Try these "busters" to exercise your brain ... they should help you grasp the concepts underlying the properties of alternating currents, etc. To gain the maximum effect you should attempt to answer them before looking at the answers!
[1] A capacitor is connected to a varying source of emf. Given the behavior of the emf, E, shown:
which one of the plots below shows how the current changes? Why?
[2] A capacitor is connected a varying source of emf as shown below.
[3] Consider the alternating emf below:
Which of the following "phasors" describes this alternating voltage? Why?
[4] An inductor with no resistance is connected across a source of varying emf as shown below:
What can you say about the work done by the source during the different time intervals (a) - (d)? Is positive, negative or zero?
[5] In the circuit below when does the bulb glow brightest, i.e., at low frequencies, at high frequencies, at some particular frequency that depends on R, C and L, or does it have the same brightness at all frequencies?
Explain.
[6] Suppose you are building an a.c. generator and you have a fixed length of wire, L, to make the coil. You would like to produce the largest emf so you have to decide whether the coil you build should contain a small or a large number of turns. How would you evaluate your options?
[7] A light bulb and a parallel plate capacitor, with a dielectric material between the plates, are connected in series and connected to a 60Hz ac wall outlet. What happens to the brightness of the bulb if the dielectric is removed from between the plates?
[8] No equations or formulas, please! The two ends of a long straight wire are connected to an alternating source of emf and the current measured; assume it is I. The wire is disconnected and wound into a coil with many turns and reconnected to the same alternating source. Is the current the same as, or larger, or smaller than I?
[9] No equations or formulas, here either!
An air-cored inductor, i.e., a reglar coil, is connected in series with a light bulb and connected to a 60Hz ac wall outlet. What happens to the brightness of the bulb when a piece of iron is inserted inside the inductor? Explain your reasoning.
[10] This is a great question and related to #9, but be careful! An air-cored inductor is connected in series with a light bulb and connected to a battery. What happens to the brightness of the bulb when a piece of iron is inserted inside the inductor, left there a short while and then removed? Explain your reasoning.
[11] Imagine two a.c. circuits; in each case the a.c. generators are the same, same frequency, same emf, etc. In circuit I only a resistor is connected across the generator; in circuit II the same resistor is in series with an inductor. Which circuit, if either, consumes on average more power? What if, in circuit II, the inductor and resistor are connected in parallel instead of in series? Explain your reasoning.
[12] An inductor and a capacitor are connected in parallel across the terminals of an a.c. generator. Explain what happens to the current drawn from the generator as the frequency changes from a very high value to a very small value?
[13] Is it possible for two series RCL circuits to have the same resonant frequecies and yet have (a) different R values and (b) different C and L values? Explain your reasoning.
[14] Suppose you have an a.c. generator, connected to a series RCL circuit, whose frequency is greater than the resonant frequency of the RCL combination. If you wanted to match the resonant frequency of the circuit to the generator, would you add a second capacitor in series or in parallel with the original capacitor? Explain your answer.
[15] This problem is nowhere near as difficult as you might imagine! Discuss the operation of the circuit below at very high frequency and at very low frequency.
[16] Currents pass through two identical bulbs X and Y, as shown below.
Which bulb, if either one, is brighter than the other? Explain your answer.