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Exam 20: Electromagnetic Induction

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Question 1

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A square conducting loop is situated such that it is vertical, in the xz-plane. The Earth's gravitational field points downward (in the -z direction) . The loop is dropped and maintains its orientation in the xz-plane. A magnetic field pointing in the +y direction exists and is uniform with respect to time, but it varies as a function of z: B_y(z) = B₀(10 m − z) . The loop is dropped and allowed to fall under the influence of gravitation. You observe the loop from the +y direction (i.e., looking toward the −y direction) . Which of the following occurs?

A) The situation is unclear because we do not know the initial conditions.
B) There is a clockwise induced current.
C) There is a counter-clockwise induced current.
D) There is no induced current.

E) None of the above
F) All of the above

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Question 2

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A 20 turn coil of area 10 cm² is placed in a magnetic field so that the normal to the coil's area is in the same direction as the field. If the field has a value of 0.25 T, what is the total flux through the coil?

A) 0.0
B) 5.0 × 10^-3 T·m²
C) 25 × 10^-5 T·m²
D) 50 T·m²
E) 25 T·m²

F) C) and D)
G) B) and E)

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Question 3

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The south end of a bar magnet is pushed downward toward a wire loop in the plane of the paper. In which direction is the induced current, and which way is the induced magnetic field?

A hollow copper pipe is held vertically, and a magnet is dropped down the pipe. The magnet is oriented with its north pole down. What direction do the eddy currents flow as viewed from above the pipe? If copper bar of approximately the same size and weight as the magnet is dropped down the tube instead, will it fall faster or slower than the magnet?

A conducting rod is free to slide on horizontal, frictionless rails separated by 2.0 m. At a particular time, the rod is sliding at 4.0 m/s to the right, as shown. A uniform magnetic field is directed into the page. If a current of 40 mA is generated in the circuit at this time, and the resistance R is 0.36 Ω, what is the magnitude of the magnetic field between the rails?

The back emf of an electric motor is greatest

A conducting ring is placed such that it is lying flat in the xy-plane. A magnetic field exists in the +z direction, and its strength is decreasing with time. Which of the following occurs?

Faraday's law of induction states that the emf induced in a rectangular loop of wire is proportional to

An ideal transformer supplies 100 kW of power to a 120 V circuit. If the input voltage is 10,000 V, what is the input current?

What electric field has the same energy density as a 1.0 T magnetic field?

A series LR circuit includes a 9.00 V battery, a 30.0 mH inductor, and a resistance of 5.00 Ω. What is the time constant of this circuit?

What is the magnetic energy density in a volume where the magnetic field is 4.0 mT?

A series LR circuit includes a 9.0 V battery, a resistance of 0.50 Ω, and an inductance of 0.80 H. What is the current 2.0 s after the switch is closed?

If the number of turns is tripled for an inductor of fixed length and fixed cross-sectional area, what happens to its inductance?

An LR circuit is prepared for an experiment, initially with its switch open. The time constant is that time after switch closure at which the energy stored in the inductor reaches approximately what percentage of its final value?

The maximum induced emf in a generator whose coils are rotating at 240 rpm is 35 V. If, instead, it is desired to generate a maximum emf of 45 V but with half the magnetic field, what rotation rate must be used?

An ideal transformer has 200 turns on its primary and 12 turns on its secondary. If the power input to the transformer is 120 kW, what is the power output?

In a transformer, the relationship between the primary and secondary currents depends on all of the following quantities except

A solenoid of length 4.00 cm and cross-sectional area 2.00 cm² has 80 turns. What is its self-inductance?

A conducting rod is free to slide on horizontal metal rails without friction. At a particular time, it is sliding with velocity v to the right, as shown. A uniform magnetic field B is directed into the page, the separation of the rails is L, and the resistance of the circuit is R. In what direction in the diagram is the current flowing in the resistor at this time?