A very long insulating cylinder of charge of radius cm carries a uniform linear density of nC/m. If you put one probe of a voltmeter at the surface, how far from the surface must the other probe be placed so that the voltmeter reads V?
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25. Electric Potential
Electric Potential
Problem 70
Textbook Question
FIGURE P25.70 shows a thin rod of length L and charge Q. Find an expression for the electric potential a distance x away from the center of the rod on the axis of the rod.


1
Step 1: Begin by understanding the setup. The rod has a total charge Q distributed uniformly along its length L. We need to find the electric potential at a point located a distance x from the center of the rod along its axis. The electric potential is a scalar quantity, so we can integrate contributions from each infinitesimal charge element along the rod.
Step 2: Divide the rod into infinitesimal charge elements dq. Since the charge is uniformly distributed, the linear charge density λ is given by λ = Q / L. The infinitesimal charge element dq can then be expressed as dq = λ dx', where dx' is a small segment of the rod.
Step 3: The distance from an infinitesimal charge element at position x' on the rod to the point where the potential is being calculated is r = |x - x'|. This distance will vary depending on the position of the charge element along the rod.
Step 4: The electric potential dV due to an infinitesimal charge element dq is given by dV = (1 / (4πϵ₀)) * (dq / r), where ϵ₀ is the permittivity of free space. Substitute dq = λ dx' and r = |x - x'| into this expression.
Step 5: Integrate dV over the length of the rod to find the total potential V. The limits of integration will be from -L/2 to L/2, as the rod is centered at the origin. The integral becomes V = (λ / (4πϵ₀)) ∫[-L/2, L/2] (dx' / |x - x'|). Solve this integral to obtain the final expression for the electric potential.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electric Potential
Electric potential is the amount of electric potential energy per unit charge at a point in an electric field. It is a scalar quantity measured in volts (V) and represents the work done to move a unit charge from a reference point to a specific point in the field without any acceleration. Understanding electric potential is crucial for calculating the potential due to charged objects, such as the thin rod in this problem.
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Electric Potential
Linear Charge Density
Linear charge density (λ) is defined as the amount of charge per unit length along a charged object, expressed in coulombs per meter (C/m). For a uniformly charged rod, λ can be calculated by dividing the total charge (Q) by the length (L) of the rod. This concept is essential for determining the contribution of each infinitesimal segment of the rod to the total electric potential at a point along its axis.
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Intro to Density
Integration in Physics
Integration is a mathematical technique used to calculate the total effect of a continuous distribution of quantities, such as charge or mass. In the context of electric potential, integration allows us to sum the contributions of infinitesimal charge elements along the length of the rod to find the total potential at a specific point. This process is fundamental in deriving expressions for electric potential from continuous charge distributions.
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Finding Moment Of Inertia By Integrating
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