Charges and are located at and , respectively. What is the net electric flux through a sphere of radius centered (a) at the origin and (b) at ?
Ch 24: Gauss' Law
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Non è quello che usi tu?Cambia libro di testo
Capitolo 24, Problema 26
A spark occurs at the tip of a metal needle if the electric field strength exceeds N/C, the field strength at which air breaks down. What is the minimum surface charge density for producing a spark?
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Understand the relationship between electric field strength and surface charge density. The electric field near the surface of a conductor is related to the surface charge density (σ) by the formula: , where is the permittivity of free space.
Identify the given values in the problem. The electric field strength at which air breaks down is , and the permittivity of free space is a constant: .
Rearrange the formula to solve for surface charge density (σ). Using , multiply both sides by to isolate σ: .
Substitute the known values into the formula. Replace with and with .
Perform the multiplication to find the minimum surface charge density (σ). The result will be in units of , which represents the surface charge density required to produce a spark.

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Electric Field Strength
Electric field strength is a measure of the force per unit charge experienced by a charged particle in an electric field. It is expressed in newtons per coulomb (N/C) and indicates how strong the electric field is at a given point. In this context, the electric field strength of 3.0 x 10⁶ N/C is the threshold at which air becomes ionized, allowing a spark to occur.
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Intro to Electric Fields
Surface Charge Density
Surface charge density refers to the amount of electric charge per unit area on a surface, typically measured in coulombs per square meter (C/m²). It is crucial for determining how much charge is needed on the needle's surface to create an electric field strong enough to exceed the breakdown threshold of air, leading to a spark.
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Surface Charge Density
Breakdown Voltage
Breakdown voltage is the minimum voltage that causes a portion of an insulator to become electrically conductive. In the case of air, when the electric field strength exceeds 3.0 x 10⁶ N/C, the air molecules become ionized, allowing current to flow and resulting in a spark. Understanding this concept is essential for calculating the necessary surface charge density to achieve this electric field strength.
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Dielectric Breakdown
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