AP test on magnetics

Grades were pretty good but your papers still show more facility with plugging into formulae then with understanding ideas. Lots of errors in the figure MCs and lots of really wrong stuff in the problems. For example:

1) many of you used whatever linear dimension you could get your hands on in F= IL x B. The L is of course the length of the wire carrying the test current that is in the magnetic field produced by the source current.
2) Some of you used the source current in the above equation. Using the source current and the field from the source current is like trying to find the force of the current on itself.

3) In the extra credit, many of you tried to use the solenoid equation on the current produced by the beam to find the field that was need to ACT ON THE PROTONS in the beam. Those of you who made this mistake are using the test particles as if they were the source of the fields acting on them. It should be obvious that the current is not causing itself to turn in circles. The beam of protons is in a field created by much stronger currents and it is this field causing the protons to bend their path. The solution is found from equating M acentripetal ( i.e. M v^2/r) with the magnetic force QvB to get Mv/r = QB and solve for B

4) perhaps even worse was trying to use the Faraday induction FORMULA to find the voltage it took to accelerate the protons up to speed. This class is NOT THE FORMULA OF THE MONTH CLUB. First of all the constant magnetic field can only change direction not add speed, and if the field is not changing why would there be an EMF. Second of all you should know that delta KE =- delta PE and that PE is equal to QDeltaV or Mv^2/2=QDV so DV =(M/Q )(v^2/2)

5) In the equation for EMF you should know that N means how many windings or turns the coil consists of NOT how many rotations it makes while moving. When will you start thinking and stop memorizing jumbles of letters?