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Multiple-choice questions may continue on
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001 10.0 points
A wire loop of radius R carrying a counterclockwise (wh
...
This print-out should have 17 questions.
Multiple-choice questions may continue on
the next column or page – find all choices
before answering.
001 10.0 points
A wire loop of radius R carrying a counterclockwise (when viewed from the right) current I is moving to the right along the x-axis
at a speed v. It passes around a second, stationary wire loop of radius a where a R.
Choose the plot that correctly displays the
qualitative behavior of the induced current
I(t) in the stationary loop. On the plots, the
+I axis represents clockwise current, while
the −I axis represents counter-clockwise current (when viewed from the right on the +xaxis).
1. VIII:
2. I:
correct
3. V:
4. II:
5. IV:
6. VI:
7. III:Version 085 – midterm 03 – chiu – (56565) 2
8. VII:
Explanation:
I is the correct choice. Before the current
loop passes the wire ring, the flux through
the ring points to the right and is increasing; to resist this change in flux, a clockwise
(positive) current is induced in the ring. After the loop passes the ring, the flux still
points to the right but is decreasing, inducing a counter-clockwise (negative) current in
the ring. To get a sense of the functional form
of the change in flux, note that we are entitled
to use
Bloop ∝ 1
(x2 + R2)3/2
since a R. (Since we are only interested in the qualitative behavior, we can
leave off the constant factors and concern ourselves only with proportionality.) Since the
area of the ring is not changing, dΦB/dt =
Aring dBloop/dt. Again ignoring the constant
factor Aloop, we calculate dBloop/dt,
dBloop
dt ∝
d
dt (x2 +1R2)3/2
dBloop
dt ∝ −
3 2
2x
(x2 + R2)5/2
dx
dt
dBloop
dt ∝ −
3xv
(x2 + R2)5/2
Examining this result, we see that the presence of x in the numerator means our plot
must pass through 0, while to either side of
t = 0 we should expect some polynomiallike behavior. Combined with what we know
about the sign of the current for times t < 0
and t > 0, I is the only correct option.
002 10.0 points
A very long thick wire of radius R carries a
current I, as in the following figure.
I1 R
Find the magnitude of the magnetic field
inside the wire, a distance r from the center
of the wire, where r < R.
Assume current density is the same at every
point inside the wire
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