Electrostatics

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```					Electrostatics
Level 1 Physics
Objectives and Essential
Questions
 OBJECTIVES                          ESSENTIAL QUESTIONS
 How are objects charged?
 At the end of this unit, the
student should be able to:           What are the principles of
charging by induction?
 Define electrostatics and the
nature of electric charge
 What is Coulomb’s Law?
 State Coulomb’s 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
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
Electric Charge
The magnitude of the charge on             •The symbol for charge is q
the proton exactly equals the              •The unit for charge is Coulomb (C)
magnitude of the charge on the             •A single charged particle (1 electron
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
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
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.
Law of Conservation of
Electric Charge
During any process, the net electric charge
of an isolated system remains constant
(is conserved).
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).
Charging
There are three ways to charge an object:

FRICTION                               CONDUCTION

INDUCTION
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
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                Figure 2d: When negative object
touch. Some negative charge               is removed, it will not be as negative
will transfer over to uncharged           as it was. Both objects have some
metal object                              negative charge.
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
Electric Force
The electric force is similar to the gravitational force. Remember

1
Fg Mm                Fg  2
r
Coulomb’s 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

1       q1q2
FE  q1q2 FE  2 FE 2
r         r
k  constant of proportionality
Nm 2
k  Coulomb constant  8.99 x10     9

C2
qq
FE  k 1 2 2  Coulomb' s Law
r

For calculation ease, one may use 9.00 x 109 Nm2/C2 for the constant
Electric Forces and Newton’s
Laws
Electric Forces obey Newton’s 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  mg
q1q2            q1q2
Fe            k 2  mg  r  k
e                   r              mg
mg
r=?                     (1.6 x10 19 ) 2
(8.999 )           31
         5.1 m
e                             (9.11x10 )(9.8)

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