A very long straight current-carrying wire produces a magnetic field of 20 mT at a distance d from the wire. To measure a field of 5 mT due to this wire, you would have to go to a distance from the wire of

Question

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The magnetic field produced by a long straight wire is proportional to the inverse of the distance from the wire. Therefore, if the field is reduced by a factor of 4 (from 20 mT to 5 mT), the distance from the wire must be multiplied by a factor of 4 in order to maintain the same proportionality. Therefore, the distance from the wire must be 4d.

A Very Long Straight Current-Carrying Wire Produces a Magnetic Field 2\sqrt { 2 }2​

A Very Long Straight Current-Carrying Wire Produces a Magnetic Field $\sqrt { 2 }$