The magnetic field of a "shunt-wound" dc motor is produced by field coils placed in parallel to the armature coils. Suppose that the field coils have a resistance of $66.0 ~\Omega$ and the armature coils $5.00 ~\Omega$. The back emf at full speed is $105 ~\mathrm{V}$ when the motor is connected to a $115$-$\mathrm{V}$ line. (a) Draw the equivalent circuit for the situations when the motor is just starting and when it is running full speed. (b) What is the total current drawn by the motor at start up? (c) What is the total current drawn when the motor runs at full speed?

Can you tell whether the current in an LRC circuit leads or lags the applied voltage from a knowledge of the power factor, cos $\phi?$

The counter emf in a motor is 72 V when operating at 1800 rpm. What would be the counter emf at $2500$ rpm if the magnetic field is unchanged?

A motor has an armature resistance of 3.75 $\Omega$. If it draws 9.20 A when running at full speed and connected to a $120$-$\mathrm{V}$ line, how large is the counter emf?

If you are given a fixed length of wire, how would you shape it to obtain the greatest self-inductance? The least?

A 450-loop circular armature coil with a diameter of 10.0 cm rotates at $60$ rev/s in a uniform magnetic field of strength $0.75 ~\mathrm{T}$. What is the rms voltage output of the generator? What would you do to the rotation frequency in order to double the rms voltage output?

How would you arrange two flat circular coils so that their mutual inductance was (a) greatest, (b) least (without separating them by a great distance)?

A simple generator bas a 720-loop square coil 21.0 cm on a side. How fast must it turn in a 0.650-$\mathrm{T}$ field to produce a $120$-$\mathrm{V}$ peak output?

A transformer designed for a 120-V ac input will often "burn out" if connected to a 120-V de source. Explain. [Hint: the resistance of the primary coil is usually very low.]

Show that the rms output of an ac generator is $V_{rms} = NAB\omega/\sqrt{2}$.

The primary of a transformer on a telephone pole has a resistance of 0.10 $\Omega$ and the input voltage is $2400 ~\mathrm{V}$ ac. Can you estimate the current that will flow? Will it be $24,000 ~\mathrm{A}?$ Explain.

Since a magnetic microphone is basically like a loudspeaker, could a loudspeaker (Earlier Section) actually serve as a microphone? That is, could you speak into a loudspeaker and obtain an output signal that could be amplified? Explain. Discuss, in Light of your response, how a microphone and loudspeaker differ in construction.

A simple generator is used to generate a peak output voltage of 24.0 V. The square armature consists of windings that are $6.0$ cm on a side and rotates in a field of $0.420 ~\mathrm{T}$ at a rate of $60$ rev/s. How many loops of wire should be wound on the square armature?

A 16-cm-diameter circular loop of wire is in a 1.10-T magnetic field. It is removed from the field in $0.15$ s. What is the average induced emf?