An infinitely long cylindrical conductor has radius and uniform surface charge density . In terms of , what is the magnitude of the electric field produced by the charged cylinder at a distance from its axis? Then, express the result in terms of and show that the electric field outside the cylinder is the same as if all the charge were on the axis.
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Gauss' Law
Problem 31b
Textbook Question
"(II) Suppose the thick spherical shell of Problem 29 is a conductor. It carries a total net charge Q and at its center there is a point charge +q. What total charge is found on the outer surface of the shell?"

1
Understand the problem: We are dealing with a spherical conducting shell that carries a net charge Q, and there is a point charge +q at its center. Conductors in electrostatics have specific properties, such as the redistribution of charges to maintain electrostatic equilibrium. We need to determine the total charge on the outer surface of the shell.
Step 1: Recall the property of conductors in electrostatics. Inside a conductor, the electric field must be zero in electrostatic equilibrium. This means that any charge inside the conductor will induce an equal and opposite charge on the inner surface of the shell to cancel the electric field within the conducting material.
Step 2: Analyze the inner surface of the shell. The point charge +q at the center will induce a charge of -q on the inner surface of the shell. This ensures that the electric field inside the conducting material of the shell is zero.
Step 3: Account for the net charge Q on the shell. The shell as a whole has a net charge Q. Since the inner surface has a charge of -q, the outer surface must have a charge that balances the total charge of the shell. The total charge on the shell is the sum of the charges on the inner and outer surfaces.
Step 4: Calculate the charge on the outer surface. The outer surface must carry a charge of Q + q to ensure that the total charge on the shell is Q. This is because the inner surface has a charge of -q, and the sum of the charges on the inner and outer surfaces must equal the net charge Q.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Gauss's Law
Gauss's Law states that the electric flux through a closed surface is proportional to the enclosed electric charge. This principle is crucial for analyzing charge distributions, especially in symmetric situations like spherical shells. It allows us to determine the electric field and charge distribution on conductors by considering the net charge and the geometry of the system.
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Gauss' Law
Charge Distribution in Conductors
In electrostatic equilibrium, charges in a conductor redistribute themselves such that the electric field inside the conductor is zero. This means that any excess charge resides on the surface of the conductor. For a spherical shell with a point charge at its center, the inner surface will acquire a charge equal in magnitude but opposite in sign to the point charge, while the outer surface will carry the remaining charge.
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Electric Fields in Conductors
Superposition Principle
The Superposition Principle states that the total electric field created by multiple charges is the vector sum of the electric fields produced by each charge individually. This principle is essential for understanding how the presence of the point charge affects the charge distribution on the spherical shell, allowing us to calculate the total charge on the outer surface by considering both the point charge and the net charge of the shell.
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Superposition of Sinusoidal Wave Functions
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