# Ampere Law in a hole Should give zero?

• Neverthelessif
In summary, the conversation discussed the calculation of magnetic fields at the center of a cable and a hole drilled in the cable, using Ampere's law. The question was raised about the magnetic field being zero at a point where there is no current flowing through an ampere loop. It was explained that the sum of individual contributions must be considered and that even if there is no current flow in a certain portion, it does not necessarily mean that the magnetic field is zero in that area.
Neverthelessif
Hi all,

I just did an exercice and there is one point that I don't understand (about Ampere's law).

## Homework Statement

The exercice was about a cable with given diameter a, in which a small hole with diameter b is drilled in. The axis of the cable and the hole are parallel and the distance between the axis is given as r. There is a uniform steady current density flowing through it.
The question is to calculate the magnetic field at the center of cable and center of the hole.

2. Useful equations

Ampere's Law

Don't worry, I won't ask you to answer the exercice for me since I more or less finished it, I was just wondering: if I choose as Ampere's loop to be a circle centered in the hole, and with a radius smaller than b, why can't we say that the magnetic field equals ZERO because there is no current flowing through that circle anyhow?

What is to be done is to add up both magnetic fields by superposition and therefore we indeed have the sum of the magnetic filed of the hole being ZERO, plus the magnetic field of the cable which is NOT zero at that point.

So here is the question, black on white: How come we can have a non-zero magnetic field at a point in space around which we can draw an Ampere loop/surface through which there is zero current going through?

Thank you very much for your time :)

"What is to be done is to add up both magnetic fields by superposition and therefore we indeed have the sum of the magnetic filed of the hole being ZERO, plus the magnetic field of the cable which is NOT zero at that point."

You are interested in the sum of individual contributions. Think about if you had a wire with current flow and were trying to determine the resultant magnetic field. You could have any number of ampere loops in which the wire is not enclosed and their resulting contribution is still zilch.
In your problem with the ampere loop from r = 0 to r = b is telling you that there is no contribution to the magnetic field from this portion of space since there is no current flow there.

## 1. What is Ampere's Law in a hole?

Ampere's Law in a hole states that the magnetic field inside a hole or cavity in a conductor is always zero.

## 2. Why does Ampere's Law in a hole give zero?

Ampere's Law in a hole gives zero because the magnetic field lines inside a hole would have to form closed loops, which is not possible in a conductor where the magnetic field lines must always be continuous.

## 3. What is the significance of Ampere's Law in a hole?

Ampere's Law in a hole is significant because it helps us understand the behavior of magnetic fields in conductors and can be used to calculate the magnetic field in situations where a hole or cavity is present.

## 4. Is Ampere's Law in a hole always true?

Yes, Ampere's Law in a hole is always true in ideal conductors. In real-world situations, there may be some small deviations due to imperfections in the conductor.

## 5. Can Ampere's Law in a hole be applied to non-conducting materials?

No, Ampere's Law in a hole only applies to conductors. In non-conducting materials, the magnetic field inside a hole or cavity may not be zero due to the presence of magnetic materials or external magnetic fields.

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