## ELECTRIC FIELDS

·         Most people will be familiar with the Van de Graff generators and hair standing on end. Here is a more accurate explanation of why. The force one point Q1 exerts on another point charge q2 distance r away is:

·         F = kQ1q2

r2

·         K = 1/(4pe0) where e0 is the permittivity of free space so

ü       F =  1      Q1q2

4pe0        r2

·         Around electrical charges we can imagine an electric field, a region in which an electric charge experiences a force.

ü       F = QE

·         The direction of the field naturally depends on the charge, the arrows are drawn to represent the direction of the force on a small positive charge +q, so the field lines for a point positive charge go radially outwards, those for a negative charge radially inwards. Hence a repulsive force between two like-charged particles is taken to be positive.

·         As E=F/q, E=( kq1q2/r2)/q;

ü       E =   1     Q

4pe0       r2

·         Energy = F x d so, similar to gravitation, potential energy Ep is given by

ü       Ep =  1     Q1q2

4pe0       r

·         Electrical potential is the potential energy per unit charge (V=energy/q), so V=(kq1q2/r)/q;

ü       V =   1     Q

4pe0       r