Transcript Ch 33 Electric Fields

Ch 33 Electric Fields
Physics
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Electric Field
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Electric Field: An area of influence around a charged object.
The magnitude of the field is proportional to the amount of electrical force exerted on
a positive test charge placed at a given point in the field.
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Electric field = [electric force]/[test charge] or E = [F]/[qo]
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The SI unit of electric field is the Newton per coulomb (N/C).
The electric field around a charged object is a vector and can be represented with
electric field lines that point in the direction of the force exerted on a unit of positive
charge.
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1. Visit: http://www.its.caltech.edu/~phys1/java/phys1/EField/EField.html
2. Visit: http://lectureonline.cl.msu.edu/~mmp/kap18/RR447app.htm
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Select Medium, q1 = +1 & q2 = -1.
Sketch diagram 
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Select Medium, q1 = +1 & q2 = +1.
Sketch diagram 
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Calculating Electric Field Strength
F kqo q1
E
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qo qo d
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For a point charge or other spherical charge distribution.
The magnitude of the force can be calculated as above.
q = charge on the surface of object
qo = test charge
d = distance between center of charged object & qo
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Example Problems
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Deepika pulls her wool sweater over her head, which charges her body as the
sweater rubs against her cotton shirt. a) What is the electric field at a location where
a 1.60 x 10-19 C-piece of lint experiences a force of 3.2 x 10-9 N as it floats near
Deepika? b) What will happen if Deepika now touches a conductor such as a door
knob?
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A fly accumulates 3.0 x 10-10 C of positive charge as it flies through the air.
What is the magnitude and direction of the electric field at a location 2.0 cm
away from the fly?
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Ch 33 The Electric Field & Potential
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Experiments with beams of negative particles were performed in Britain
by Joseph John ("J.J.") Thomson, and led to his conclusion in 1897 that
they consisted of lightweight particles with a negative electric charge,
nowadays known as electrons. Thomson was awarded the 1906 Nobel
Prize. Charge to Mass Ratio.
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In 1909, Robert Millikan and Harvey Fletcher performed the oil-drop
experiment to measure the elementary electric charge (the charge of
the electron).
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By repeating the experiment for many droplets, they confirmed that the
charges were all multiples of some fundamental value, and calculated it
to be 1.5924 × 10−19 coulombs, within one percent of the currently
accepted value of 1.602176487 × 10−19 coulombs. They proposed that
this was the charge of a single electron
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.The mass of the electron was subsequently determined (9.11 x 10-31 kg)
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Won Nobel Prize in 1923
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Electric Shielding
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If the charge on a conductor is not moving, the electric field inside the conductor is exactly zero.
The absence of electric field within a conductor holding static charge does not arise from the
inability of an electric field to penetrate metals.
It comes about because free electrons within the conductor can “settle down” and stop moving only
when the electric field is zero.
Consider a charged hollow metal sphere. Because of mutual repulsion, the electrons spread as far
apart from one another as possible, distributing themselves uniformly over the surface of the
sphere.
The forces on a test charge located inside a charged hollow sphere cancel to zero.
If a conductor is not spherical, then the charge distribution will not be uniform. The exact charge
distribution over the surface is such that the electric field everywhere inside the conductor is zero.
A Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of
such material. Such an enclosure blocks out external static electrical fields. Faraday cages are
named after physicist Michael Faraday, who built one in 1836.
An external static electrical field will cause the electrical charges within the conducting material to
redistribute themselves so as to cancel the field's effects in the cage's interior. This effect is used,
for example, to protect electronic equipment from lightning strikes and other electrostatic
discharges
Faraday Cage Video
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33.4 Electrical Potential Energy
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The electrical potential energy of a charged particle is increased when
work is done to push it against the electric field or something else that
is charged.
A charged object can have potential energy by virtue of its location in an
electric field.
Suppose you have a small positive charge located at some distance from a
positively charged sphere. If you push the small charge closer to the
sphere, you expend energy to overcome electrical repulsion.
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“The Pulley”
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The work is equal to the energy gained by the charge.
The energy a charge has due to its location in an electric field is called
electrical potential energy.
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33.5 Electric Potential Difference
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Electric potential is not the same as electrical potential energy.
Electric potential is electrical potential energy per charge.
The concept of electrical potential energy per charge has a special name, electric
potential: electric potential difference = electrical potential energy/charge.
The SI unit of measurement for electric potential difference is the volt.
Since potential energy is measured in joules and charge is measured in coulombs,
1 volt = 1 joule/coulomb.
Since electric potential difference is measured in volts, it is commonly called voltage.
Voltage = “Potential Difference”
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V = Potential Difference (Volts)
W
q = Charge (Coulombs)
V
W = Work (Joules)
q
If a field exists between two charged parallel plates that is uniform except
near the plate edges, then the potential difference equals the Electric Field plates times
the plate separation. V = Ed
Units:
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Example Problems
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An electron in Tammie’s old black and white TV is accelerated toward the screen
across a potential difference of 22,000 V. How much kinetic energy does the electron
lose when it strikes the TV screen?
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Joe shuffles his feet across the living room rug, building up a charge on his body. A
spark will jump when there is a potential difference of 9000 V between the door and
the palm of Joe’s hand. This happens when his hand is 0.3 cm from the door. At this
point, what is the electric field between Joe’s hand and the door?
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33.6 Electrical Energy Storage
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The energy stored in a capacitor
comes from the work done to
charge it.
Electrical energy can be stored in a
common device called a capacitor.
The simplest capacitor is a pair of
conducting plates separated by a small
distance, but not touching each other.
When the plates are connected to a
charging device such as a battery,
charge is transferred from one plate to
the other.
The greater the battery voltage and
the larger and closer the plates, the
greater the charge that is stored.
C= q/V
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