Download LECTURE 4 Announcements

LECTURE 19
Pick up
• lecture notes
• Practice test/review on
Sunday, March 4th
at 11 am (note change of time) ,
NSH 123
• Practice Questions will be
posted on the Web, under
Exams
Magnetic fields caused by currents
0 2 I
B
4 R
N
S
N
S
BEarth
Concept Test: After closing the switch in the
above experiment, the current flows
a.) from the switch towards the compass
b.) from the compass towards the switch
c.) not enough information to decide
Concept Test: A battery establishes a steady
current around the circuit below. A compass needle
is placed successively at points P,Q, and R. The
Relative deflection of the needle, in descending
order,is
P
1.
2.
3.
4.
5.
P, Q, R
Q, R, P
R, Q, P
P, R, Q
Q, P, R
Q
R
Concept Test: Two very long fixed wires cross
each other perpendicularly (see figure). They do
not touch, but are very close to each other, as
shown. Indicate all the points where the net
magnetic field is zero
I
I
Force between two parallel wires
 0 I1
B1 
2 R
F
 0 I1 I 2
 I 2 B1 
l
2 R
I
Repulsive
I
Attractive
I
Problem A rectangular loop is placed next to a
straight wire as shown in the figure. What is the
direction and the magnitude of the net magnetic
force acting on the loop?
I
d
I
w
l
Ampere’s Law
 
 B  dl  0 IC
Example: Infinitely long wire
B(2r )  0 I C
0 I
B
2 r
Ampere’s Law: Applications
Toroid with N turns:
Calculate and sketch the
magnetic field around the
toroid as a function of
the radial distance r from
its center.
B(r)
a
b
r