PHYS 212 Lecture 5: lecture 5

2) One electron-volt, eV, is equivalent to 1.6*10^-19 Joules of energy. Ignoring gravity, if two, infinitely separated, protons (mass = 2*10^-27 kg) are shot directly toward one another, each with 1 TeV of kinetic energy (T = 10^12), how close together (what separation distance in meters) will they get, before the Coulomb force (proton charge = 1.6*10^-19 Coulombs) of repulsion stops their motion and then makes them infinitely separate again? (Hint: Use conservation of energy: as the protons approach each other, their kinetic energy is being converted into electrical potential energy, such that at their closest point of approach all of the initial kinetic energy will have been converted into electrical potential energy.)

KEY TERMS

1. work (4.1)                 6. potential energy                                      11. watt

2. joule                         7. gravitational potential energy                   12. horsepower

3. foot-pound                8. conservation of total energy (4.3)             13. kilowatt-hour

4. energy (4.2)              9. conservation of mechanical energy           14. alternative energy source (4.6)

5. kinetic energy           10. power (4.4)                                           15. renewable energy sources

For each of the following items, fill in the number of the appropriate key term from the preceding list.

c. _ _ _ _ Unit of electrical energy

A charge q that moves between two points A and B, and has anelectric force acting on it ( and perhaps several other forces) haswork W done on it by the electric force. The electrostatic force isconservative and so we can define a change in electric potentialenergy, Delta U = -W. We defined electric field using the relationF^^\-> = q E^^\->, and in a similar way we define electricpotential commonly called voltage as
Delta V = V_B - V_A = -W/q=(Delta U)/q or equivalently q Delta V =q (V_B - V_A) = -W=Delta U

A ball has mass 3.0 µg and charge -6 nC moves from point A to pointB. It has the same speed at points A and B, and the work done bythe electric field during the move is -1.5 mJ.
(a) What is the change in electric potential energy of the ball?1
(b) What is the change in voltage between points A and B? 2
(c) At which point does the ball have a higher electric potentialenergy?
(d) Which point has the higher voltage?
(e) Has the kinetic energy of the ball changed?