Download Electric Forces and Fields

Electric Forces
and Fields
Electric Charge
Positive and Negative
Charge is conserved
Millikan Oil Drop Experiment
Charge on the electron/proton
Conductors vs. Insulators
Insulators and conductors can be charges by contact
Conductors can be charged by induction
Insulators can be charged by polarization
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Coulomb’s Law




The force of electrical attraction or repulsion
between charges.
F = kcq1q2/r2
Kc = Coulomb’s Law Constant = 8.99 x 109 nm2/c2
Note: Attractive force would be negative and
repulsive positive.
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17-2 Coulomb’s Law
q 1q 2
Fe = kc 2
r
kc = Coulomb’s Constant
kc = 8.99x109 Nm2/C2
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1780’s
Charles Coulomb was
experimenting to find how
charges create forces
The force depends on
Charge
Distance
He found the following
relationship
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17-2 Coulomb’s Law
n+
+n
e-
Find the force between
the nucleus and an
electron in a helium atom
if the are separated by
1.00 nm.
q1 = 2(1.60x10-19) C
q2 = 1.60x10-19 C
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17-2 Coulomb’s Law
P+
A proton is being levitated
by a charged floor with a
charge of 3.6 picoCoulomb.
How high does the proton
hover above the ground?
(pico = 10-12)
Q = 3.6pC
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17-2 Coulomb’s Law
1
q1 = - 6 mC
2
2m
q2 = +2 mC
3
1m
q3 = - 3 mC
Find the net force on q3
1
3
2
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What would
change if this
were the
original
orientation?
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17-2 Coulomb’s Law
1
q1 = - 6 mC
2
2m
q2 = +2 mC
3
x
q3 = - 3 mC
What is the value of x if q2
is in equilibrium?
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17-2 Coulomb’s Law
1
q1 = - 6 mC
2
y
q2 = +2 mC
3
x
q3 = - 3 mC
Q2 is equilibrium with q1
and q3. What is the value of
x and y if the distance
between q1 and q3 is 10.0
m?
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Electric Field Strength


E = F/q (N/c)
For a point charge:
E = kq/r2
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Example: What is the force on an electron
due to an electric field of 50 N/C?
If the mass of an electron is 9.11 x 10-31 kg,
what would be the resulting acceleration of
this electron?
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Electric Field Lines
Electric lines of force
Number of lines is proportional to
field strength
Like vs. unlike charges
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Review Questions
1) An electron is pushed due to an electric field of
50N/C.
If the mass of an electron is 9.11 x 10-31 kg, what
would be the resulting acceleration of this electron?
2) What would the net electric field be for an object
directly between a 6.0nC and a -7.5nC charge that is
3.2m apart?
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Review Question 2
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An electron is released from rest from the
negative plate between parallel plates separated
by a distance of 15 cm. It strikes the other plate
in 1 E –7 sec. Determine the electric field
between the plates.
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Review Question 3 (honors)
Given the isosceles triangle above with identical
charges, Q, on either corners of the bottom vertices.
1)
What is the net electric field acting at position C?
2)
Where should a new charge, +2Q, in order to
make the net electric field at point C zero?
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