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\(\mathrm{A}\) series ac circuit contains a \(250 \Omega\) resistor, a \(15 \mathrm{mH}\) inductor, a \(3.5 \mu \mathrm{F}\) capacitor, and an ac power source of voltage amplitude \(45 \mathrm{~V}\) operating at an angular frequency of \(360 \mathrm{rad} / \mathrm{s}\). (a) What is the power factor of this circuit? (b) Find the average power delivered to the entire circuit. (c) What is the average power delivered to the resistor, to the capacitor, and to the inductor?

Short Answer

Expert verified
The power factor for this AC circuit is 0.3482. The average power delivered to the total circuit is 0.695 W. The average power delivered to the resistor is 0.9788 W, while the power delivered to both the capacitor and the inductor are 0 W.

Step by step solution

01

Calcuate the Impedance

The first step is to calculate the total impedance of the circuit. This can be done using the formula: \(Z = \sqrt{R^2 + (X_L - X_C)^2}\), where \(Z\) is the impedance, \(R\) is the resistance, \(X_L\) is the inductive reactance, and \(X_C\) is the capacitive reactance. For this circuit, \(X_L = \omega L = 360 \cdot 15*10^{-3} = 5.4 \Omega\), \(X_C = 1/(\omega C) = 1/(360 \cdot 3.5*10^{-6}) = 716.19 \Omega\), and \(R = 250 \Omega\). Substituting these into the formula, \(Z = \sqrt{(250)^2 + (5.4 - 716.19)^2} = 717.464 \Omega\).
02

Calculate the Power Factor

The power factor is calculated using the formula: \(PF = cos(\theta) = R/Z\). Substituting \(R = 250 \Omega\) and \(Z = 717.464 \Omega\) in the formula, \(PF = cos(\theta) = 250/717.464 = 0.3482\).
03

Compute the Total Average Power

The total average power delivered to the circuit is given by the formula: \(P_{avg} = IV_{rms} \cdot PF\), where \(I\) is the current, \(V_{rms}\) is the root mean square voltage, and \(PF\) is the power factor. We still need to calculate the current using \(I = V_{0}/Z\), where \(V_0\) is the amplitude of the source voltage, and root mean square voltage \(V_{rms} = V_{0} / \sqrt{2}\). After substituting \(V_{0} = 45 V\) and \(Z = 717.464 \Omega\), we find \(I = 45/717.464 = 0.0627 A\) and \(V_{rms} = 45 / \sqrt{2} = 31.82 V\). Then, replace these values into the power formula to find the total average power: \(P_{avg} = 0.0627 \cdot 31.82 \cdot 0.3482 = 0.695 W\).
04

Determine Power Delivered to Each Component

We already determined that the total average power is 0.695 W. Now, determine the power delivered to each component individually. For the resistor, \(P_R = I^2R = (0.0627)^2 \cdot 250 = 0.9788 W\). Since there is no power dissipated in the ideal capacitor and inductor, \(P_C = P_L = 0 W\) respectively.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Impedance Calculation
Understanding the impedance of an AC circuit is critical for assessing how much the circuit resists the flow of alternating current. Think of impedance as a combination of traditional resistance and the unique 'reactance' that comes from capacitors and inductors. In our series AC circuit exercise, the resistance (R), inductive reactance ((X_L)), and capacitive reactance ((X_C)) have distinct values which all contribute to the total impedance ((Z)).

To calculate impedance, we use the formula: \[Z = \sqrt{R^2 + (X_L - X_C)^2}\]The inductive reactance is related to the inductor's value and the frequency of the source using \[X_L = \omega L\], and similarly, capacitive reactance is \[X_C = 1/(\omega C)\]. Plugging in their respective values from our exercise, the impedances of the inductor and capacitor were 5.4\(\Omega\) and 716.19\(\Omega\) respectively.
With these numbers, we calculated a total impedance of 717.464 \(\Omega\), demonstrating how the presence of reactance components significantly changes the nature of the circuit compared to a simple resistive one. When aiming to simplify impedance calculation for students, it's beneficial to emphasize the role of the frequency and reactance, reminding them that unlike resistance, reactance can vary with frequency.
Power Factor
The power factor is a measure of how effectively an AC circuit converts the input voltage into useful work. More technically, it's defined by the cosine of the phase angle (\(\theta\)) between the circuit's voltage and current - essentially describing the alignment of these two waveforms. Power factor values range from -1 to 1, where the extremes represent complete reactive power (with energy entirely stored in the circuit's fields and then returned to the source) and 1 indicating that all the power is being consumed (or 'active').

In our solution, the power factor is calculated using \[PF = cos(\theta) = R/Z\], where R is the resistance and Z the total impedance. After calculation, we determined a power factor of 0.3482, which suggests a less than ideal power transfer efficiency, with a significant portion of the apparent power in the circuit not being used for actual work. Helping students grasp the power factor concept can be aided by contrasting it with the efficiency of purely resistive circuits (where PF equals 1) and explaining its impact on power delivery.
Average Power Delivered
The average power delivered in an AC circuit is the measure of the circuit's actual power usage over time. We can determine this by leveraging the root mean square (RMS) values of voltage and current, which reflects the effective voltage and current considering the AC waveform's shape.

Using the formula \[P_{avg} = IV_{rms} \times PF\], we identify the real power doing work in the system. For the given exercise, we arrived at an average power of 0.695 W. This number is significantly lower than what might be estimated by simply considering the source voltage and resistance, primarily because of the phase differences and the power factor contribution in AC circuits. It's helpful to point out to students that the average power delivered isn't solely a product of voltage and current, but also of how in-phase these two parameters are, which is encapsulated by the power factor.

As for the individual components, the resistor consumed 0.9788 W, which is all the power because capacitors and inductors in ideal scenarios store and return power without dissipating it. Therefore, their average power contributions are zero. This offers an excellent opportunity to discuss how real-world components may differ, as they can have parasitic resistances that would actually dissipate some power.

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Most popular questions from this chapter

An \(L-R-C\) series circuit is constructed using a \(175 \Omega\) resistor, a \(12.5 \mu \mathrm{F}\) capacitor, and an \(8.00 \mathrm{mH}\) inductor, all connected across an ac source having a variable frequency and a voltage amplitude of \(25.0 \mathrm{~V}\). (a) At what angular frequency will the impedance be smallest, and what is the impedance at this frequency? (b) At the angular frequency in part (a), what is the maximum current through the inductor? (c) At the angular frequency in part (a), find the potential difference across the ac source, the resistor, the capacitor, and the inductor at the instant that the current is equal to one-half its greatest positive value. (d) In part (c), how are the potential differences across the resistor, inductor, and capacitor related to the potential difference across the ac source?

A transformer connected to a \(120 \mathrm{~V}\) (rms) ac line is to supply \(13,000 \mathrm{~V}\) (ms) for a neon sign. To reduce shock hazard, a fuse is to be inserted in the primary circuit; the fuse is to blow when the rons current in the secondary circuit exceeds \(8.50 \mathrm{~mA}\). (a) What is the ratio of secondary to primary turns of the transformer? (b) What power must be supplied to the transformer when the \(\mathrm{rms}\) secondary current is \(8.50 \mathrm{~mA} ?\) (c) What current rating should the fuse in the primary circuit have?

You have a \(200 \Omega\) resistor, a \(0.400 \mathrm{H}\) inductor, and a \(6.00 \mu \mathrm{F}\) capacitor. Suppose you take the resistor and inductor and make a series circuit with a voltage source that has voltage amplitude \(30.0 \mathrm{~V}\) and an angular frequency of \(250 \mathrm{rad} / \mathrm{s}\). (a) What is the impedance of the circuit? (b) What is the current amplitude? (c) What are the voltage amplitudes across the resistor and across the inductor? (d) What is the phase angle \(\phi\) of the source voltage with respect to the current? Does the source voltage lag or lead the current? (e) Construct the phasor diagram.

An \(L-R-C\) series circuit has \(R=400 \Omega, L=0.600 \mathrm{H},\) and \(C=5.00 \times 10^{-8} \mathrm{~F}\). The voltage amplitude of the source is \(80.0 \mathrm{~V}\). When the ac source operates at the resonance frequency of the circuit, what is the average power delivered by the source?

Cell phones that use \(4 \mathrm{G}\) technology receive signals broadcast between \(2 \mathrm{GHz}\) and \(8 \mathrm{GHz}\) (a) If you want to create a simple \(L-R-C\) series circuit to detect a \(4.0 \mathrm{GHz}\) cell phone signal, what is the relevant value of the product \(L C,\) where \(L\) is the inductance and \(C\) is the capacitance? (b) If you choose a capacitor that has \(C=1.0 \times 10^{-15} \mathrm{~F}\), what inductance do you need? (c) Suppose you want to wind your own toroidal inductor and fit it inside a box as thin as your cell phone. Based on the size of your phone, estimate the largest cross- sectional area possible for this. (d) Assume the largest allowable radius of the toroid is \(1.0 \mathrm{~cm}\) and estimate the lowest number of windings needed to create your inductor, assuming the material inside has a relative permeability of 1 .

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