Intro to Electrostatics - Net Charge
March 7
Today
1.
Intro to Electrostatics
> Net Charge
> Charging by Conduction/Induction
Period 6
2.
Electrostatic Force/Coulomb's Law (?)
3.
HW: Electrostatics Problems 1
1
Intro to Electrostatics - Net Charge
March 7
Charge; q or Q (C):
A material property, similar to mass. Protons [p] (+) and Electrons [e-] (-)
have charge.
Most objects are neutrally charged; this means there are an equal number
of p and e-.
An object obtains a Net Charge by either gaining (-) or losing (+) electrons;
protons are never gained/lost.
Friction is one way of transferring electrons from one object to another.
*Charge is conserved; you cannot create or destroy charges, you can only
move them from one object to another.
Period 6
2
Intro to Electrostatics - Net Charge
March 7
Net Charge
If an object has either gained (-) or lost (+) electrons, we say it has
acquired a Net Charge. The Net Charge is determined by the excess or
extra number of charges that it has gained/lost.
We determine net charge in the following way:
q = ne
where q (could also be Q) represents the Net Charge, n is the extra
number of protons/electrons, and e represents the "Elementary Charge";
the amount of charge of 1 p, or 1 e-.
*e = 1.6 E-19 C
C stands for "Coulomb" [coo-low], the SI unit for
charge
*The charge of 1 p = 1 e; the charge of 1 e- = -1 e.
*To find n, we usually will take #p - #e-
Period 6
3
Intro to Electrostatics - Net Charge
March 7
Example 1: A Helium nucleus contains 2 protons and 2 neutrons. What is
the net charge of this nucleus?
Example 2: An ion has 30 protons and 28 electrons. What is the net charge
of this ion?
*Example 3: An ion has a net charge of -4.8 E-19 C, and it has 54 protons.
How many electrons does it have?
Period 6
4
Intro to Electrostatics - Net Charge
March 7
Conductor:
• Metals; some non-metals (semi-conductors)
• "Free" e- in valence shell are free to move from atom to atom
Insulator:
• Most non-metals
• Valence e- are tightly bound; not free to move
Period 6
5
Intro to Electrostatics - Net Charge
March 7
Charging by Conduction:
• Requires physical contact between 2 objects
• Electrons move from one object to the other, towards the larger + charge
• The total charge before and after MUST be the same
• Charges tend to be balanced after the transfer of eContact
Before
Period 6
After
Before
Contact
After
Before
Contact
After
6
Intro to Electrostatics - Net Charge
March 7
Charging by Induction:
• Objects do not touch
• Charges will be added to/removed from the "system"
• *Allows for charging of the opposite sign
• 4 step process:
1.
Bring a charged object near a (usually neutral) CONDUCTOR
Charged Object
+
Neutral Conductor (metal)
2.
"Ground" the conductor, AWAY from the charged object
Neutral Conductor (metal)
Charged Object
+
3.
Remove the "ground"
Charged Object
+
4.
No longer Neutral Conductor (metal)
Remove the charged object
No longer Neutral Conductor (metal)
Period 6
7