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Energy of concentric shells * (a) Concentric spherical shells of radius \(a\) and \(b\), with \(a

Short Answer

Expert verified
The energy stored in the electric field of the two concentric spherical shells, whether transferred immediately or gradually, is given by \(\frac{Q^2}{8\pi\epsilon_{0}a}\)

Step by step solution

01

Calculation of stored energy for part (a)

The energy stored in the system of two charged spheres can be given by the formula \(U = \frac{1}{2}QV\), where \(Q\) is the charge and \(V\) is the potential difference between the two spheres(same as potential of the inner sphere due to its own charge because potential of the outer sphere due to its own charge is zero at the location of the inner sphere). The potential \(V\) for a charged sphere is given by \(V = \frac{Q}{4\pi\epsilon_{0}a}\), where \(\epsilon_{0}\) is the permittivity of free space. Substituting and simplifying we get \(U = \frac{Q^2}{8\pi\epsilon_{0}a}\)
02

Calculation of work for part (b)

The work done \(dw\) in transferring a small charge \(dq\) from the outer shell to the inner shell is equal to the product of the small charge and the potential \(V(q)\) at that moment. Hence, \(dw = dq * V(q)\). Now we need to integrate this to find the total work done or total energy transferred. \(\int dw = \int dq * V(q)\), from \(q=0\) to \(Q\). This gives us the total amount of energy or work done in moving the charge bit by bit, where \(V(q) = \frac{q}{4\pi\epsilon_{0}a}\). After performing the integral, we discover that the result is the same as in the part (a), \(U = \frac{Q^2}{8\pi\epsilon_{0}a}\)
03

Conclusion

Therefore, the energy stored in the electric field of the system for both case (a) and case (b) is found to be the same: \(\frac{Q^2}{8\pi\epsilon_{0}a}\). This shows that the energy stored in a two concentric spherical shell system only depends on the charge and size of the system, not on how the charge was transferred or distributed between the shells.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Concentric Spherical Shells
Concentric spherical shells are spherical structures where one sphere is placed inside another without touching each other. Imagine two perfectly round balls rearranged, one inside the other. Each shell can carry a charge, and in our context, the inner shell carries a charge \(Q\), while the outer shell carries a charge \(-Q\). These charges are uniformly distributed over their respective surfaces.

Due to their spherical symmetry, the behavior of charge and the electric field between these shells can be predicted using the principles of electrostatics. The electric field is generally concentrated in the space between the shells and is affected by the charges present. This system is a great way to model various phenomena in electrostatics, helping students understand the importance of symmetry in simplifying complex electric fields.

The setup of concentric spherical shells can be seen in many practical applications, such as capacitors and insulation systems, highlighting their role in the safe and efficient management of electric fields.
Electrostatics
Electrostatics is the branch of physics that deals with the properties and behavior of static electricity. It focuses on forces, fields, and energy associated with stationary or slow-moving electric charges. In the context of concentric spherical shells, electrostatics is essential to determine how charges distribute themselves and how the electric field forms between the shells.

One fundamental concept in electrostatics is Coulomb's Law, which describes the force between two charges. Another key aspect is the electric field, represented as the force per unit charge. For a spherically symmetric charge distribution, like in concentric shells, it simplifies calculations of the field and potential.

Understanding electrostatics is vital for predicting how charges interact in static systems. This includes calculating the potential energy between charged objects, enabling us to determine the energy stored in electric fields, a crucial factor to consider in designs such as capacitors and electrical insulation.
Potential Energy
Potential energy in the context of electrostatics is the energy a charged object possesses due to its position in an electric field. When dealing with concentric spherical shells, it’s important to calculate the potential energy to understand the energy stored within the system's electric field.

The potential energy due to electric forces is derived from the work done in bringing a charge from infinity to a specific point in space. In the scenario of spherical shells, you can calculate the stored energy using the expression \(U = \frac{1}{2} QV\), where \(Q\) is the charge and \(V\) is the potential difference induced by these charges.

By understanding potential energy, students learn how energy is conserved and transferred within electric fields. This concept simplifies the analysis of complex electrostatic systems by providing a scalar quantity, thereby avoiding vector calculations of electric forces over the entire space.
Permittivity of Free Space
The permittivity of free space, denoted by \(\epsilon_{0}\), is a fundamental constant in physics that characterizes the ability of the classical vacuum to support electric fields. It's crucial in the study of electrostatics and affects how electric fields behave between charged objects like concentric spherical shells.

The value of \(\epsilon_{0}\) is approximately \(8.85 \times 10^{-12}\) F/m (farads per meter), and it features prominently in equations like Coulomb's law and the formulae for electric field calculations, such as in our concentric shells' potential \(V = \frac{Q}{4\pi\epsilon_{0}a}\).

In practice, the permittivity of free space helps define how much resistance a vacuum offers to the electric field. It's essential when calculating potential difference and produced energy in systems involving electrical charges. Understanding \(\epsilon_{0}\) gives students insight into the factors that determine how electric fields exist and propagate in empty space, providing a deeper view into fundamental electrostatic principles.

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Most popular questions from this chapter

Maximum field from a ring ** A charge \(Q\) is distributed uniformly around a thin ring of radius \(b\) that lies in the \(x y\) plane with its center at the origin. Locate the point on the positive \(z\) axis where the electric field is strongest.

\mathrm{~ F o r c e s ~ o n ~ t h r e e ~ s h e e t s ~ * ?}\( Consider three charged sheets, \)A, B\(, and \)C .\( The sheets are parallel with \)A\( above \)B\( above \)C\(. On each sheet there is surface charge of uniform density: \)-4 \cdot 10^{-5} \mathrm{C} / \mathrm{m}^{2}\( on \)A, 7 \cdot 10^{-5} \mathrm{C} / \mathrm{m}^{2}\( on \)B\(, and \)-3 \cdot 10^{-5} \mathrm{C} / \mathrm{m}^{2}\( on \)C$. (The density given includes charge on both sides of the sheet.) What is the magnitude of the electrical force per unit area on each sheet? Check to see that the total force per unit area on the three sheets is zero.

Potential energy of a cylinder A cylindrical volume of radius \(a\) is filled with charge of uniform density \(\rho\). We want to know the potential energy per unit length of this cylinder of charge, that is, the work done per unit length in assembling it. Calculate this by building up the cylinder layer by layer, making use of the fact that the field outside a cylindrical distribution of charge is the same as if all the charge were located on the axis. You will find that the energy per unit length is infinite if the charges are brought in from infinity, so instead assume that they are initially distributed uniformly over a hollow cylinder with large radius \(R\). Write your answer in terms of the charge per unit length of the cylinder, which is \(\lambda=\rho \pi a^{2}\). (See Exercise \(1.83\) for a different method of solving this problem.)

Intersecting sheets ** (a) Figure \(1.49\) shows the cross section of three infinite sheets intersecting at equal angles. The sheets all have surface charge density \(\sigma .\) By adding up the fields from the sheets, find the electric field at all points in space. (b) Find the field instead by using Gauss's law. You should explain clearly why Gauss's law is in fact useful in this setup. (c) What is the field in the analogous setup where there are \(N\) sheets instead of three? What is your answer in the \(N \rightarrow \infty\) limit? This limit is related to the cylinder in Exercise 1.68.

Field in the end face Consider a half-infinite hollow cylindrical shell (that is, one that extends to infinity in one direction) with uniform surface charge density. Show that at all points in the circular end face, the electric field is parallel to the cylinder's axis. Hint: Use superposition, along with what you know about the field from an infinite (in both directions) hollow cylinder.

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