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Ch 31: Electromagnetic Fields and Waves
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 31, Problem 35b

A wire with conductivity σ carries current I. The current is increasing at the rate dI/dt. Evaluate the displacement current for a copper wire in which the current is increasing at 1.0×106 A/s.

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Understand the concept of displacement current: Displacement current arises in situations where there is a time-varying electric field, such as in a capacitor or in this case, a wire with a time-varying current. It is given by the equation: Id=ε0dΦEdt, where ε0 is the permittivity of free space and dΦE/dt is the rate of change of the electric flux.
Relate the displacement current to the time-varying current in the wire: For a wire, the displacement current can be expressed as Id=ε0dIdt, where dI/dt is the rate of change of the current.
Substitute the given values into the formula: The problem states that the rate of change of current is dI/dt=1.0106 A/s. The permittivity of free space is a constant: ε0=8.85×1012 F/m.
Perform the substitution: Using the formula Id=ε0dIdt, substitute ε0=8.85×1012 and dI/dt=1.0106.
Simplify the expression to find the displacement current: Multiply the values of ε0 and dI/dt to calculate the displacement current. Ensure the units are consistent and the result is expressed in amperes (A).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Conductivity (σ)

Conductivity is a measure of a material's ability to conduct electric current. It is defined as the ratio of the current density to the electric field strength. In the context of a wire, higher conductivity indicates that the wire can carry more current with less resistance, which is crucial for understanding how the wire behaves under varying current conditions.
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Displacement Current

Displacement current is a concept introduced by James Clerk Maxwell to account for changing electric fields in situations where there is no actual flow of charge, such as in capacitors. It is defined mathematically as the rate of change of the electric displacement field and is essential for understanding electromagnetic waves and the continuity of current in circuits where the electric field varies with time.
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Displacement vs. Distance

Rate of Change of Current (dI/dt)

The rate of change of current, denoted as dI/dt, describes how quickly the electric current in a conductor is increasing or decreasing over time. This rate is significant in analyzing dynamic electrical systems, as it influences the magnetic fields generated around the conductor and is directly related to the displacement current in scenarios where the current is not constant.
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Current in a Circuit with a Changing Magnetic Field
Related Practice
Textbook Question

In FIGURE P31.32, a circular loop of radius r travels with speed v along a charged wire having linear charge density λ. The wire is at rest in the laboratory frame, and it passes through the center of the loop. What electric and magnetic fields would an experimenter in the loop's frame calculate at distance r from the current of part c?

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Textbook Question

A 1.0 μF capacitor is discharged, starting at t = 0 s.The displacement current between the plates is Idisp=(10 A)exp(t2.0 μs)I_{\(\text{disp}\)}=(10\(\text{ A}\))\(\exp\]\left\)(-\(\frac{t}{2.0\text{ }\)}\(\mu\[\text{s}\]\right\)). What was the capacitor’s initial voltage (ΔVC)₀?

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Textbook Question

A simple series circuit consists of a 150 Ω resistor, a 25 V battery, a switch, and a 2.5 pF parallel-plate capacitor (initially uncharged) with plates 5.0 mm apart. The switch is closed at t = 0 s. Find the electric flux and the displacement current at t = 0.50 ns.

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Textbook Question

An electron travels with v=(5.0×106i^)m/s\(\vec{v}\) = (5.0 \(\times\) 10^6\,\(\hat{i}\))\,\(\text{m/s}\) through a point in space where E=(2.0×105i^2.0×105j^)V/m\(\vec{E}\) = (2.0 \(\times\) 10^5\,\(\hat{i}\) - 2.0 \(\times\) 10^5\,\(\hat{j}\))\,\(\text{V/m}\) and B=0.10k^T\(\vec{B}\) = -0.10\,\(\hat{k}\)\,\(\text{T}\). What is the force on the electron?

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Textbook Question

A 10 A current is charging a 1.0-cm-diameter parallel-plate capacitor. What is the magnetic field strength at a point 2.0 mm radially from the center of the capacitor?

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Textbook Question

FIGURE P31.38 shows the electric field inside a cylinder of radius R=3.0R=3.0 mm. The field strength is increasing with time as E=1.0×108t2E=1.0\(\times\)10^8t^{2} V/m, where t is in s. The electric field outside the cylinder is always zero, and the field inside the cylinder was zero for t<0t<0. Find an expression for the electric flux ΦeΦ_e through the entire cylinder as a function of time.

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