A model-train transformer plugs into 120 V ac, and draws 0.75 A while supplying $15 ~\mathrm{A}$ to the train.

(a) What voltage is present across the tracks? (b) Is the transformer step-up or step-down?

A step-up transformer increases 16 V to 120 V. What is the current in the secondary as compared to the primary? Assume $100$ percent efficiency.

A transformer has 420 turns in the primary and 120 in the secondary. What kind of transformer is this and, assuming $100$ percent efficiency, by what factor does it change the voltage? By what factor does it change the current?

A transformer is designed to change 120 V into 10,000 V, and there are $5000$ turns in the primary. How many turns are in the secondary, assuming $100$ percent efficiency?

Suppose you are holding a circular ring of wire and suddenly thrust a magnet, south pole first, away from you toward the center of the circle. Is a current induced in the wire? Is a current induced when the magnet is held steady within the ring? Is a current induced when you withdraw the magnet? In each case, if your answer is yes, specify the direction.

Compare the oscillations of an LC circuit to the vibration of a mass m on a spring. What do L and C correspond to in the mechanical system?

A dc generator is rated at 10 kW, 200 V, and 50 A when it rotates at $1000$ rpm. The resistance of the armature windings is $0.40 ~\Omega$. (a) Calculate the "no-load" voltage at $1000$ rpm (when there is no circuit hooked up to the generator). (b) Calculate the full-load voltage (i.e. at $50 ~\mathrm{A}$) when the generator is run at $800$ rpm. Assume that the magnitude of the magnetic field remains constant.

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?

An LRC resonant circuit is often called an oscillator circuit. What is it that oscillates?

Under what conditions is the impedance in an LRC circuit a minimum?

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 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?

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?