(II) A 75-W incandescent lightbulb is designed to operate with an applied ac voltage of 120 V rms. The bulb is placed in series with an inductor L, and this series combination is then connected to a 60.0-Hz 240-V rms voltage source. For the bulb to operate properly, determine the required value for L. Assume the bulb has resistance R and negligible inductance.
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31. Alternating Current
Inductors in AC Circuits
Problem 7c
Textbook Question
Suppose that a circular parallel-plate capacitor has radius r0 = 3.0 cm and plate separation d = 5.0 mm. A sinusoidal potential difference V = V0 sin (2𝝅ft) is applied across the plates, where V0 = 180 V and f = 60 Hz. Plot B0(r) in tesla for the range 0 ≤ r ≤ 10 cm.

1
Understand the problem: The goal is to determine the magnetic field B₀(r) as a function of the radial distance r for a circular parallel-plate capacitor with sinusoidal voltage applied. The magnetic field is induced due to the time-varying electric field between the plates. We will derive the expression for B₀(r) and discuss how to plot it for the given range of r.
Step 1: Recall the relationship between the displacement current and the magnetic field. The displacement current density J_d is given by: , where is the electric displacement field. The magnetic field B₀(r) is related to the displacement current through Ampère's law with Maxwell's correction: , where is the displacement current.
Step 2: Calculate the electric displacement field D. The electric field E between the plates is given by: , where is the potential difference and is the plate separation. Substituting the sinusoidal voltage , we get: . The displacement field is related to the electric field by: , so: .
Step 3: Compute the displacement current density J_d. Taking the time derivative of D, we get: . Substituting E, we find: .
Step 4: Use Ampère's law to find B₀(r). For a circular loop of radius r within the capacitor plates, the displacement current enclosed is: , where is the area of the loop: . Substituting J_d, we get: . Using Ampère's law, the magnetic field is: . Substituting I_d, we get: .
Step 5: Plot B₀(r). To plot B₀(r) for the range 0 ≤ r ≤ 10 cm, substitute the given values: , , , and constants and . Evaluate B₀(r) at different values of r and plot the results.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Capacitance and Electric Field
A parallel-plate capacitor consists of two conductive plates separated by a distance, creating an electric field between them. The capacitance, defined as the ability to store charge per unit voltage, is influenced by the plate area and separation. The electric field strength (E) between the plates is directly proportional to the voltage applied and inversely proportional to the distance between the plates.
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Intro to Electric Fields
Magnetic Field due to a Time-Varying Electric Field
According to Maxwell's equations, a time-varying electric field generates a magnetic field. In this scenario, the sinusoidal potential difference creates an oscillating electric field, which in turn induces a magnetic field around the capacitor. The relationship between the electric and magnetic fields is crucial for understanding electromagnetic wave propagation.
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Electric Field due to a Point Charge
B-field in a Capacitor
The magnetic field (B) generated by a capacitor can be calculated using the formula B = (μ₀/2π) * (I/r), where I is the current and r is the radial distance from the center. For a sinusoidal voltage, the current varies with time, leading to a corresponding variation in the magnetic field. The plot of B₀(r) will show how the magnetic field strength changes with distance from the center of the capacitor.
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Intro to Capacitors
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