Download A magnetic field is perpendicular to the plane of a flat coil

AP Physics B: Flux and Lenz’s Law, Transformers
Name_______________
Lenz’s Law: Induced current will always flow in the direction that opposes the
change in magnetic flux that produced it.
LENZ'S LAW
To determine the direction of the current produced when electric potential is induced, we use Lenz's Law: the induced
current flows in a direction that opposes the change that induced the current. This is more easily understood through an
example.
In the following example the permanent magnet moves to the left. What is the direction of the current through the resistor?
The movement of the north end of the permanent magnet away from the solenoid induces electric potential in the
solenoid. To oppose the motion of the magnet, the left end of the solenoid becomes south, attracting the magnet. The
attraction is not strong enough to prevent the movement; it just offers resistance to the movement.
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1. A magnetic field is perpendicular to the plane of a flat coil so φ=0°. Since the
magnitude of the field is increasing, an emf will be induced in the coil unless
something is done to prevent it. Except for one option, all of the following are
possible options that could be used to keep the emf at the zero level. Which option
could not be used?
A. Decrease the area of the coil.
B. Increase the angle between the field and the normal to the plane of the coil.
C. Simultaneously decrease the area of the coil and increase the angle between the
field and the normal to the plane of the coil.
D.Reduce the time interval during which the magnitude of the field increases.
2. The drawing shows three flat coils, one square and two rectangular, each being
pushed into a region where there is a uniform magnetic field directed into the
screen. Outside of this region the magnetic field is zero. In each case the magnetic
field within the region has the same magnitude, and the coil is being pushed at the
same velocity v. Each coil begins with one side just at the edge of the field region.
Consider the magnitude of the emf induced as each coil is pushed from the starting
position shown in the drawing until the coil is just completely within the field region.
Rank the magnitudes of the emfs in descending order (largest first). Hint, find the
area for each, then think about the time it takes to travel through the field, then find
ΔΦ/Δt for each.
A.
EA, EB, EC
B.
EA, EC, EB
C.
EB, EA and EC (a tie)
D.
EC, EA and EB (a tie)
3.0 A long, vertical, straight wire carries a current I. The wire is perpendicular to the
plane of a circular metal loop and passes through the center of the loop (see the
drawing). The loop is allowed to fall and maintains its orientation with respect to the
straight wire while doing so. In what direction does the current induced in the loop
flow? ( hint, there will be current in the loop only if a magnetic field is passing
through the loop with a vertical component of B).
There is no induced current.
Around the loop from A to B to C to A
Around the loop from C to B to A to C
4.
A metal ring is dropped from rest below a bar magnet that is fixed in position
as suggested in the figure. An observer views the ring from below. Which one
of the following statements concerning this situation is true?
A.
As the ring falls, an induced current will flow counterclockwise as viewed by
the observer.
B.
As the ring falls, an induced current will flow clockwise as viewed by the
observer.
C.
As the ring falls, there will be an induced magnetic field around the ring that
appears counterclockwise as viewed by the observer.
D.
As the ring falls, there will be an induced magnetic field around the ring that
appears clockwise as viewed by the observer.
E.
Since the magnet is stationary, there will be no induced current in the ring.
5.
Two conducting loops carry equal currents I in the same direction as shown in the
figure. If the current in the upper loop suddenly drops to zero, what will happen to the
current in the bottom loop according to Lenz's law?
A.
B.
C.
D.
E.
The current will decrease.
The current will increase.
The current will not change.
The current will also drop to zero.
The current will reverse its direction.
6.
A conducting loop has an area of 0.065 m2 and is positioned such that a uniform
magnetic field is perpendicular to the plane of the loop. When the magnitude of the
magnetic field decreases to 0.30 T in 0.087 s, the average induced emf in the loop is 1.2
V. What is the initial value of the magnetic field?
A)
0.42 T
B)
0.75 T
C)
0.87 T
D)
1.2 T
E)
1.9 T
7. A uniform magnetic field passes through two areas, A1 and A2. The angles
between the magnetic field and the normals of areas A1and A2 are 30.0° and 60.0°,
respectively. If the magnetic flux through the two areas is the same, what is the ratio
A1/A2?
A)
0.577
B)
0.816
C)
1.00
D)
1.23
E)
1.73
8. A neon sign requires 12,000 V for its operation. It operates from a 220-V receptacle.
(a) What type of transformer, step-up, or step-down is needed? (b) What is the turns ratio
Ns/Np needed of the transformer?
9. Kevin likes to watch bugs get zapped in his patio “bug zapper”. Kevin’s uses an ac
voltage of 4320 V to electrocute the insects, while it is plugged into a standard 120.0 V
ac outlet. If the primary coil has 21 turns, how many turns are in the secondary coil?
10.0 The secondary coil of a step-up transformer provides the voltage that operates an
electrostatic air filter. The turns ratio of the transformer is 50:1 ( S/P as this is step-up).
The primary coil is plugged into a standard 120-V outlet. The current in the secondary
coil is 1.7 x 10-3 A. Find the power consumed by the air filter. Remember that P=IV.
Answer: 10.2 W
11. A copper ring is placed inside a solenoid, with the normal to the ring being parallel to
the axis of the solenoid. The copper ring gets hot, but nothing touches it. Explain why.
12. A magnetic field is perpendicular to the plane of a single-turn circular coil. The
magnitude of the field is changing, so that an emf of 0.80 V and a current of 3.2 A are
induced in the coil. The wire is then re-formed into a single-turn square coil, which is
used in the same magnetic field (again perpendicular to the plane of the coil and with a
magnitude changing at the same rate). What emf and current are induced in the square
coil? ( Hint: R stays the same, so Esquare/Ecircle=Isquare/Icircle)
Answer: 0.63V, 2.5 A