A series RLC circuit consists of a 50 Ω resistor, a 3.3 mH inductor, and a 480 nF capacitor. It is connected to a 5.0 kHz oscillator with a peak voltage of 5.0 V. What is the instantaneous current i when ε = ε0?
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31. Alternating Current
Series LRC Circuits
Problem 81
Textbook Question
An inductance coil draws 2.2 A dc when connected to a 45-V battery. When connected to a 60.0-Hz 120-V (rms) source, the current drawn is 3.8 A (rms). Determine the inductance and resistance of the coil.

1
Step 1: Understand the problem. The coil has both resistance (R) and inductance (L). When connected to a DC source, only the resistance affects the current. When connected to an AC source, both resistance and inductive reactance (X_L) affect the current. Use this information to set up equations for R and L.
Step 2: Use Ohm's Law for the DC circuit. The current in the DC circuit is given by I = V / R. Rearrange this to find the resistance: R = V / I, where V = 45 V and I = 2.2 A.
Step 3: For the AC circuit, calculate the total impedance (Z). The relationship between voltage, current, and impedance in an AC circuit is given by V_rms = I_rms * Z. Rearrange to find Z: Z = V_rms / I_rms, where V_rms = 120 V and I_rms = 3.8 A.
Step 4: Relate impedance (Z), resistance (R), and inductive reactance (X_L). In an AC circuit, the impedance is given by Z = √(R² + X_L²). Use the value of R from Step 2 and the value of Z from Step 3 to solve for X_L: X_L = √(Z² - R²).
Step 5: Calculate the inductance (L) using the formula for inductive reactance: X_L = 2πfL, where f = 60.0 Hz. Rearrange to find L: L = X_L / (2πf). Use the value of X_L from Step 4 to determine L.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Inductance
Inductance is a property of an electrical component, typically a coil, that quantifies its ability to store energy in a magnetic field when an electric current flows through it. It is measured in henries (H) and is crucial in AC circuits, as it affects how the current and voltage relate to each other over time, particularly in response to changes in current.
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Ohm's Law
Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R) of the conductor. This fundamental relationship is expressed as V = IR, and it is essential for analyzing both DC and AC circuits, allowing for the calculation of current, voltage, or resistance when the other two quantities are known.
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Resistance and Ohm's Law
RMS Current and Voltage
RMS (Root Mean Square) values are used to express the effective value of alternating current (AC) and voltage. The RMS value of an AC signal is equivalent to a DC value that would deliver the same power to a load. Understanding RMS is vital for analyzing AC circuits, as it allows for the comparison of AC and DC quantities and is used in calculations involving power and impedance.
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RMS Current and Voltage
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