**Deriving the equation for electric field strength between parallel plates**

By definition electric field strength is force per unit positive charge;

* (1)*

Work done is the product of force and distance, . This can be rearranged for ;

* (2)*

Additionally, potential difference is the work done per unit charge, . This can be rearranged for ;

* (3)*

Substituting equation *(2)* and *(3)* into *(1)* gives:

Work done cancels to give:

This then leaves:

**Showing that is equivalent to **

The same process as above can be used but by using units instead of quantities:

So, by definition electric field strength is force per unit positive charge and so:

* (1)*

Work done is the product of force and distance, . This can be rearranged for ;

* (2)*

Additionally, potential difference is the work done per unit charge, . This can be rearranged for ;

* (3)*

Substituting *(2)* and *(3)* into *(1)* gives:

Work done cancels to give:

This then leaves: