electrostatics. electrostatics electricity at rest
TRANSCRIPT
Electrostatics
ELECTROSTATICS
Electricity at rest
A Bit of History
Ancient Greeks– Observed electric and magnetic phenomena as
early as 700 BC Found that amber, when rubbed, became electrified and
attracted pieces of straw or feathers Magnetic forces were discovered by observing
magnetite attracting iron
A Bit More History
William Gilbert– 1600– Found that electrification was not limited to amber
Charles Coulomb– 1785– Confirmed the inverse square relationship of
electrical forces
History Final
Hans Oersted– 1820– Compass needle deflects when placed near an
electrical current
Michael Faraday– A wire moved near a magnet, an electric current
is observed in the wire
Properties of Electric Charges
Two types of charges exist– positive and negative– Named by Benjamin Franklin
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•Like charges repel •Opposite charges attract
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Charges continued
The natural order is balanced charges Net charge of zero
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•Unbalanced charges are possible
Question #2
•The charge on sphere 2 is three times the charge on sphere 1
• Which force diagram is correct?• A,B,C,D, or E (none of them)
More Properties of Charge
Positive charge carrier is the proton– Protons do not move from one material to another
Held in Nucleus
Negative charge carrier is the electron (e-)– An object becomes charged (+ or -) by gaining or
losing electrons
More Properties of Charge
Electric charge is always conserved– Charge is not created, only exchanged– Charging occurs through the exchange of
electronsLose an electron
– Gain a positive charge
Gain an electron– Gain a negative charge
Properties of Charge, final
The SI unit of charge is the Coulomb (C) Charge is quantized
– All charges are a multiple of the fundamental unit of charge, symbolized by (e)
– Electrons have a charge of e-
e- = -1.602 x 10-19 C– Charge of -2 = 2* e- = 2* (-1.602 x 10-19 C)
– Protons have a charge of e+
e+ = 1.602 x 10-19 C– Charge of +2 = 2* e+ = 2* (1.602 x 10-19 C)
Conductors
Conductors: materials in which the electric charges move freely– Copper, aluminum and silver are good conductors– When a conductor is charged in a small region,
the charge readily distributes itself over the entire surface of the material
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Insulators
Insulators : materials in which electric charges do not move freely– Glass and rubber are examples of insulators– When insulators are charged by rubbing, only the
rubbed area becomes charged There is no tendency for the charge to move into other
regions of the material
semiconductors : characteristics between those of insulators and conductors– Silicon and germanium are examples
Charging…
Three ways– Friction
Mechanical motion (rubbing)
– Conduction (or Contact)Direct contact (no rubbing)
– InductionCharge alteration without any contact
Charging by Friction
Self-explanatory… (demo)
Charging by Conduction
A charged object (the rod) is physically touches the other uncharged, object (the sphere)
The same type of charge is CONDUCTED from the rod to the sphere
Charging by Conduction
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Charging by Induction
• Induced charge - NO physical contact between charged & uncharged object
• OPPOSITE charge is INDUCED
Temporary charge Induction
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ELECTRICALLY POLARIZED
Permanent charge Induction
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GROUNDING
Another way to Induce a charge
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Question #3
An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other.
The force on the electron is:a) Greater than that on the alpha particleb) Less than that on the alpha particlec) Same as that on the alpha particled) I haven’t a clue…
Answer #3: (c) Same
The force on the electron the same as that on the alpha particle - Newton’s Third Law.
Question #4
An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other.
The particle with the most acceleration is thea) Alpha particleb) Electronc) Neither - they have the same accelerationd) I haven’t a clue…
Answer #4: (b) Electron
The particle with the most acceleration is the ELECTRON. Newton’s Second Law (F=ma)
Question #5
An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. As the particles get closer to each other, each experiences an increase in:
a) forceb) speedc) accelerationd) All of thesee) None of these
Answer #5: (d) ALL
As the particles get closer, the FORCE and thus the ACCELERATION and also the SPEED
Electrical Field
Gravitational Field - A force field that exists around any object with mass– Interacts with mass
Electric field - A force field that exists around a charged object– Interacts with charges– How do we know it exists?
If another charged object enters this electric field, the field exerts a force on the second charged object
direction of movement determines charge of the field
Visualizing an Electric Field
Michael Faraday developed the concept of drawing Electric Field Lines– Vector quantity– Proximity of field lines indicates field strength– Arrows indicate direction of field
Direction indicates the charge– Out of positive– Into negative
Electric Field Lines
Point Charge– Field lines radiate
equally in all directions Radiate out on positive
– Proximity to each other indicates field strength
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Negative Point Charge– Lines point inward
Towards the charge
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Electric Field Line Patterns
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Electric dipole - consists of two equal and opposite charges
– Add field lines Connected field lines
indicates opposite charge
– Matching numbers of field lines indicates similar charge values
The high density of lines between the charges indicates the strong electric field in this region
Electric Field Lines
Electric Field Line Patterns
Two equal but like point charges
Zoomed out (far away)– the field would be appear
to be one charge Zoom in (close-up) No connections indicate
like charges; (repulsion) Low density of field lines
between the charges indicates a weak field in region “C”
Electric Field Lines
Electric Field Patterns
Unequal and unlike charges
Note that two lines leave the +2q charge for each line that terminates on -q
Electric Field Lines, cont.
Electric Field, cont.
How do we know they are there?– Interact with charges
How do we know what charge they are?– Experimenting (testing)– test charge, placed in the field, will experience a force
Electric Field Testing, cont.
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Electric Field Testing, cont.
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Direction of Electric Field
The electric field produced by a negative charge is directed toward the charge
– A positive test charge would be attracted to the negative source charge
Direction of Electric Field, cont
The electric field produced by a positive charge is directed away from the charge
– A positive test charge would be repelled from the positive source charge
Electric Field
Mathematically,
The electric field is a vector quantity
Question #9
What is the magnitude of the electric field 0.50 meters away from a -3C point charge?
a) 1.08 x 105 N/Cb) -1.08 x 105 N/Cc) 5.4 x 104 N/Cd) -5.4 x 104 N/Ce) I don’t have a clue…
Answer #9: (a) 1.08x105 N/C
What is the magnitude of the electric field 0.50 meters away from a -3C point charge?
a) 1.08 x 105 N/Cb) -1.08 x 105 N/Cc) 5.4 x 104 N/Cd) -5.4 x 104 N/Ce) I don’t have a clue…
Question #9
What is the magnitude of the electric field 0.50 meters away from a -3C point charge?
a) 1.08 x 105 N/Cb) -1.08 x 105 N/Cc) 5.4 x 104 N/Cd) -5.4 x 104 N/Ce) I don’t have a clue…
Electrostatic Forces
• If like charges repel and opposites attract…• That means there is motion• If there is motion there must be a force (F = ma)
• Newton’s Second Law
• There must be a way to calculate the electrostatic force!
Coulomb’s Law
F = electrostatic force or electrical forceke = electrostatic force constant
aka – proportionality constantaka – Coulomb’s Constant= 9.0x109 Nm2/C2
q1 = charge for particle 1q2 = charge for particle 2r = radius (distance between charges)
Question #4
Two charges (+20 C and -10 C) are 3 m apart. What is the magnitude of the force between them?
a) 0.2 Nb) 0.6 Nc) 22.22 N d) 2.0 x 10 11 N
F = ?q1 = +20uCq2 = -10uC r = 3mke = 9x109 Nm2/C2
F
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CxCxCNmxF
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10101020/109
0.2 N
Balloon on a Ceiling
Which is stronger the force of gravity, or electrical force?
Two point charges are separated by a distance r
The like charges produce a repulsive force between them
The force on q1 is equal in magnitude and opposite in direction to the force on q2
Vector Nature of Electric Forces
FORCE
OPPOSITE
EQUAL
Two point charges are separated by a distance r
The unlike charges produce a repulsive force between them
The force on q1 is equal in magnitude and opposite in direction to the force on q2
Vector Nature of Electric Forces
FORCE
OPPOSITE
EQUAL
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Question #6
If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the MAGNITUDE of the force between them?
a) 0.2 Nb) 0.6 Nc) 22.22 N d) 2.0 x 10 11 Ne) I don’t have a clue
Answer #6: (a) 0.2 N
Question #7
If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the DIRECTION of the force between them?
a) Away from each otherb) Towards each otherc) One chases the other d) Nothing - they don’t move at alle) I don’t have a clue
Answer #7: (a) Away
If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the DIRECTION of the force between them?
Like charges repel
Question #10
What is the electrostatic force acting on a 2 nC charge placed in a 335 N/C electric field?
a) 0 Nb) 6.7 x 10-4 Nc) 6.7 x 10-7 Nd) 6.7 Ne) I don’t have a clue…
Answer #10: (c) 6.7 x 10-7 N
What is the electrostatic force acting on a 2 nC charge placed in a 335 N/C electric field?
Electrical Shielding
Electrical charges spread over the surface of a conductor in such a way that the net charge INSIDE (at the center) of the conductor is zero
Electrical Potential Energy
Energy possessed by a charge by virtue of its location
A function of the charge sizes involved and their proximity to one another
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Electrical PE >> KE
Electric Potential
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Electrostatics
The End…