Transcript Document

The Electric Force
Topics
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Electrical Forces
Electric Charges
Conservation of Charge
Coulomb’s Law
Conductors and Insulators
Superconductors
Charging
Charge Polarization
Electric Field
Electric Potential
Electrical Energy Storage
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)
Electrical Attraction
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
“Electrostatic” Force
• Two charges, Q1 and Q2, separated by distance r exert a force on
each other:
F = (k·Q1·Q2) / 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 Newton’s gravitation in form
• Electron and proton attract each other 1040 times stronger
electrically than gravitationally!
– Good thing charge is usually balanced!
Coulomb Law Illustrated
• Like charges repel
• Unlike charges attract
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If charges are of same magnitude (and same separation),
all the forces will be the same magnitude, with different
directions.
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
Charge Separation
• Can separate charges by rubbing:
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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
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
Lightning
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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
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Thunder is bang produced by the extreme
pressure variations induced by the formation
and collapse of the plasma conduit
Electric Charges
Charge polarization
Charge polarization
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).
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Run
Away!
Electric Charges
Electric Shielding
Electric Potential
Electric Potential
electric potential = electric potential energy/charge
Electrical Energy Storage
Class Problem
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Two oppositely charged particles, an alpha
particle with two positive charges and a lessmassive 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:
As the particles get closer to each other,
each experiences an increase in:
g) force
h) speed
a) greater
i) acceleration
b) the same
j) all of these
c) less
k) none of these
The particle with the most acceleration is the:
d) alpha particle
e) electron
f) same for each
Class Problem
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The answers are b, e and j:
By 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.
Class Problem
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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 the:
a) opposite direction
b) same direction
c) … but it won't bend at all
Class Problem
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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
rod—whether 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.
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.
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.
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.
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.