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Recall: Newton’s Law of Gravitation expresses the magnitude of the gravitational force exerted by each of two masses on the other: FG.12 = Gm1m2/r122
where r12 is the distance between the two centers of mass, and G is a universal
constant (G = 6.67 x 10-11 N·m2/kg2).
Coulomb’s Law
Likewise, Coulomb’s Law expresses the magnitude of the electrostatic force
exerted by each of two point charges on the other: |FE.12| = k|q1||q2|/r122
where r12 is the distance between the two charges, and k is a universal constant
(k = 8.99 x 109 N·m2/C2).
Note the magnitude of the constant, k, indicating how much more powerful
electrostatic force is than gravitational force. When two electrons (indeed, any
two small bodies with both mass and net charge) encounter one another, the
force that generally governs—overwhelmingly—is the electrostatic force.
4/5/17
OSU PH 213, Before Class #2
1
Two uniformly charged spheres are firmly anchored and electrically
insulated from each other. The net charge on sphere 2 is three times
the net charge on sphere 1. Which force diagram correctly shows the
magnitude and direction of the electrostatic force felt by each sphere?
4/5/17
OSU PH 213, Before Class #2
2
Two uniformly charged spheres are firmly anchored and electrically
insulated from each other. The net charge on sphere 2 is three times
the net charge on sphere 1. Which force diagram correctly shows the
magnitude and direction of the electrostatic force felt by each sphere?
Newton’s Third Law
always applies!
4/5/17
OSU PH 213, Before Class #2
3
Coulomb’s Law speaks only of the magnitude of the electrostatic
force. What about its direction? Gravitational force is always
attractive. But with electrostatic force, the direction depends on the
charge types: Opposite charge types attract; like charge types repel.
Electrostatic forces are vectors that you can resolve into components,
add, sum to zero, etc.—just like any other forces. But you have to
keep track of the directions very carefully, because in some cases the
forces are attractive; in other cases they are repellant.
And note: The signs we use to indicate vector directions along the xand y-axes are NOT the same as the signs we use to indicate the
types of electrical charge we’re dealing with. A negative charge
does not necessarily exert (or experience) a force that acts in the
negative direction.
4/5/17
OSU PH 213, Before Class #2
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Practice more with magnitude:
A point charge (q1) is located at the origin. It exerts a force of known
magnitude F12 on an electron (q2) located at the point (4,0).
Write an expression for the magnitude F13 of the force it exerts on a
charge q3 = +2e, located at (0,2). (Assume all coordinates in meters.)
(See After class 2 for the solution.)
4/5/17
OSU PH 213, Before Class #2
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Now follow-up with a vector calculation (including direction):
In the previous situation, what’s the net force (magnitude and
direction) exerted on q1 by the other two charges?
(See After class 2 for the solutions.)
4/5/17
OSU PH 213, Before Class #2
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