The magnitude of the magnetic field B a distance r from a long straight wire is B = μ 0 I 2 π r where μ 0 is the permeability constant of free space, and I the current in the wire. The magnetic field is measured at four different distances from a long straight wire while the current is held constant. Order the magnitudes of the measured magnetic field at each distance, from lowest to highest.

The magnetic field is lowest for largest distance and highest when distance is least.

Explanation:

The magnitude of magnetic field strength at a distance 'r' from a long straight wire carrying current 'I' is given as:

Now, as per question, the distance 'r' is varied while keeping the current constant in the wire.

As seen from the above formula, the magnitude of magnetic field strength for a constant current varies inversely with the distance 'r'. This means that, as the value of 'r' increases, the magnitude of magnetic field strength decreases and vice-versa.

Therefore, the magnitude of magnetic field strength is maximum when the distance 'r' is least and the magnetic field is minimum for the largest distance.

Example:

If are the magnitudes of magnetic field strengths for distances respectively such that . Now, as per the explanation above, the order of magnitudes of magnetic field strength is: