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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 38a

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.
Illustration of an electric field inside a cylinder with radius R, showing field lines and increasing strength over time.

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Step 1: Recall the formula for electric flux (Φₑ), which is defined as the integral of the electric field (E) over a surface area (A). For a uniform electric field inside the cylinder, the flux is given by Φₑ = E × A, where A is the cross-sectional area of the cylinder.
Step 2: Write the expression for the cross-sectional area of the cylinder. The area A is given by A = πR², where R is the radius of the cylinder. Substituting R = 3.0 mm = 3.0 × 10⁻³ m, we have A = π(3.0 × 10⁻³)².
Step 3: Substitute the given time-dependent electric field E = 1.0 × 10⁸t² V/m into the formula for electric flux. The flux becomes Φₑ = (1.0 × 10⁸t²) × A.
Step 4: Replace A with its expression in terms of R. The flux now becomes Φₑ = (1.0 × 10⁸t²) × π(3.0 × 10⁻³)².
Step 5: Simplify the expression for Φₑ to obtain the final time-dependent formula. The result will be Φₑ = (1.0 × 10⁸t²) × π × (9.0 × 10⁻⁶), which can be further simplified to Φₑ = 9.0π × 10²t².

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Key Concepts

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

Electric Field

An electric field is a region around a charged object where other charged objects experience a force. It is quantified by the electric field strength (E), measured in volts per meter (V/m). In this scenario, the electric field inside the cylinder is given as a function of time, indicating that it changes as time progresses.
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Intro to Electric Fields

Electric Flux

Electric flux (Φₑ) is a measure of the quantity of electric field lines passing through a given area. It is calculated as the product of the electric field strength and the area through which it passes, adjusted for the angle between the field lines and the normal to the surface. The formula is Φₑ = E × A × cos(θ), where θ is the angle between the electric field and the normal to the surface.
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Gauss's Law

Gauss's Law relates the electric flux through a closed surface to the charge enclosed by that surface. It states that the total electric flux through a closed surface is equal to the enclosed charge divided by the permittivity of free space (ε₀). This principle is essential for calculating electric flux in scenarios with symmetrical charge distributions, such as the cylinder in this problem.
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Related Practice
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At one instant, the electric and magnetic fields at one point of an electromagnetic wave are E=(200i^+300j^50k^) V/m\(\overrightarrow{E}\)=(200\(\hat{i}\)+300\(\hat{j}\)-50\(\hat{k}\))\(\text{ V/m}\) and B=B0(7.3i^7.3j^+ak^) μT\(\overrightarrow{B}\)=B_0(7.3\(\hat{i}\)-7.3\(\hat{j}\)+a\(\hat{k}\))\(\text{ }\[\mu\]\text{T}\). What are the values of aa and B0B_0?

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