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Question on Ampere's Law and Solenoid

Physics Asked on November 17, 2021

Just came across magnetic field inside a solenoid and amperes law. Amperes law says that dot product of magnetic field and distance from current would give us the $mu_p$ times the current enclosed. If we put an amperes loop inside a solenoid, would the magnetic field be zero as the current enclosed is zero?

2 Answers

There is a catch, Ampere's law has not said that the field would be zero. Instead, it says line integral of the field along a loop will be equal to zero; fields inside the solenoid can't be zero.

Answered by Jack Rod on November 17, 2021

Amperes law says that dot product of magnetic field and distance from current would give us the $mu_p$ times the current enclosed.

This is not what Ampere's law says. Ampere's law refers to the line integral $$ointmathbf Bcdottext dmathbf l=mu_0I_text{enc}$$

The line integral is not a single dot product, nor is $text dmathbf l$ a distance from the current source. Rather, the line integral is an infinite, continuous sum of dot products and $text dmathbf l$ is an infinitesimal displacement along the Amperian loop.

If we put an amperes loop inside a solenoid, would the magnetic field be zero as the current enclosed is zero?

Since $I_text{enc}=0$ we know that the line integral $ointmathbf Bcdottext dmathbf l=0$. However, in general this doesn't tell us what $mathbf B$ is along the Amperian loop, as the value of an integral does not uniquely determine the value of the integrand along the region of integration.

For a uniform, infinite solenoid though you can use Ampere's law and symmetry arguments to determine the field within the solenoid, and then it would be easy to see how a line integral along an Amperian loop within the solenoid evaluates to $0$, but this latter step would then be using what $mathbf B$ is rather than determining it.

Answered by BioPhysicist on November 17, 2021

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