they were transferred to the material.

Charge Distribution

• One of these isolated charged spheres is copper and the other is rubber. The diagram below depicts the distribution of - charge over the surface of two spheres. Which one is rubber and which one is copper?

Example #3• Two charge spheres are brought into contact

with one another and then separated. One of the spheres has a charge of -5 C while the other has a charge of +8 C prior to them being brought into contact with one another. – What is the total charge of the system? – What is the charge on each of the spheres afterwards?– How many elementary charges would have to be added

or subtracted from each sphere to make them neutral?

+++++++

++

+

+

++

++

+

++

__

_

_

___

_

_

_

___

_

_

++

++

+ ++++++++

+

+++

+++

++

__

_

____

_

_

_

____

_++

+++

+

++ +

+

_+

_

__+_

_

+

+

_+_+

++ ++ +

Example 3, cont.:

+ ++++++++

+

+++

+++

++

__

_

____

_

_

_

____

_+ ++ +

_+

_

__+_

_

+

+

_+_+

++ ++ +

+

Example 3, cont.:

• There are two ways of finding the number of elementary charges that would have to be added or subtracted from each sphere.

• or

of Charge• When two neutral bodies are rubbed together, they

can become charged through .– As per , one body will

become charged while the other body will become charged.

– Electrons will be from one object and on the other.

• When a charged body is brought in close proximity to one that is neutral, the neutral one will develop an in charge distribution.

+

+

+

++

--

- --+-

-

- -++

+- - - - - - - -

❘

Separation of Charge and Lightning

Lightning

Lightning

Charge by

• Electrons flow from one sphere to the other due to of charge.

Charge by Induction of an Electroscope

Types of Charge

• Positively charged objects have a deficiency of electrons.

• Negatively charged objects have a surplus of electrons.

• Note that the type of charge is in reference to electrons and not protons.– Why?

• Because electrons exist outside the positive nucleus and can be readily transferred from one object to another.