The Electric Force

1
The Electric Force
2
Topics

• Electrical Forces
• Electric Charges
• Conservation of Charge
• Coulombs Law
• Conductors and Insulators
• Superconductors
• Charging
• Charge Polarization
• Electric Field
• Electric Potential
• Electrical Energy Storage

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Electric Charge

• Recall that fundamental particles carry something
  called electric charge
• protons have exactly one unit of positive charge
• electrons have exactly one unit of negative
  charge
• Electrical current is flow of charge (electrons)
• Electromagnetic force is one of the basic
  interactions in nature
• like charges experience repulsive force
• opposite charges attracted to each other (like
  gravity)

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Electrical Attraction
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Charge Balance

• Neutral atoms made of equal quantities of
  positive and negative charges
• Neutral carbon has 6 protons, 6 electrons, (
  neutrons)
• Electrons can be stripped off atoms
• Occupy the vulnerable outskirts of atoms
• Usually charge flows in such a way as to maintain
  neutrality
• Excess positive charge attracts excess negative
  charge

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Electrostatic Force

• Two charges, Q1 and Q2, separated by distance r
  exert a force on each other F (kQ1Q2) /
  r2
• k is a constant (9?109), Q is in Coulombs, r in
  meters
• One unit of charge (proton) has Q 1.6?10-19
  Coulombs
• Looks a lot like Newtons gravitation in form
• Electron and proton attract each other 1040 times
  stronger electrically than gravitationally!
• Good thing charge is usually balanced!

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Coulomb Law Illustrated

• Like charges repel
• Unlike charges attract

If charges are of same magnitude (and same
separation), all the forces will be the same
magnitude, with different directions.
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Coulomb Force Law, Qualitatively

• Double one of the charges
• force doubles
• Change sign of one of the charges
• force changes direction
• Change sign of both charges
• force stays the same
• Double the distance between charges
• force four times weaker
• Double both charges
• force four times stronger

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Charge Separation

• Can separate charges by rubbing
• feet on carpet
• atmosphere across ground
• silk on glass
• balloon on hair!
• Insulators keep charges where they are (no flow)
• Conductors distribute charge equally on surface

• Electrons loosely bound conductor
• Electrons tightly bound insulator
• Semiconductors
• Super Conductors

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Static Electricity

• Rubbing action redistributes charge (unbalanced)
• If enough charge builds up, we get discharge
• Air spark is actually due to breakdown of air
• neutral air molecules separate into ions
  (electrons are stripped away)
• current can then flow through the plasma-fied
  air
• In essence, air becomes a wire for a short bit
• this happens at 3 million volts per meter
• 1 cm spark then at 30,000 volts
• typical finger-spark may involve a few billion
  electrons

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Lightning

• Lightning is an unbelievably huge discharge
• Clouds get charged through air friction
• 1 kilometer strike means 3 billion volts!
• Main path forms temporary wire along which
  charge equalizes
• often bounces a few times before equal
• Thunder is bang produced by the extreme pressure
  variations induced by the formation and collapse
  of the plasma conduit

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Electric Charges
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Charge polarization
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Charge polarization
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Electric Field

• Can think of electric force as establishing
  field telling particles which way to move and
  how fast

Electric field lines tell a positive charge
which way to move. For example, a positive
charge itself has field lines pointing away from
it, because this is how a positively-charged test
-particle would respond if placed in the
vicinity (repulsive force).

Run Away!

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Electric Charges
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Electric Shielding
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Electric Potential
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Electric Potential
electric potential electric potential
energy/charge
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Electrical Energy Storage
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Class Problem
As the particles get closer to each other, each
experiences an increase ing) forceh)
speedi) accelerationj) all of thesek) none
of these

• Two oppositely charged particles, an alpha
  particle with two positive charges and a
  less-massive electron with a single negative
  charge are attracted to each other. Compared to
  the force that the alpha particle exerts on the
  electron, the electron exerts a force on the
  alpha particle that is a) greaterb) the
  samec) lessThe particle with the most
  acceleration is thed) alpha particlee)
  electronf) same for each

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Class Problem

• The answers are b, e and jBy Newton's 3rd Law,
  the particles pull on each other with equal and
  opposite forces. By Newton's 2nd Law, for the
  same force the particle with less mass undergoes
  more acceleration. By Coulomb's Law, as the
  separation distance is decreased, the force
  increases. By Newton's 2nd Law, as the force
  increases the acceleration increases. Since the
  particles accelerate toward each other, their
  speeds increase also.

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Class Problem

• A thin stream of water bends toward a negatively
  charged rod. When a positively charged rod is
  placed near the stream, it will bend in thea)
  opposite directionb) same directionc) but
  it won't bend at all

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Class Problem

• The answer is b. If you answered a, you likely
  thought the bending was due to positively charged
  water. But the water, even with many ions,
  normally has no appreciable net charge. The
  interaction between the charged rod and the water
  stream is mainly due to the dipole nature of
  water molecules. H2O molecules are electric
  dipoles, positive on the hydrogen side and
  negative on the oxygen side. Like compasses that
  align along a magnetic field, H2Os align along
  the electric field of the nearby rodwhether the
  rod is negative or positive. For both magnets and
  charges, the closest aligned pole or charge is
  always opposite in sign. Opposites attract, so
  net attraction is the result.

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Class Problems
To say that electric charge is quantized is to
say that the charge on an object 1) will
interact with neighboring electric charges. 2)
is sometimes positive. 3) is a whole-number
multiple of the charge of one electron. 4) can
be neither created nor destroyed. 5) may occur
in an infinite variety of quantities. To say
that electric charge is conserved is to say that
electric charge 1) is a whole number multiple of
the charge of one electron. 2) may occur in an
infinite variety of quantities. 3) will interact
with neighboring electric charges. 4) can be
neither created nor destroyed. 5) is sometimes
negative. When a car is struck by lightning,
the resulting electric field inside the car is
1) normally huge for a time longer than the
lightning stroke itself. 2) normally huge, but
for a brief time. 3) zero. 4) small enough to
be safe for an occupant inside.
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Class Problems
An uncharged pith ball is suspended by a nylon
fiber. When a negatively charged rubber rod is
brought nearby, without touching it, the pith
ball 1) is repelled by the rod. 2) becomes
charged by induction. 3) becomes polarized. 4)
in unaffected. 5) None of the above choices are
correct. After a capacitor is fully charged,
the total number of electrons it contains 1) is
much greater. 2) is slightly greater. 3) is
slightly less. 4) is much less. 5) remains
unchanged. When the distance between two
charges is halved, the electrical force between
the charges 1) halves. 2) quadruples. 3)
doubles. 4) is reduced by 1/4.. 5) None of the
above choices are correct.
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Class Problems
A negatively charged rod is held near a metal can
that rests on a dry wood table. If you touch the
opposite side of the can momentarily with your
finger the can is then 1) positively charged.
2) partially discharged. 3) completely
discharged. 4) negatively charged. 5)
discharged only on the side touched. The
electric field around an isolated electron has a
certain strength 1 cm from the electron. The
electric field strength 2 cm from the electron is
1) the same. 2) half as much. 3) twice as
much. 4) four times as much. 5) None of the
above choices are correct. If you use 10 J of
work to push a coulomb of charge into an electric
field, its voltage with respect to its starting
position is 1) more then 10 V. 2) less than 10
V. 3) 10 V. 4) None of the above choices are
correct.
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Class Problems
Two charges separated by one meter exert 1-N
forces on each other. If the charges are pushed
to 1/4 meter separation, the force on each charge
will be 1) 4 N. 2) 8 N. 3) 16 N. 4)
2 N. 5) 1 N. The electrical force on a
2-C charge is 60 N. What is the value of the
electric field at the place where the charge is
located? 1) 240 N/C 2) 20 N/C 3) 120
N/C 4) 60 N/C 5) 30 N/C A proton and
an electron are placed in an electric field.
Which undergoes the greater acceleration? 1)
proton 2) electron 3) Both accelerate equally.
4) Neither accelerates.