Download Static Electricity - Kania´s Science Page

UCSD
Spring 2008
Physics 10
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Physics 10
UCSD
The Electric Force
Electric Charge
Electric Fields
Electron Beams
Physics 10
UCSD
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
• Electromagnetic force is one of the basic
interactions in nature
– like charges experience repulsive force
– opposite charges attracted to each other (like gravity)
• Electrical current is flow of charge (electrons)
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Physics 10
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Charge Balance
• Neutral atoms are made of equal quantities of
positive and negative charges
– Neutral carbon has 6 protons, 6 electrons, (& neutrons)
• Electrons can be stripped off of atoms
– Electrons occupy the vulnerable outskirts of atoms
• Usually charge flows in such a way as to maintain
neutrality
– Excess positive charge attracts excess negative charge
– Your body has 51028 positive charges and 51028
negative charges, balanced within millions or billions
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Physics 10
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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
– charge is free to “move about the cabin”
– why do the charges collect on the surface?
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Physics 10
UCSD
Induced Charge
• Charge can also be coaxed to redistribute itself
within an object
Result: Attraction!
+
+
+
–
–
+
+
+
–
–
–
–
–
– –
–
– – – – –
– – – –
–
Charged rod approaches sphere
+ charge attracted to – charge in rod
– charge repelled by rod
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Physics 10
UCSD
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-field” 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
• hold onto key to reduce pain of spark
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Physics 10
UCSD
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
• www.stormchasing.nl/lightning.html
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Physics 10
UCSD
Lightning Rods
• Perform two functions
– provide safe conduit for lightning away from house
– diffuse situation via “coronal discharge”
Charges are attracted to tip of rod, and
“electric field” is highly concentrated
there.
Charges “leak” away, diffusing charge
in what is sometimes called “St. Elmo’s
Fire”, or “coronal discharge”
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Physics 10
UCSD
ST. ELMO’S FIRE
Spring 2008
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UCSD
Static Electricity Lab
Physics 10
• Tear a piece of paper into small pieces, starting
about 15 cm away bring an unrubbed ebonite
(gray) rod close until almost touching. Record
results. Repeat for iron filings and small stream of
water flowing from the beaker (over the pan).
• Repeat the process with an ebonite (gray) rod
rubbed with the fur material.
• Repeat the rubbed procedures with the cotton
material and the nylon material.
• Then repeat all procedures (rubbed and unrubbed)
using the clear plastic rod, metal rod and the
comb.
• Summarize and explain all results!
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Physics 10
UCSD
“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!
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Physics 10
UCSD
Coulomb Law Illustrated
• Like charges repel
• Unlike charges attract
+
+
r
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+
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If charges are of same magnitude (and same separation),
all the forces will be the same magnitude, with different
directions.
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Physics 10
UCSD
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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Physics 10
UCSD
Electric Force a lot like Gravity
• Same 1/r2 dependence; charge takes place of mass.
• Does this mean electricity is product of geometry,
just like gravity (general relativity)?
• no because gravity is always pulling down, but
electrostatic force can act in any direction
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Physics 10
UCSD
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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Physics 10
UCSD
Electric Field
• Calculating Electric Field (unit of
Newton/coulomb)
• Think of it as the ability to pull negative charge
towards it.
• If you know the force and the charge you are
attracting, solve for it using
• E=F/q
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Physics 10
UCSD
Example Electric Fields Around Charges
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Physics 10
UCSD
Electric Fields in Circuits
• Point away from positive terminal, towards negative
• Channeled by conductor (wire)
• Electrons flow opposite field lines (neg. charge)
E
electrons & direction of motion
E
E
Electric field direction
E
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Electron Beams; Cathode Ray Tubes (CRTs)
• Televisions, Oscilloscopes, Monitors, etc. use an
electron beam steered by electric fields to light up
the (phosphorescent) screen at specified points
screen
+ + + + + + +
cathode emitter
E-field
electron beam
- - - - - - -
metal plates
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Physics 10
UCSD
Magnet, Static and Electric Field Quiz
Physics- Kania
Name:
1. If you wrap a current around a vertical metal bar and the
current is going around the bar to the right as you view it, what
direction is the north pole of the magnetic field created?
2.Why can iron be made to behave as a magnet but wood cannot?
3.Explain how a compass works.
4.Describe charging by induction.
5.Describe how static electricity is created in clouds.
6.In the static lab, why was the metal bar not able to be induced
into picking up paper, but the ebonite bar was?
7.What happens to the Electrostatic force between two charges if
the distance between them is doubled?
8.What happens to the Electrostatic force between two charges if
one of the charges is doubled and the distance between them is
doubled?
9.Describe the main difference between electric field lines and
magnetic field lines.
10.Describe St. Elmo’s Fire.
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Assignments
• Selected readings from Hewitt Chaps. 23, 24, 25,
26 (specific pages listed on assignments page)
• HW 6 due 5/23: 22.E.1, 22.E.5, 22.E.11, 22.E.16,
22.E.20, 22.E.30, 22.E.33, 22.P.1, 23.E.3, 26.E.7,
26.E.9, 26.E.11
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