Electricity

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Electricity

• Electrostatics and Fields

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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)

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• There are only two possible types of charge.
• Positive charge rubbing glass rod with silk
• Negative charge rubbing rubber rod with fur
• Unlike charges attract and like charges repel
• Charged objects also tend to attract neutral
  objects

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Repulsion/Attraction of Charges
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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

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Charging Objects
Rubbing provides energy which Helps to remove
electrons from The cloth
Equal number Of electrons And protons
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• 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

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Conduction and Induction

• There are other ways to give an object an
  electrical charge
• Insulators vrs conductors

Conductor (gold, copper) Allows electrons to move
Through it
Charged/neutral
Insulator (rubber, glass) Does not allow
electrons To move through it
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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
Container Minimizes air resistance
Gold leaves
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Conduction
Electrons move into the electroscope And move
down the leaves causing Them to repel
Electrons move up into the rod, Leaving the
protons behind to Repel each other
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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 protons Behind to repel
each other
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Induction

• The electrons flow downward away from the neg.
  rod and they repel each other

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Using Induction to Determine Charge
Start with an electroscope with neg. charge
e- move up (attracted To pos. charge) and the
leaves move closer together
e- move down (away from the neg. rod) the leaves
move farther apart
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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

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Grounding
When grounded an electroscope will lose or gain
electrons until it becomes neutral
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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.

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• 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

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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

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Examples

• Two positive charges each of quantity10 µC are
  separated by a distance of 5.00x10-8 m. What
  force will each charge experience?

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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?

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Example

• Three charges are arranged at the corners of a
  right triangle as shown below. Calculate the net
  force exerted on charge B.

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• Both charges A and C exert a force on B. The net
  force on B will therefore be the resultant of the
  force that A and C exert on it. Even though A and
  C exert a force on each other these forces have
  no effect on B so we can ignore them