A spaceship is coming directly toward you while youare in the International Space Station. You are toldthat the spaceship is shining sodium light (with anintense yellow doublet of wavelengths 588.9950 and589.5924 nm). You have an apparatus that can resolvetwo closely spaced wavelengths if the difference is 0.55 nm. If you fi nd that you can just resolve thedoublet, how fast is the spaceship traveling with respectto you?

Lecture 7 Potential from a distribution of charges X V = 1 qi 4⇡✏0 ri §Smooth distribution 1 X qi 1 Z ⇢ V = = dV 4⇡✏0 i ri 4⇡✏ 0 r § of point charges is usually much simpler thanup calculating the electric field •It’s a scalar Electric Potential from Two Oppositely Charged Point Charges §The electric field lines from two oppositely charge point charges are a little more complicated §The electric field lines originate on the positive charge and terminate on the negative charge §The equipotential lines are always perpendicular to the electric field lines §The red lines represent positive electric potential §The blue lines represent negative electric potential §Close to each charge, the equipotential lines resemble those from a point charge 22 Electric Potential from Two Identical Point Charges §The electric field lines from two identical point charges are also complicated §The electric field lines originate on the positive charge and terminate at infinity § Again, the equipotential lines are always perpendicular to the electric field lines §There are only positive potentials §Close to each charge, the equipotential lines resemble those from a point charge 23 Example: Superposition of Electric