COULOMB’S LAW
Force between two electric charges 1 and 2 placed at distance  from each
other is given by Coulomb’s Law:
 = 
1 2
2
Here, positive force means repulsion (charges of the same sign), negative attraction (opposite charges). The electric charge in SI is measured in units called
Coulomb (C). 1 Coulomb is a huge charge: a single lightning transfers a charge of
about 5. The constant  is called (you guessed right!) ... Coulomb constant:
 ≈ 8988 · 1010
 2
 2
≈ 1010 2
2


HOMEWORK 2 (due February 26)
Problem 1
4 identical charges  are placed at the corners of a square. The square has
side length . Find the total electrostatic force acting on each of the charges
(remember that force is a vector!). Which way is it directed?
Problem 2
An electroscope is made of two beads of mass  each, suspended from the
same point on weightless strings of length . When the device is charged with,
its total charge  is equally divided between the two beads. As a result, the
beads repel each other and the strings deviate from the vertical by angle .
Find the total charge of the two beads,  if that angle is  = 450 . Derive the
formula for  and evaluate it if  = 1,  = 5. Can you derive a general
formula that would work at arbitrary , not only 450 ?
Problem 3 (Review)
A sealed aluminum sphere of radius  = 1 floats in the water with exactly
half of it submerged. Find the volume of the void inside the sphere.
1