When does work equal negative electric potential energy

[sanhuy]

Homework Statement

Let EPE be change in electric potential energy

so if a positive charge is moving towards the negative terminal with the electric field it would mean that it is losing PE therefore work = - EPE, and if a negative charge is moving against the electric field towards the positive terminal it would gain potential energy? therefore W = + Delta EPE

can someone confirm if this concept is right and i should use it for life.

Homework Equations

W = EPE = q*DELTA V

The Attempt at a Solution

 

[sanhuy]

 

[ehild]

View attachment 76386 View attachment 76386

Yes, you are right.

Let we have capacitor plates A and B d distance apart. Let be the potential U_A at A (on the left) and U_B at B and UA>UB. The electric field is E=(U_A-U_B)/d, and points to the right, away from the positive plate. The force acting on charge q is F= qE, and the work done by the field when q moves from A to B is W=q(U_A-U_B). We define change of potential by U_final -U_initial, and the change of potential energy is ΔEPE=q(U_final -U_initial). U_initial=U_A, U_final=U_B , so ΔEPE=q(U_B-U_A) which is opposite to the work done by the electric field. W = -ΔEPE.