PHY 242 Laboratory for 10/22/98

Exam II will take place during the lab period on Thursday, Oct. 22. It will consist of 6 to 8 problems and cover Chapters 28 through 32. Here are the usual warnings and disclaimers....

Below are a few sample problems from a previous exam (1992). You are responsible for all material covered in lecture and homework assignments. DO NOT ASSUME that a subject not covered in the sample exam below will not show up on Thursday's exam!

During the exam, you will be provided with an equation sheet. You will be allowed to use only the equations on the sheet, or others that you derive from those on the sheet. I would suggest that you create your own equation sheet from the equations in the summary at the end of each chapter and use only that when trying the problems below. Resist the temptation to simply look for a similar problem in the text or notes (the standard "get the homework done" technique). That will better prepare you for the exam. If you can't solve the problems, come see me. And don't wait until Wednesday night!!

Sample Physics 242 Exam II

1. The electron gun in a picture tube fires electrons with speed 3 x 10⁶ m/s. They are deflected upward by the yoke with a uniform magnetic field.
   
   a) In the diagram indicate and state the direction of this magnetic field.
   b) If the field strength is 500 mT, what is the acceleration of the electron?
   For a bonus, estimate the degree of deflection if the region of the yoke's field is 10 cm long.

2. A flat antenna lead consists of two wires separated by 1 cm. The wires carry 2 A each in opposite directions as shown. Show the B-field created by the right wire at the left wire and the force it exerts on the left wire. What is the force per unit length on the left wire?
   
   
3. A conducting cylindrical shell of inner radius a and outer radius b carries a uniformly distributed current I out of the page.
   a) What is the magnetic field for a < r < b?
   b) In reality, will the current be uniformly distributed? If not, where will the current density be greatest? Briefly state your arguments.

4. A long copper bar which carries current density j has thickness t (which is very small) and width w. What is the magnetic field in the plane of the bar at a distance d from the near edge? (Hint: Start with the field due to a long wire and let I --> dI. )
   

5. Find the magnetic field at the center of the double loop below. Assume a current I and assune the "cut" region, that connects the inner and outer loops, is very small . Show the current direction and state the direction of B.
   

6. A vertical bar of length L is moved back and forth in a uniform magnetic field B (into the page). The position of the bar as function of time is x = Asin wt. Find the emf induced across the bar as a result of its motion.
   

7. A 100 µF capacitor and a 10 mH inductor are connected in series a switch. The capacitor is initially charged with a 100 VDC potential. What is the maximum current in the circuit and how long after the switch is closed did the maximum current first appear?

8. A 10 pF capacitor is used in a tuning circuit with a variable inductor and .1W load resistor.
   

   a) What value must the inductor have to tune to 96.1 MHz?
   b) If the antenna signal is 20 mV rms at this frequency, what is the rms voltage across the load resistor?
   c) What is the rms voltage across the inductor? the capacitor?