Name ______
Phys212 Test 3, Fall 2004
For #1-#6, circle the best answer.
(4 pts each, no partial credit)
1.An electron moves in a magnetic field as shown. What is the direction of the force on the electron?
A:
B:⊙
C:
D:
E:
F:
G:
H:
2.In each of the figures below, there is a magnetic field directed INTO the page and a thin conducting bar moving through the field so that one side of the bar generates a + charge and the other side generates a – charge. Circle the figure which has the correct direction for the velocity of the car.
+ + -
+ -
- - +
+ - +
+ -
- + -
3.A wire carries a current in the direction shown. What is the direction of the magnetic field at P?
A:
B:⊙
C:
D:
E: P
F:
G:
H:
4.The primary purpose of Lenz’s law is to:
A:Induce a change in the overall current around a loop.
B:Induce a change in the overall force on a loop.
C:Induce a change in the overall magnetic field through a loop.
D:Induce a change in the overall magnetic flux through a loop.
E:Prevent a change in the overall current around a loop.
F:Prevent a change in the overall force on a loop.
G:Prevent a change in the overall magnetic field through a loop.
H:Prevent a change in the overall magnetic flux through a loop.
5.A current is established in the wire loop as shown. Which direction should a magnetic field be applied to the loop so that the loop will have a tendency to flip over (so that the top of the loop moves out of the page and the bottom of the loop moves into the page)?
A:
B:⊙
C:
D:
E:
F:
G:clockwise
H:counter clockwise
6.In the most general sense, all magnetic fields are produced by:
(circle the best answer)
A:batteries
B:induced flux
C:negative charges
D:north and south poles
E:nuclei (protons and neutrons) of atoms
F:moving charges
G:positive charges
Show your work and/or explain your answers.
You should draw a meaningful figure for ALL problems.
Make sure you answer the question and use appropriate units if needed!!!
(11pts) (P27.39, example 63)
7.A bar with length of 40 cm and a mass of 200 g slides down two posts as shown and at constant velocity. There is a magnetic field of 3 T pointing into the paper. The two conducting posts have a voltage applied to them so that a current will flow through the bar. What is the amount and direction of the current in the bar so that it falls with constant velocity?
(11pts) (28.33)
8.A coaxial cable is designed so that the inner conductor carries 5 A of current and the outer conductor carries 7 A of current. Find the magnitude and direction of the magnetic field at a distance of 12 cm from the center of the cable. Assume the cable is very long. The figure below shows a cut-away view of the coaxial cable.
8 cm⊙
⊙⊙
2 cm
⊙⊙
⊙⊙
(11pts) (examples 68, 69)
9.A wire of length 3d has a current of I in it. Starting with the Biot-Savart Law, find the magnetic field at the point indicated.
d2d
a
P
(11pts) (29.20, example 81)
10.A conducting bar rests on two conducting rails so that the bar makes electrical contact with the rails. A 1.1 m long bar is pulled to the right at a steady 7 m/s. The bar is in an area in which the magnetic field is a uniform 1.3 T. What is the magnitude and direction of the induced current in the bar if the resistor is 50 ? Which direction does the current flow through the resistor?