Simple Nature - Light and Matter

Inductors tend to be big, heavy, expensive circuit elements, so asimple surge protector would be more likely to consist of a capacitorin parallel with the computer. (In fact one would normally justconnect one side of the power circuit to ground via a capacitor.)The capacitor has a very high impedance at the low frequency ofthe desired 60 Hz signal, so it siphons off very little of the current.But for a high-frequency signal, the capacitor’s impedance is verysmall, and it acts like a zero-impedance, easy path into which thecurrent is diverted.The main things to be careful about with impedance are that(1) the concept only applies to a circuit that is being driven sinusoidally,(2) the impedance of an inductor or capacitor is frequencydependent.Discussion QuestionA Figure z on page 607 shows the voltage and current for a capacitor.Sketch the q-t graph, and use it to give a physical explanation of thephase relationship between the voltage and current. For example, why isthe current zero when the voltage is at a maximum or minimum?B Figure ac on page 609 shows the voltage and current for an inductor.The power is considered to be positive when energy is being put into theinductor’s magnetic field. Sketch the graph of the power, and then thegraph of U, the energy stored in the magnetic field, and use it to givea physical explanation of the P-t graph. In particular, discuss why thefrequency is doubled on the P-t graph.C Relate the features of the graph in figure ac on page 609 to the storytold in cartoons in figure m/2-3 on page 598.10.5.8 PowerHow much power is delivered when an oscillating voltage is appliedto an impedance? The equation P = IV is generally true,since voltage is defined as energy per unit charge, and current isdefined as charge per unit time: multiplying them gives energy perunit time. In a DC circuit, all three quantities were constant, butin an oscillating (AC) circuit, all three display time variation.A resistorFirst let’s examine the case of a resistor. For instance, you’reprobably reading this book from a piece of paper illuminated bya glowing lightbulb, which is driven by an oscillating voltage withamplitude V o . In the special case of a resistor, we know that I andV are in phase. For example, if V varies as V o cos ωt, then I will bea cosine as well, I o cos ωt. The power is then I o V o cos 2 ωt, which isalways positive, 6 and varies between 0 and I o V o . Even if the timevariation was cos ωt or sin(ωt+π/4), we would still have a maximumpower of I o V o , because both the voltage and the current would reach6 A resistor always turns electrical energy into heat. It never turns heat intoelectrical energy!610 Chapter 10 Fields