A rod of length lies along the -axis with its center at the origin. The rod has a nonuniform linear charge density , where a is a constant with the units C/m2. Determine the constant a in terms of and the rod's total charge .
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24. Electric Force & Field; Gauss' Law
Gauss' Law
Problem 44
Textbook Question
The three parallel planes of charge shown in FIGURE P24.44 have surface charge densities ─ ½ η , η , and ─ ½ η. Find the electric fields to in regions A to D. The upward direction is the + y-direction.


1
Step 1: Understand the problem setup. The three parallel planes of charge have surface charge densities of -½η, η, and -½η, respectively. The regions A, B, C, and D are defined as the spaces above the top plane, between the planes, and below the bottom plane. The goal is to calculate the electric field in each region, considering the upward direction as the +y-direction.
Step 2: Recall the formula for the electric field due to a plane of charge. The electric field produced by a plane of charge with surface charge density σ is given by: , where ε₀ is the permittivity of free space. The direction of the field depends on the sign of the charge density: positive charge produces a field pointing away from the plane, and negative charge produces a field pointing toward the plane.
Step 3: Analyze the contributions to the electric field in region A (above the top plane). Only the top plane contributes to the field in this region. Use the formula for the electric field due to a plane of charge, and note that the field points downward because the top plane has a surface charge density of -½η.
Step 4: Analyze the contributions to the electric field in region B (between the top and middle planes). Both the top and middle planes contribute to the field in this region. Calculate the field due to each plane separately, considering their charge densities and directions, and then add the fields vectorially.
Step 5: Repeat the process for regions C (between the middle and bottom planes) and D (below the bottom plane). For region C, consider the contributions from the middle and bottom planes. For region D, only the bottom plane contributes to the field. Use the formula and add the fields vectorially, keeping track of the directions based on the charge densities.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electric Field due to a Plane of Charge
The electric field generated by an infinite plane of charge is uniform and directed away from the plane if the charge is positive, and towards the plane if the charge is negative. The magnitude of the electric field (E) created by a plane with surface charge density (σ) is given by E = σ / (2ε₀), where ε₀ is the permittivity of free space. This principle is crucial for analyzing the contributions of each charged plane in the given problem.
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Electric Field due to a Point Charge
Superposition Principle
The superposition principle states that the total electric field at a point due to multiple charge distributions is the vector sum of the electric fields produced by each distribution independently. In this scenario, the electric fields from each of the three charged planes must be calculated separately and then combined to find the resultant electric field in each region (A to D).
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Superposition of Sinusoidal Wave Functions
Direction of Electric Fields
The direction of the electric field is determined by the nature of the charge creating it. For positive charges, the field lines point away from the charge, while for negative charges, they point towards the charge. Understanding the orientation of the electric fields in relation to the specified regions (A to D) is essential for correctly determining the net electric field in each area.
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Intro to Electric Fields
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