Electrostatics
Electrostatics
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The study of electrical charges that can be
collected and held in one place.
To understand electricity we must understand
the atom.
Protons – positive charge, held in the nucleus by
the strong nuclear force.
Electrons – negative charge, found outside of
the nucleus
Neutrons – neutral charge, also found in the
nucleus.
Elementary charge
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The charge on both a proton and an
electron is known as the elementary
charge (e). (cover of reference tables)
Protons carry a positive elementary charge
Electrons carry a negative elementary
charge.
Charged Particles
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Most atoms contain an equal number of
protons and electrons. Therefore they are
electrically neutral.
An atom with more protons than electrons
will have a positive net charge.
An atom with more electrons than protons
will have a negative net charge.
Obtaining Charges
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The transfer of electrons to or from an
object, causing a deficiency or excess of
electrons.
An object loses electrons it will be +
An object gains electrons it will be –
Grounding an object allows for excess
electrons be released or more electrons to
be gained to achieve a neutral charge.
Charges Interacting
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Two objects that are both positively
charged (+) (+), or both negatively
charged (-) (-), repel one another.
Two objects that have opposite charges
respectively, (+) (-), or (-) (+), attract
one another.
Neutral Objects
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A positively charged object will attract
either a negatively charged object or a
neutral object.
A negatively charged objects will attract a
positively charged object or a neutral
object.
This is caused by an induced
charge…..more to come on that later.
Conservation of Charge
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Your hair and a comb are both neutral.
Running the comb through your hair causes
electrons to be stripped off of your hair and
added to the comb.
Your hair’s charge is now _____
The comb’s charge is now ______
The number of electrons lost by your hair is
equal to the number of electrons gained by the
comb.
elost = egained
Insulators and Conductors
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Conductor – material which readily transfers charge.
Insulator – material which does not transfer charge
easily.
Insulators and conductors can be charged by contact.
This occurs when there is a transfer of electrons
between two objects which are rubbed together.
Example: Rubber and fur.
Fur has a low affinity for electrons.
Rubber has a high affinity for electrons.
Electroscope – a device that can detect the presence
of an electric charge and the charges sign. (ex. Pith
ball)
Charging by Conduction
Neutral pith ball
Positively charged rod moves toward pith ball.
Upon contact, the electrons flow from the pith ball into the rod.
Now both objects have a net positive charge.
Transferring Charge
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Add the two charges and divide by two to find
the final charge.
Object A
Object B
Object C
+14e
-46e
Neutral
If A comes in contact with B what will each
charge be? A =
B=
If B comes in contact with C what will each
charge be? B =
C=
If C comes in contact with A what will each
charge be? C =
A=
Charging By Induction
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Induction – charging an object by bringing
it near another charged object.
Two neutral conducting spheres
The negatively charged PVC pipe repels
the electrons in the sphere to one side.
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Moving the spheres apart, sphere A is left
with a positive net charge.
Measuring Charge
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Charge (q) – measured in a unit called the
Coulomb (C). The smallest unit of charge
called the elementary charge is equal to the
charge on a single electron (e).
e = -1.6 x 10-19 C
A proton has a charge of + 1.6 x 10-19 C
****A charge will always be a multiple of
this number since we can’t have ½ of an
electron or proton****
Coulomb’s Law
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Two charged objects may experience motion
toward or away from one another.
Therefore there must be a force acting on these
objects.
The electrostatic force is as follows:
Fe = k (q1q2)
r2
where k is the electrostatic constant = 8.99 x 109 N
m2/C2
The electrostatic force is dependant upon the
magnitude of the charges as well as the distance
between the objects.
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What equation does this appear similar too?
Fe >>Fg
Fe always acts on a straight line between
objects.
Electric force must be a field force since there
is no contact between the objects.
Practice
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Example Problem: Find the electrostatic force
between these two point charges. The distance
between them is 2.3 x 10-3 m.
Fe = k (q1q2)
r2
Fe = 12760 N Attraction
Magnitude and Direction
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This force, like other forces is a vector.
(magnitude and direction)
When finding the magnitude of the force,
ignore the positive or negative charge.
The direction will have to be found by
interpreting the charges relations.
F1-2 in the previous question is to the left
because 1 is attracting 2.
Practice #1
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How many excess electrons are on a
sphere with a charge of -9.20x10-17C?
575 electrons
Practice #2
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Two charges q1 and q2 are a distance d apart
and exert a force F on each other. What will the
new force be if:
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q1 is doubled and q2 is cut in half.
F
q1 is tripled and q2 is doubled.
6F
q2 is cut in half and d is tripled.
1/18 F
Practice #3
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Two negative charges of -24C each are
separated by 6.0cm. What force exists
between the charges?
1400N repulsive force
Practice #4
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Determine the magnitude of the
electrostatic force between a proton and
an electron that are separated by a
distance of 7.5x10-8m
4.1 x 10-14N