Coulomb’s Law
8.02T Notes
The magnitude of the electrostatic force between two
stationary point charges q1 and q2 separated by a
distance r is given by
F =k
Wednesday and Friday
September 5 and 7, 2001
where
k=
The Electric Force
The electric force F12 is the force on q2 due to q1:
F12 = k
where
r̂
q1q2
rˆ
2
r
q1 q2
r2
1
4πε o
≈ 8.99 x 109
Nm 2
C2
The Electric Force Is Huge
Usually hidden because matter is mostly neutral. To see
its effects clearly we must separate charge, which we
will do today in the Sticky Tape Experiment.
is the unit vector directed from q1 to q2.
Link to larger version of the above animation
1
The Electric Field E
The E field at a point is the ratio of the electric force on
a small test charge to the amount of the test charge:
E=
F
qtest
The electric field E at r due to a charge q is
E=k
where
q
rˆ
r2
r̂ is the unit vector directed from q to the observer.
Visualizing Vector Fields 2
Field Lines
Drawn so that the tangent to the field line is parallel to
the electric field at each point on the field line.
Visualizing Vector Fields 1
“Vector Field Diagram”
Put small arrows in the direction of the field on a
uniform grid.
The length of the arrows are all the same.
The color of the arrows are coded for field strength.
Vector Field Diagram Java Applet
Visualizing Vector Fields 3
“Iron Filings” representation
• Put random patterns oriented along the field
• Gives much greater spatial resolution
• Does not indicate field magnitude or direction
Field Line Java Applet
2
Direction of Force
Like charges repel and unlike charges attract.
Another way to state this:
The field of unlike charges PULL
them together.
Direction of Force
The fields of unlike charges PULL them together.
The field of like charges PUSH
them apart.
Link to larger version of the above animation
Direction of Force
The fields of like charges PUSH them apart.
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