Download 16&17 Static Electricity Notes

Electricity
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Static Electricity
• Charge comes in two forms, which Ben Franklin
designated as positive (+) and negative (-).
• Charge is quantized.
– The smallest possible stable charge, designated as e,
is the magnitude of the charge on 1 electron or 1
proton.
– A proton has charge of e, and an electron has charge
of -e.
– e is referred to as the “elementary” charge.
– e = 1.602 x 10-19 coulombs.
– The coulomb is the SI unit of charge.
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Sample Problem
• A certain static discharge delivers -0.5
coulombs of electrical charge. How many
electrons are in this discharge?
• q=ne
• n = q/e
• n = (-0.5 C) / (-1.602 x 10-19 C)
• n = 3,121,098,626,716,604,245
• OR 3.12 x 10 18
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Sample Problem
• 1. How much positive charge resides in
two moles of hydrogen gas (H2)?
• 2. How much negative charge?
• 3. How much net charge?
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Sample Problem
• The total charge of a system composed of
1800 particles, all of which are protons or
electrons, is 31x10-18 C.
• How many protons are in the system?
• How many electrons are in the system?
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Coulomb’s Law and
Electrical Force
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Demo #1
• 1. Demonstrate how you can pick up the
tissue without touching it in any way with
your body.
• 2. What is occurring on the atomic level
that lets you do this?
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The atom
• The atom has positive charge in the
nucleus, located in the protons. The
positive charge cannot move from the
atom unless there is a nuclear reaction.
• The atom has negative charge in the
electron cloud on the outside of the atom.
Electrons can move from atom to atom
without too much difficulty.
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So…
• You charge the balloon by rubbing it on
hair or on a sweater, and the balloon
becomes negative. How can it pick up a
neutral tissue?
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The Electroscope
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The electroscope is
Made from a metal
Or other conductor,
And may be contained
Within a flask.
The vanes are free
to move.
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Demo #2
1.Rub the plastic rod with
the fur. Bring the rod
toward the pole of the
electroscope. What
happens to the vanes?
2.Explain your
observations.
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Demo #3
1.Rub the glass rod with the
silk. Bring the rod toward
the pole of the
electroscope. What
happens to the vanes?
2.Explain what you observe.
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Demo #4
1. What happens when
you touch the
electroscope with the
glass rod?
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Electric Force
• Charges exert forces on each other.
• Like charges (two positives or two
negatives) repel each other resulting in a
repulsive force.
• Opposite charges (a positive and a
negative) attract each other, resulting in an
attractive force.
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Coulomb’s Law - form 1
• Coulomb’s law tells us how the magnitude of the
force between two particles varies with their
charge and with the distance between them.
• Coulomb’s law applies directly only to spherically
symmetric charges.
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Coulomb’s Law - form 2
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Spherically Symmetric Forces
Newton’s Law of Gravity
FG = Gm1m2
r2
Coulomb’s Law
FE = kq1q2
r2
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Sample Problem
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Sample Problem
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Superposition
• Electrical force, like all forces, is a
vector quantity.
• If a charge is subjected to forces from
more than one other charge, vector
addition must be performed.
• Vector addition to find the resultant
vector is sometimes called
superposition.
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The Electric Field
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Gravitational Fields
 S F = ma
• GmEmm = ma
• (2rE) 2
• a=GmE
•
4rE 2
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The Electric Field
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Why use fields?
• Forces exist only when two or
more particles are present.
• Fields exist even if no force is
present.
• The field of one particle only can
be calculated.
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Field around a + charge
**The arrows in a field
are not vectors, they
are “lines of force”.
**The lines of force
indicate the direction
of the force on a
positive charge
placed in the field.
**Negative charges
experience a force
in the opposite direction.
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Field around a - charge
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Field between charged plates
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Field vectors from field lines
• The electric field at a given point is not
the field line itself, but can be
determined from the field line.
• The electric field vector is always
tangent to the line of force at that point.
• Vectors of any kind are never curved!
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Field Lines and Path
of Moving Charge
• The electric field lines do not represent the
path a test charge would travel.
• The electric field lines represent the
direction of the electric force on a test
particle placed in the field.
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Field Vectors from Field Lines
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Force from an Electric Field
• The force on a charged particle placed
in an electric field is easily calculated.
• F = Eq
– F: Force (N)
– E: Electric Field (N/C)
– q: Charge (C)
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Sample Problem
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Sample Problem
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Sample Problem
• A proton traveling at 440 m/s in the +x direction
enters an electric field of magnitude of 5400 N/C
directed in the +y direction. Find the acceleration.
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For Spherical Electric Fields
• The electric Field surrounding a point
charge or a spherical charge can be
calculated by: E = k q / r2 where
– E: Electric Field (N/C)
– k: 8.99 x 109 N m2 / C2
– q: Charge (C)
– r: distance from center of
charge q (m)
• Remember that k = 1/4peo
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Principle of Superposition
• When more than one charge contributes to
the electric field, the resultant electric field
is the vector sum of the electric fields
produced by the various charges.
• Again, as with force vectors, this is
referred to as superposition.
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Keep in mind…
• Electric field lines are NOT vectors, but
may be used to derive the direction of
electric field vectors at given points.
• The resulting vector gives the direction of
the electric force on a positive charge
placed in the field.
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Sample Problem
• A particle bearing -5.0 mC is placed at -2.0
cm and a particle bearing 5.0 mC is placed
at 2.0 cm. What is the field at the origin?
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Sample Problem
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Sample Problem
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Electric Potential Energy
• The energy contained in a
configuration of charges.
• Like all potential energies, when it goes up
the configuration is less stable; when it
goes down, the configuration is more
stable.
• Unit: Joule
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Electric Potential Energy
• increases when charges are brought into
less favorable configurations.
DU>0
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Electric Potential Energy
• decreases when charges are brought into
more favorable configurations.
DU<0
•
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Electric Potential Energy
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Work and Charge
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Work and Charge
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Electric Potential
• Electric potential is hard to understand, but
each to measure.
• We commonly call it “voltage”, and its unit
is the Volt.
• 1V=1J/C
• Electric potential is easily related to both
the electric potential energy and to the
electric field.
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Electrical Potential and
Potential Energy
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Electrical Potential and
Potential Energy
DV = D U / q
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Sample Problem
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Electrical Potential in Uniform
Electric Fields
• The electric potential is related in a simple
way to a uniform electric field.
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Sample Problem
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Sample Problem Fr75
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