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Ch 30: Inductance
Young & Freedman Calc - University Physics 15th Edition
Young & Freedman Calc15th EditionUniversity PhysicsISBN: 9780135159552Not the one you use?Change textbook
Chapter 30, Problem 17c

A solenoid 25.0 cm long and with a cross-sectional area of 0.500 cm2 contains 400 turns of wire and carries a current of 80.0 A. Calculate: the total energy contained in the coil's magnetic field (assume the field is uniform);

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First, understand that the energy stored in the magnetic field of a solenoid can be calculated using the formula for the energy density of a magnetic field: \( u = \frac{1}{2} \frac{B^2}{\mu_0} \), where \( B \) is the magnetic field and \( \mu_0 \) is the permeability of free space.
Calculate the magnetic field \( B \) inside the solenoid using the formula \( B = \mu_0 n I \), where \( n \) is the number of turns per unit length (\( n = \frac{N}{L} \)), \( N \) is the total number of turns, \( L \) is the length of the solenoid, and \( I \) is the current.
Substitute the values into the formula for \( n \): \( n = \frac{400}{0.25} \) turns/m. Then calculate \( B \) using \( B = \mu_0 n I \).
Once \( B \) is calculated, find the energy density \( u \) using \( u = \frac{1}{2} \frac{B^2}{\mu_0} \).
Finally, calculate the total energy stored in the solenoid by multiplying the energy density \( u \) by the volume of the solenoid: \( E = u \times \text{Volume} \), where the volume is \( A \times L \) and \( A \) is the cross-sectional area.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Magnetic Field in a Solenoid

The magnetic field inside a long solenoid is uniform and can be calculated using the formula B = μ₀nI, where B is the magnetic field, μ₀ is the permeability of free space, n is the number of turns per unit length, and I is the current. This concept is crucial for understanding how the solenoid generates a magnetic field.
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Magnetic Energy Density

Magnetic energy density is the energy stored per unit volume in a magnetic field, given by the formula u = B²/(2μ₀), where u is the energy density, B is the magnetic field, and μ₀ is the permeability of free space. This concept helps in determining how much energy is stored in the magnetic field of the solenoid.
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Total Magnetic Energy in a Solenoid

The total energy stored in the magnetic field of a solenoid can be calculated by multiplying the magnetic energy density by the volume of the solenoid. The volume is found by multiplying the cross-sectional area by the length of the solenoid. This concept is essential for calculating the total energy contained in the solenoid's magnetic field.
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