Physics · Magnetism And Matter · NEET
Always use (1 + chi), never chi alone. The full formula is mu = mu_0 * (1 + chi). Adding 1 accounts for the vacuum (free space) part; the chi part is only the extra help the material gives. If chi = 599, you use 1 + 599 = 600, not 599. Forgetting the +1 is the single most common mistake in this numerical, and for large chi like ferromagnets it barely changes the answer, but NTA writes options to punish it anyway.
Because the total magnetic field B inside a material comes from two sources: the external field (through free space) plus the field created by the material itself. In symbols B = mu_0(H + M) and M = chi*H, so B = mu_0*H*(1 + chi) = mu*H. The '1' is the vacuum contribution and 'chi' is the material's extra contribution. That is why mu_r = 1 + chi and never just chi.
No. Permeability mu depends only on the material, so mu = mu_0(1 + chi). The value of H is a distractor (extra data). You would only need H if the question asked for magnetisation M = chi*H, intensity of magnetisation, or the field B = mu*H. Learn to spot which quantity is asked: for mu, ignore H.
Just reverse the formula. From mu_r = 1 + chi you get chi = mu_r - 1. And mu_r = mu / mu_0. So chi = (mu / mu_0) - 1. Example: if mu = 4*pi*10^-4 and mu_0 = 4*pi*10^-7, then mu_r = 1000, so chi = 1000 - 1 = 999. This reverse form is tested just as often as the forward one.
An iron rod of susceptibility 599 is subjected to a magnetising field of 1200 A/m. The permeability of the material of the rod is (mu_0 = 4*pi*10^-7 T m/A):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
mu = mu_0 * (1 + chi), where mu_0 = 4*pi*10^-7 T m/A is the permeability of free space and chi is the magnetic susceptibility. The relative permeability is mu_r = 1 + chi.
No. Relative permeability mu_r = 1 + chi is a pure number (no units). Permeability mu = mu_0 * mu_r carries the same units as mu_0 (T m/A). Read the question carefully — NTA asks for one or the other.
mu_0 = 4*pi*10^-7 T m/A, which is about 1.256*10^-6 T m/A. Questions usually give it as 4*pi*10^-7, so keep pi in your answer and match it to the options.
Yes. mu = mu_0(1 + chi) still holds. For diamagnets chi is a tiny negative number like -10^-5, so mu_r is slightly less than 1 and mu is slightly less than mu_0. That is why diamagnets weakly repel field lines.
