Physics 208 GH Quiz 3

Lecturer: Dr. Min Xu

Nov 22, 2002
LAST NAME: ___________________________________________________________________________________________________ FIRST NAME: __________________________________________________________________________________________________ SS# (last 4 digits): _____________________________________________________________________________________________

General Instruction

You have 1 hour 30 minutes to answer 4 questions on this exam. Work each problem only on the sheet provided for this problem, using the back if necessary. Please write the name and recitation section on each sheet at the top of the page.

You must indicate the reasoning and show all the steps.

If you are unable to obtain the answer to part of a problem and you need the answer to solve the subsequent parts, use an appropriate symbol for the answer to the previous part and solve in terms of that 1symbol.

1. (25 points) A square 1-turn coil with sides of length 40cm carries a current of 3A. It lies in the x-y plane in a uniform magnetic field B = (0.3x^ + 0.4^z )T, where ^x and ^z are unit vectors along x and z axes, respectively. The current flow in the coil is counter clockwise viewing from the positive z-axis.


(a) (10 points) Find the magnetic moment of the coil, including its direction.
(b) (10 points) Find the torque exerted on the coil.
(c) (5 points) Draw a neat diagram showing the coil, current flow, magnetic field, magnetic moment and torque.

2. (25 points) A conducting rod with length L = 30.0cm moves in a magnetic field B of magnitude 0.45T directed into the plane of the figure. The rod moves with velocity v = 5m/s in the direction shown.


(a) (5 points) What is the motional emf induced in the rod?
(b) (5 points) What is the potential difference between the ends of the rod?
(c) (5 points) Which end of the rod is at higher potential?
(d) (5 points) When the charges in the rod are in equilbrium, what is the magnitude and direction of the electric field within the rod?
(e) (5 points) When the charges in the rod are in equilbrium, which end of the rod has an excess of positive charge?

PIC
Figure 1: Figure for question 2.

3. (20 points) A charge moves in a circular path if its initial velocity v is perpendicular to the uniform magnetic field B. Find the radius of this circular orbit. Show intermediate steps.

4. (30 points) A long, straight wire carries a current of 10A. An electron is traveling in the vicinity of the wire.


(a) (10 points) What is the direction of the magnetic field produced by the current? Draw a neat diagram showing the magnetic field lines due to the current. Remember the density of the magnetic field lines should be proportional to the magnitude of the magnetic field.
(b) (10 points) At the instant when the electron is 10cm from the wire and traveling with a speed of 1×105m/s parallel to the wire in the same direction as the current, what are the direction and magnitude of the force that the magnetic field of the current exerts on the electron?
(c) (10 points) Show schematically the path followed by the electron after the instant given in (b).