The counter emf in a motor is 100 V when the motor is operating at 1000 rpm. How would you change the motor's magnetic field if you wanted to reduce the counter emf to $65 ~\mathrm{V}$ when the motor was running at $2500$ rpm?

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 particle with mass $1.81 \times 10^{-3} \mathrm{kg}$ and a charge of $1.22 \times 10^{-8} \mathrm{C}$ has, at a given instant, a velocity $\vec{v}=\left(3.00 \times 10^{4} \mathrm{m} / \mathrm{s}\right) \hat{\jmath}$. What are the magnitude and direction of the particle’s acceleration produced by a uniform magnetic field ${\vec{B}}=(1.63 \mathrm{T}) \hat{\imath}+ (0.980 \mathrm{T}) \hat{\jmath}$?

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?

What is the difference between magnetic flux and magnetic field?