Math Problem Statement

The circuit in the figure consists of switch S, a 5.80 V ideal battery, a 10.0 MΩ resistor, and an airfilled capacitor. The capacitor has parallel circular plates of radius 4.00 cm, separated by 3.50 mm. At time t = 0, switch S is closed to begin charging the capacitor. The electric field between the plates is uniform. At t = 170 μs, what is the magnitude of the magnetic field within the capacitor, at radial distance 2.20 cm?

Solution

To solve this problem, we need to calculate the magnetic field within the capacitor at a specific radial distance due to the displacement current as the capacitor charges. Let's break down the steps:

Given dataTo solve this problem, we need to calculate the magnetic field within the capacitor at a specific radial distance due to the displacement current as the capacitor charges. Let's break down the steps:

Given

  • Battery voltage V=5.80VV = 5.80 \, \text{V}
  • Resistor R=10.0MΩ=10.0×106ΩR = 10.0 \, \text{M}\Omega = 10.0 \times 10^6 \, \Omega
  • Capacitor separation d=3.50mm=3.50×103md = 3.50 \, \text{mm} = 3.50 \times 10^{-3} \, \text{m}
  • Radius of capacitor plates a=4.00cm=0.04ma = 4.00 \, \text{cm} = 0.04 \, \text{m}
  • Time t=170μs=170×106st = 170 \, \mu\text{s} = 170 \times 10^{-6} \, \text{s}
  • Radial distance within capacitor r=2.20cm=0.022mr = 2.20 \, \text{cm} = 0.022 \, \text{m}

Approach:

  1. Calculate the capacitance of the parallel-plate capacitor: C=ϵ0AdC = \frac{\epsilon_0 \cdot A}{d} where A=πa2A = \pi a^2 is the area of the plates and ϵ0=8.854×1012F/m\epsilon_0 = 8.854 \times 10^{-12} \, \text{F/m}.

  2. Find the current in the RC circuit, which decreases over time as the capacitor charges. The current at any time tt is given by: I(t)=VRetRCI(t) = \frac{V}{R} \cdot e^{-\frac{t}{RC}}

  3. Calculate the displacement current IdI_d, which equals the current I(t)I(t) because the electric field changes as the capacitor charges.

  4. Determine the magnetic field at the distance rr using Ampère's law for displacement current: B2πr=μ0Idr2a2B \cdot 2 \pi r = \mu_0 I_d \frac{r^2}{a^2} Solving for BB: B=μ0Idr2πa2B = \frac{\mu_0 I_d r}{2 \pi a^2} where μ0=4π×107Tm/A\mu_0 = 4 \pi \times 10^{-7} \, \text{T}\cdot\text{m/A}.

Step-by-Step Solution:

Step 1: Calculate Capacitance

C=ϵ0πa2d=(8.854×1012)π(0.04)23.5×103C = \frac{\epsilon_0 \cdot \pi a^2}{d} = \frac{(8.854 \times 10^{-12}) \cdot \pi \cdot (0.04)^2}{3.5 \times 10^{-3}}

Step 2: Calculate Initial Current

I0=VR=5.8010.0×106I_0 = \frac{V}{R} = \frac{5.80}{10.0 \times 10^6}

Step 3: Calculate Current at t=170μst = 170 \, \mu\text{s}

I(t)=I0etRCI(t) = I_0 \cdot e^{-\frac{t}{RC}}

Step 4: Calculate Magnetic Field

Substitute Id=I(t)I_d = I(t) into the equation for BB: B=μ0Idr2πa2B = \frac{\mu_0 I_d r}{2 \pi a^2}

I’ll now compute each step for the final result.The magnitude of the magnetic field within the capacitor at a radial distance of 2.20 cm is approximately 4.19×1013T4.19 \times 10^{-13} \, \text{T} (teslas).

Would you like further details on any part of this solution, or do you have additional questions?

Here are some related questions you might find interesting:

  1. How does the displacement current relate to the real current in this RC circuit?
  2. What would happen to the magnetic field if the radial distance rr was closer to or further from the center?
  3. How does the magnetic field change over time as the capacitor charges?
  4. What is the physical significance of the displacement current in a capacitor?
  5. How would changing the resistor's value affect the charging time and magnetic field?

Tip: In circuits with capacitors, the displacement current can be as significant as the real current when analyzing the magnetic effects within the capacitor plates.

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Math Problem Analysis

Mathematical Concepts

Electromagnetism
Capacitance
RC Circuit Dynamics
Displacement Current
Magnetic Field Calculation using Ampère's Law

Formulas

Capacitance of a parallel-plate capacitor: C = ε₀A/d
Current in an RC circuit: I(t) = (V/R) * e^(-t/(RC))
Displacement current: I_d = I(t)
Magnetic field using Ampère’s law: B * 2πr = μ₀ * I_d * (r² / a²)

Theorems

Ampère's Law for Displacement Current
Exponential Decay in RC Circuits

Suitable Grade Level

Undergraduate Physics