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Electric Charges
electricity comes from the Greek word elektron, which means amber.
Amber is petrified tree resin and the Greeks noticed that if you rubbed an amber rod with a cloth it would attract small bits of leaves
Today rubbing a balloon, a rubber rod, a glass rod or any number of substances causes attraction of other objects (Charging an object)
There are only two possible types of charge.Positive charge – rubbing glass rod with silkNegative charge – rubbing rubber rod with fur
Unlike charges attract and like charges repel
Charged objects also tend to attract neutral objects
What Happens when a object becomes charged?
Atoms have three subatomic particles: protons (+), electrons (-) and neutrons
In neutral atoms the # of protons = The # of electrons
The electrons, with addition of some energy, can be removed and transferred to other objects
Rod + fur – electrons move from fur to rod
Charging Objects
Equal number Of electrons And protons
Rubbing provides energy whichHelps to remove electrons from The cloth
Electrons move, but protons do not! Positively charged objects usually end up that way because they have lost electrons leaving them with more protons that electrons
Remember – we are just moving electrical charge from place to place
Conduction and Induction
There are other ways to give an object an electrical charge
Insulators vrs conductors
Charged/neutral Conductor (gold, copper)Allows electrons to move Through it
Insulator (rubber, glass)Does not allow electronsTo move through it
Conduction
Bring a neutral object in contact with a previously charged object. Both objects will end up with the same charge although it will be smaller than the initial charge
Electroscope – used to illustrate the behaviour of
electrical charges Metal knob
Gold leaves
ContainerMinimizes air resistance
Conduction
Electrons move up into the rod,Leaving the protons behind toRepel each other
Electrons move into the electroscope And move down the leaves causing Them to repel
Induction
A charged object is just brought near a neutral object. The charged object never actually touches the neutral object.
The electrons flow to the top to be close To the positive rod leaving the protonsBehind to repel each other
Using Induction to Determine Charge
Start with anelectroscopewith neg. charge
e- move up (attracted To pos. charge) andthe leavesmove closer together
e- move down (away from the neg. rod)the leaves move farther apart
Grounding
The Earth because of its immense mass can gain or lose many negative (or positive) charges and still remain neutral.
As a result when you make contact between a charged object and the Earth it will immediately lose its charge and become neutral.
This process is known as grounding. The third wire on an electrical outlet is grounded so as to avoid any dangerous charge buildup
Electrical Forces
We have seen how electrically charged objects can repel or attract other charged objects.
Is there a way to mathematically describe the attraction/repulsion? This question was answered by Charles Coulomb.
the greater the quantity of charge involved the greater the force of attraction or repulsion. Also, The force of attraction/repulsion actually depends on the square of the distance.
F = force of attraction/repulsion (Newtons or N)
q1 and q2 = quantity of charge on each object (coulombs or C)
d = distance between the two charged objects (m)
K = 9.00x109 N m2/C2
Conversions
Typically charges are usually measured in micro-coulombs (μC)
( 1C is a large quantity of charge)Is there a limit to how small a charge can be? As
it turns out there appears to be. No charge smaller than the charge found on an electron (or proton) seems to exist. This quantity of charge is called the elementary charge and has a value of:
e= 1.60 x 10-19 C
Examples
Two positive charges each of quantity10 µC are separated by a distance of 5.00x10-8 m. What force will each charge experience?
Three examples
A -30 µC charge is placed 40 cm from a second unknown charge. If the -30 µC charge experiences a net attractive force of 25.3 N, what is value of the second charge?
A 40 µC and -20 µC charge exert a force of magnitude 1000 N on each other. How far apart are the two charges?
How many protons are needed to have 1.0 C of charge?
Example
Three charges are arranged at the corners of a right triangle as shown below. Calculate the net force exerted on charge B.