A very long coaxial cable consists of an inner thick wire (radius R) carrying a
ID: 1485753 • Letter: A
Question
A very long coaxial cable consists of an inner thick wire (radius R) carrying a current of 5A, and an outer thick wire encasing the inner wire (starting at R and going out to 2R) and carrying 5A in the opposite direction. What statement is true about how the magnetic field changes as a function of distance from the cable centre, r?
Magnetic field is zero for r=0 to R, then increases linearly as we go from r=R to 2R
Magnetic field is 0 everywhere inside the cable.
Magnetic field increases going from r=0 to R, then decreases from r=R to 2R becoming 0 at r=2R.
Magnetic field increases linearly from the centre of the cable, until we reach r=2R.
A.Magnetic field is zero for r=0 to R, then increases linearly as we go from r=R to 2R
B.Magnetic field is 0 everywhere inside the cable.
C.Magnetic field increases going from r=0 to R, then decreases from r=R to 2R becoming 0 at r=2R.
D.Magnetic field increases linearly from the centre of the cable, until we reach r=2R.
Explanation / Answer
Using alpere's law.
B =miu*Ienc / 2*pi*r
for r=0 to R, I enclosed keeps increasing as r incraeses, So B will keep increasing
for r=R to 2R, I enclosed starts decresing as current in opposire direction is increasing as r is increasing.
for r>2R, I enclosed is 0 as current in upward and downward becomes equal
So, B= 0
AnsweR: C