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AC Power
47 questions By Tony R. Kuphaldt
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Question 19 of 47
Power calculation in DC circuits is simple. There are three formulae that may be used to calculate power:
$$P = IV \ \ \ \ \ \ \ \ \ \ \ P=I^2R \ \ \ \ \ \ \ \ \ \ \ P = \frac{V^2}{R} \ \ \ \ \ Power \ in \ DC \ Circuits$$
Calculating power in AC circuits is much more complex, because there are three different types of power: apparent power (S), true power (P), and reactive power (Q). Write equations for calculating each of these types of power in an AC circuit:
Reveal answer$$S = IV \ \ \ \ \ \ S = I^2Z \ \ \ \ \ \ S = \frac{V^2}{Z} \ \ \ \ \ \ Apparent \ power \ in \ AC \ circuits$$
$$P = IV cos\theta \ \ \ \ \ \ P = I^2R \ \ \ \ \ \ P = \frac{V^2}{R} \ \ \ \ \ \ True \ power \ in \ AC \ circuits$$
$$Q = IV sin\theta \ \ \ \ \ \ Q = I^2X \ \ \ \ \ \ Q = \frac{V^2}{X} \ \ \ \ \ \ Reactive \ power \ in \ AC \ circuits$$
Follow-up question #2: substitute π, f, and either L or C into the reactive power equations so that one may calculate Q without having to directly know the value of X.Follow-up question #1: algebraically manipulate each of the following equations to solve for all the other variables in them:
$$Q = I^X \ \ \ \ \ \ S = \frac{V^2}{Z} \ \ \ \ \ \ P = I^2R$$
Notes:Nothing much to comment on here, as these equations may be found in any number of texts. One thing you might consider doing to encourage participation from your students is to ask three of them to write these equations on the board in front of class, one student per power type (S, P, and Q). This would be an ideal question for your more timid students, because there is little explanation involved and therefore little chance of embarrassment.
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Question 20 of 47
Calculate the power factor of this circuit:
Reveal answerP.F. = 0.872
Notes:In order to solve for power factor, your students must find at least one formula to use for calculating it. There is definitely more than one method of solution here, so be sure to ask multiple students to share their strategies for the benefit of all.
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Question 21 of 47
Explain the difference between a leading power factor and a lagging power factor.
Reveal answerA leading power factor is one created by a predominantly capacitive load, whereas a lagging power factor is one created by a predominantly inductive load.
Notes:Ask your students to explain the phase relationships between voltage and current for each of these two conditions: a circuit with a “leading” power factor, and a circuit with a “lagging” power factor. The terms may make a lot more sense once these relationships are seen.
