Electric Potential Energy!

1. Electric Potential Energy!

2. Electric Potential Energy - the stored energy that exists as a result of the attraction or repulsion of two or more charged particles. The best way to think about it conceptually is as the potential to become kinetic energy. For example! _ + + + Repelling particles have the potential to explode apart, turning EPE into KE! Attracting particles have the potential to snap together, turning EPE into KE!

3. Electric potential energy will henceforth be denoted as UE UE Units: Joules of course! Like all energy, it is a scalar quantity Fun Fact!Batteries are world renowned for their ability to turn chemical potential energy into electric potential energy! Pick some up at a store near you!

4. Let’s start by analyzing the UE of a system of two repelling particles +q +q If we push them closer together (doing work on the system)… +q +q …the system now has more of the ability to fly apart and gain kinetic energy! By pushing the two repelling charges together, we are increasing the electric potential energy of the system. This applies to two positive charges OR two negative charges

5. Electric Cannon Whiteboard: Round 1 A cannon that works by electrostatic repulsion is loaded, as shown below. Complete the work/energy bar chart (qualitatively) +q +q +q +q

6. +q +q +q +q Positive work must be done to increase the system’s electric potential energy!

7. Round 2: Letting it Fire! Complete the associated work/energy bar chart (qualitatively) v +q +q +q +q

8. Round 2: Letting it Fire! Complete the associated work/energy bar chart (qualitatively) v +q +q +q +q

9. What happens to the electric potential energy of the system as d approaches infinity? +q +q UE approaches zero. If the particles are infinitely far apart, they do not repel at all, and the system will not have the ability to gain any KE. Remember, think of electric potential energy as the potential for the system to gain kinetic energy.

10. How about two attracting particles? What happens to the electric potential energy of the system as d approaches infinity? if we pull them infinitely far apart +q -q UEstill approaches zero! If the particles are infinitely far apart, they do not attract at all, and the system will not have the ability to gain any KE

11. Electric Nutcracker Whiteboard: Round 1 A nutcracker that works by electrostatic attraction is loaded, as shown below. Complete the work/energy bar chart (qualitatively) -q -q +q +q This one requires some really out-of-the-box thinking! Positive work is done, but as the charges get further apart, their electric potential energy approaches zero! Ahhhh!!!!

12. Positive work is done in pulling the charges apart, but as the charges get further apart, their electric potential energy approaches zero! -q -q +q +q Think about it! If the charges were pulled infinitely far apart, they would not interact at all, and have zero UE! The only way to be consistent here is to introduce the idea of negative potential energy.

13. Oppositely charged particles have negative electric potential energy! The closer they are, the more negative their potential energy is. -q +q The further apart they are, the less negative their potential energy is. +q -q By pulling them apart and doing positive work on the system, you are increasing its energy by making it less negative. This brings the charges further apart, and brings UE closer to zero.

14. Nutcracker Round 2: Let’s get Crackin! The nutcracker is now released, steadily attracting and eventually hurtling toward Mr. Nut. Complete the work/energy bar chart. -q -q +q +q Remember - The closer they are, the more negative their potential energy is. The further apart they are, the less negative their potential energy is.

15. They start out further apart, with greater (less negative) potential energy. As they attract one another, their electric potential energy energy decreases, and is converted into kinetic energy. The decrease in potential energy is shown by it becoming more negative. -q -q +q +q

16. What does negative potential energy mean conceptually? The system is bound together. If you were to let it go, it would collapse into itself. This is called a bound state. +q -q If you wanted to separate the charges out to infinity and give it zero energy, you would have to add energy to the system (do work!) Since you have to add energy to the system to give it zero energy of interaction, the system has a negative potential energy.

17. Derive the Equation for UE! You will need to use the definition of work, as well as Coulomb’s Law. Go for it!

18. Use the signs of the charges in here! (No absolute value) Two positives or two negatives will result in positive electric potential energy. Consistent with earlier logic. A positive and a negative will result in a negative electric potential energy. Also consistent with earlier logic! Kinda nice how it works out, huh?

19. Whiteboard: Calculate the Total Electric Potential Energy of the System L L +q +q +q Tip: Calculate the potential energy of each pair of charges, and add all of the potential energies together as scalars.

20. The total UE of a group of charged is the scalar sum of the UE of each pair of particles. L L +q +q +q 1 2 3 UEtotal = UE1&2 + UE2&3 + UE1&3

21. Now with Negative Scalars! L L +q -q +q