Question

What is the magnitude of the electric field at a point midway between a −7.7μC and a +6.2μC charge 10cm apart? Assume no other charges are nearby.

What is the direction of the electric field?

Answer 1

Let's start with determination of direction first, the direction of electric field from a positive charge is directed radially outwards while that from a negative charge is radially inwards as depicted in the picture below

One can say in rather simpler words that electric field moves away from positive charge and towards the negative charge.

Hence, the direction of electric field will be towards negative charge.

We can also deduce that Electric Field due to positive charge is in same direction as that of negative charge (say +x-direction). Thus we can add the magnitude of electric field due to both the charges.

We know that Electric field,

where

is the absolute permittivity (vacuum)

r = distance between the charge and point at which electric field is being calculated, here, 5cm.

So, Electric field due to positive charge,

and electric field due to negative charge is

The net electric field at a point midway between the two charges will be algebraic sum of and

towards negative charge.