Electric Field Strength and Potential

Electric Fields When two charged objects are close to each other, they both experience forces The objects will try to move as a result of this force Each charged object produces a field around itself Define Force = Electric Field Strength x Charge F = Eq; E = F/q

Electric Field If we have two charged objects, Q and q, separated by a distance r, we can show that Usually written as Q q r ε 0, permittivity of free space, = 8.85 x C 2 N -1 m -2 Although you may prefer to use k = 8.99 x 10 9 N m 2 C -2

Electric Field Strength Define Force = Electric Field Strength x Charge F = Eq; E = F/q Round a point charge Q, ε 0, permittivity of free space, = 8.85 x C 2 N -1 m -2 Note: Direction of vector is direction of the force that a +ve charge would experience  Force per unit charge

Potential If an object moves in the direction of a force, then there will be a change of energy,  E Work = Force x Distance moved in the direction of the force How much Potential Energy will a charge q have when it is a distance r from another charge Q? (Assume they are both positive) Work out how much Kinetic Energy it will gain Q q r

Potential Define the Potential, V, as the energy per unit charge. (Note it is Potential, not Potential Energy)  Energy per unit Charge

Potential We always assume that there is a reference of 0V when charges are an infinite distance apart This means that this scalar can have negative values of potential when the two charges attract each other. It can also have a positive value of the two charges repel. However we often compare the difference in potential between to points – guess what we call this? Usually we don’t go to , it’s too far

Equipotentials and Electric Field Lines Equipotentials are lines joining points of equal potential For a field round a point charge, the Potential is inversely proportional to the distance from the charge When Equipotentials are closer together then this indicates that the Field Strength is higher Electric field lines are always perpendicular to equipotentials

This is the uniform field strength between two charged parallel plates Electric Field between two charged parallel plates Between the plates –The equipotentials are equally spaced and parallel to the plates –The Electric Field Strength is uniform everywhere between the plates If a charge q is moved from one plate to the other through the uniform field then work = force x distance in dirn. of force Vq = Eq x d E = V d V Equipotentials Electric Field Lines d