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The magnitude of the force between the charges iswhere

F=k(2) kis a constant calledCoulomb's constant:

The direction of the force is along a line joining the two charges. It is repulsive if the charges have the same sign and is attractive if the charges have opposite sign.

k= 9.0x10^{9}(3)

If there are more than two charges present, then the force on any one charge must be found by adding vectorially the forces found by Coulomb's law (1.2) between each pair of charges.

It is convenient for many applications to introduce the concept of the
**electric field**, conventionally denoted by .
Suppose we have
a ``background'' distribution of charge
*Q*_{1},*Q*_{2},...,*Q*_{n} in some
region of space, and measure the force on a charge *q* placed
nearby. The electric field associated with this charge distribution is
defined through the relation

= q.
| (4) |

For a single point charge *Q* the electric field a distance *r* away is
found from Eqs.(1.2,1.4) to have the magnitude

E = k ,
| (5) |

10/9/1997