Electrostatics

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Electrostatics

• Level 1 Physics

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Objectives and Essential Questions

• OBJECTIVES
• At the end of this unit, the student should be
  able to
• Define electrostatics and the nature of electric
  charge
• State Coulombs Law and its equation to calculate
  the electrostatic force between two charges
• Explain charging of an object by contact and
  induction
• Distinguish between conductors and isulators

• ESSENTIAL QUESTIONS
• How are objects charged?
• What are the principles of charging by induction?
• What is Coulombs Law?

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Electric Charge Just the facts
Charge is a property of subatomic particles

• Facts
• Two types of charge Positive (protons) and
  Negative (electrons)
• LIKE charges REPEL and OPPOSITE charges ATTRACT
• Similar to fluids Can be in 2 states STATIC
  and DYNAMIC

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Electric Charge
The magnitude of the charge on the proton exactly
equals the magnitude of the charge on the
electron.

• The symbol for charge is q
• The unit for charge is Coulomb (C)
• A single charged particle (1 electron
• or proton) is called an elementary
• particle (symbol is e).

Particle Charge Mass
Proton 1.6 x 10-19 C 1.67 x 10-27 kg
Electron -1.6 x 10-19 C 9.11 x 10-31 kg
Neutron 0 1.67 x 10-27 kg
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Charged Objects

• It is possible to transfer electric charge from
  one object to another.
• Usually, electrons are transferred
• The body that loses electrons has an excess of
  positive charge.
• The body that gains electrons has an excess of
  negative charge.

When an ebonite rod is rubbed against animal
fur, electrons from atoms of the fur are
transferred to the rod. This gives the rod a
negative charge and leaves a positive charge on
the fur
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Conservation of Charge

• When an ebonite rod is rubbed with animal fur
• Process serves to separate electrons and protons
  already present in the materials.
• Protons/Electrons are not created or destroyed.
• As an electron is transferred to the rod, a
  proton is left behind on the fur.
• Both have identical charges (magnitudes are the
  same, signs are opposite).
• Transfer does not change the net charge of the
  fur/rod system.
• Each material contains an equal amount of protons
  and electrons to begin with, net charge of the
  system is zero initially and will remain zero
  during the entire process.

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Law of Conservation of Electric Charge
During any process, the net electric charge of
an isolated system remains constant (is
conserved).
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Conductors and Insulators

• Not only can charge exist on an object,
• but it can move through an object.
• The movement of charge is limited by
• the substance the charge is trying to
• pass through.
• There are generally 2 types of substances.

Conductors Substances that readily conduct
electric charge Insulators Substances that
conduct electric charge poorly (resist the
movement).
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Charging
There are three ways to charge an object
FRICTION
CONDUCTION
INDUCTION
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Friction

• The only reason that we are able to use
  electricity in our modern world is that
• it is possible to separate positive and negative
  charges from each other.
• Since the two objects are made of different
  materials, their atoms
• will hold onto their electrons with different
  strengths
• As they pass over each other, the electrons with
  weaker bonds are
• stripped off of that material and collect on
  the other material.

Example Rub a piece of ebonite across a piece
of animal fur. The fur does not hold on to its
electrons as strongly as ebonite. Some of the
electrons will be ripped off of the fur and stay
on the ebonite. Now the fur has a slightly
positive charge and the ebonite is slightly
negative.
NET CHARGE IS STILL ZERO ? CONSERVATION OF
ELECTRIC CHARGE
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Conduction
The two objects will come into physical contact
with each other (sometimes called charging by
contact).
Figure 2a Negatively charged metal object and
an uncharged metal sphere
Figure 2b Bring the two objects close together,
separation of charge starts occur
Figure 2c The two objects touch. Some negative
charge will transfer over to uncharged metal
object
Figure 2d When negative object is removed, it
will not be as negative as it was. Both objects
have some negative charge.
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Induction
To charge a conductor without coming in direct
contact with it.
Figure 3a Metal sphere is on insulating stand
and has a ground wire attached.
Figure 3b Bring a negative object nearby.
Causes a separation of charge in sphere and
electrons will travel down the wire.
Figure 3c Keep negative object nearby. Either
cut or remove the ground wire. The electrons
have no way to travel back up the wire
Figure 3d Now, remove the negative object.
the sphere has a net positive charge
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Electric Force
The electric force is similar to the
gravitational force. Remember
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Coulombs Law
The magnitude F of the electrostatic force
exerted by one point charge q1 on another point
charge q2, is directly proportional to the
magnitude of the charges and inversely
proportional to the square of the distance r
between them
For calculation ease, one may use 9.00 x 109
Nm2/C2 for the constant
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Electric Forces and Newtons Laws

• Electric Forces obey Newtons Laws.

Example An electron is released above the
surface of the Earth. A second electron directly
below it exerts an electrostatic force on the
first electron just great enough to cancel out
the gravitational force on it. How far below the
first electron is the second?
Fe
e
mg
r ?
5.1 m
e