1. 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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2. 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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3. 17-2 Coulomb’s Law q1q2 Fe = kc r2 1780’s
Charles Coulomb was experimenting to find how charges create forces
The force depends on
Charge
Distance
He found the following relationship
Fe = kc
q1q2 r2
kc = Coulomb’s Constant
kc = 8.99x109 Nm2/C2
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4. 17-2 Coulomb’s Law
Find the force between the nucleus and an electron in a helium atom if the are separated by 1.00 nm.
n +
+ n e-
q1 = 2(1.60x10-19) C
q2 = 1.60x10-19 C
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5. 17-2 Coulomb’s Law
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) P+
Q = 3.6pC
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6. 17-2 Coulomb’s Law 2 3 1
q1 = - 6 mC
q2 = +2 mC
q3 = - 3 mC
2 m
1 m
Find the net force on q3 2 3 1
What would change if this were the original orientation?
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7. 17-2 Coulomb’s Law What is the value of x if q2 is in equilibrium? 2 3
q1 = - 6 mC
q2 = +2 mC
q3 = - 3 mC
2 m x
What is the value of x if q2 is in equilibrium?
X = 2.45m
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8. 17-2 Coulomb’s Law 2 3 1
q1 = - 6 mC
q2 = +2 mC
q3 = - 3 mC x y
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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9. Electric Field Strength
E = F/q (N/c)
For a point charge:
E = kq/r2
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 kg, what would be the resulting acceleration of this electron?
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10. Electric Field Lines Electric lines of force
Number of lines is proportional to field strength
Like vs. unlike charges
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11. 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 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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12. Review Question 2
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.
171 N/C
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13. Review Question 3 (honors)
Given the isosceles triangle above with identical charges, Q, on either corners of the bottom vertices.
What is the net electric field acting at position C?
Where should a new charge, +2Q, in order to make the net electric field at point C zero?
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