Today’s agenda: Electric potential energy.

1. Today’s agenda: Electric potential energy. You must be able to use electric potential energy in work-energy calculations. Electric potential. You must be able to calculate the electric potential for a point charge, and use the electric potential in work-energy calculations. Electric potential and electric potential energy of a system of charges. You must be able to calculate both electric potential and electric potential energy for a system of charged particles (point charges today, charge distributions next lecture). The electron volt. You must be able to use the electron volt as an alternative unit of energy.

2. Definition and Really Important fact to keep straight. This definition is from Physics 23. The change in potential energy is defined as the negative of the work done by the conservative force which is associated with the potential energy (today, the electric force). If an external force moves an object “against” the conservative force,* and the object’s kinetic energy remains constant, then Always ask yourself which work you are calculating. *for example, if you “slowly” lift a book, or “slowly” push two negatively charged balloons together

3. Another Important Fact. Potential energies are defined relative to some configuration of objects that you are free to choose. For example, it often makes sense to define the gravitational potential energy of a ball to be zero when it is resting on the surface of the earth, but you don’t have to make that choice. “Available energy is the main object at stake in the struggle for existence and the evolution of the world.”—Ludwig Boltzmann

4. If I hold one proton in my right hand, and another proton in my left hand, and let them go, they will fly apart.(You have to pretend my hands are “physics” hands—they aren’t really there.) “Flying” protons have kinetic energy, so when I held them at rest, they must have had potential energy. The electric potential energy of a system of two point charges q1 and q2, separated by a distance r12 is Sooner or later I am going to forget and put in a 1/r2 dependence. Don’t be bad like me. This is not a definition; it is derived from the definition of potential energy. Read your text, or ask me in the Learning Center where this comes from.

5. Still Another Important Fact. Our equation for the electric potential energy of two charged particles uses the convention that the potential energy is zero when the particles are infinitely far apart. Does that make sense? It’s the convention you must use if you want to use the equation for potential energy of point charges! If you use the above equation, you are “automatically” using this convention. Homework hint: if charged particles are “far” apart, their potential energy is zero. So how far is “far?”

6. Example: calculate the electric potential energy of two protons separated by a typical proton-proton intranuclear distance of 2x10-15 m.+1.15x10-13 J To be worked at the blackboard in lecture.

7. Example: calculate the electric potential energy of two protons separated by a typical proton-proton intranuclear distance of 2x10-15 m.+1.15x10-13 J +e +e D=2x10-15 m What is the meaning of the + sign in the result?

8. Example: calculate the electric potential energy of a hydrogen atom (electron-proton distance is 5.29x10-11 m).-4.36x10-18 J To be worked at the blackboard in lecture.

9. Example: calculate the electric potential energy of a hydrogen atom (electron-proton distance is 5.29x10-11 m).-4.36x10-18 J -e +e D=5.29x10-11 m What is the meaning of the - sign in the result? Is that a small energy? I’ll have more to say about the energy at the end of the lecture.