9/24 Electric Potential Energy

Equations using “q'“

assume discrete point charges

Equations using E are more general and can handle both point charge (messier) and

charge densities (no other choice)

U_E is measured as potential energy,

like gravity

U_E is the energy of a charge based on its

radial distance from another charge

U_E =

kqq/r

Electric Potential Energy between any 2 point chages,

d distance away

When U_E is biggest when

2 charges are close AND BIG

When U_E is smallest when

2 opposite charges are close

Electrical Work is done from the POV of the force field, 

not us 

Like gravity, work done by us is opposite

of the work done by the force field (gravity)

Electrostatic work principles 

• This is the work done BY the field

• Again, work must be parallel to force

• How are the directions of force and E related?

• Components of E and F need to be parallel to displacement

Electrostatic Work is Path

Independent

Fields do positive work when the object moves

“naturally” or “spontaneously” System loses Ug or U_E

Fields do negative work when the object is forced to

move against its natural motion

Electric Potential is NOT 

Electric Potential Energy 

Electric potential is also called

Voltage (EMF)

Potential is the amount of potential energy between 

2 charges divided by one of the charges 

SI unit is

Volt (V), J/C

Electric Potential always INCREASES when

moving towards a positive charge (+V) 

Electric Potential always DECREASES when moving 

towards a NEGATIVE charge (-V) 

Relative Voltage is

change in V

Absolute Voltage is

V

Conductors are considered

equipotential surfaces

Equipotential means 

equal voltage

Equipotential lines just mean where the lines where the

voltage is the same