The magnetic field at a distance r from a very long wire carrying current is 0.8 T. The magnetic field at a distance of 4r is_____.

The magnetic field B produced by a long straight wire at a distance r from the wire is given by Ampere's law as:

B = μ0I / (2πr)

where:

• B is the magnetic field
• μ0 is the permeability of free space (4π x 10^-7 T m/A)
• I is the current in the wire
• r is the distance from the wire

From the problem, we know that the magnetic field at a distance r is 0.8 T.

We want to find the magnetic field at a distance of 4r.

We can use the same formula, but replace r with 4r:

B' = μ0I / (2π(4r))

Since the current I and the permeability of free space μ0 are constants, we can see that the magnetic field B' at a distance of 4r is 1/4 of the magnetic field B at a distance r.

Therefore, the magnetic field at a distance of 4r is 0.8 T / 4 = 0.2 T.