Physics · Magnetism And Matter · NEET
It means the maths of a magnetic dipole (bar magnet) is the same shape as the maths of an electric dipole. NCERT sets up this analogy so you do not need to derive magnetic field formulas from scratch. You take a formula you already know from electrostatics and swap two things: p becomes m, and 1/(4πε0) becomes μ0/4π. The distance dependence (1/r^3 for a dipole field) stays the same.
Only two replacements. First, the electric dipole moment vector p is replaced by the magnetic dipole moment vector m. Second, the constant 1/(4πε0) is replaced by μ0/4π. That is it. The variable r (distance) and the angles stay the same. Do not change E to B by itself — E maps to B only because the whole formula is being rewritten.
In electrostatics the field strength scales with 1/(4πε0). In magnetostatics the corresponding constant that appears in front of a dipole field is μ0/4π. So ε0 sits in the denominator for electricity, but μ0 sits in the numerator for magnetism. Writing 1/(4πμ0) would be wrong. Just memorise the pair: 1/(4πε0) goes to μ0/4π.
Electric dipole axial field is E_axial = (1/(4πε0)) x 2p/r^3. Apply the analog: B_axial = (μ0/4π) x 2m/r^3. Electric dipole equatorial field is E_equatorial = (1/(4πε0)) x p/r^3, so B_equatorial = (μ0/4π) x m/r^3. Notice the axial field is twice the equatorial field in both cases — the analogy keeps that factor of 2.
Yes. Torque on an electric dipole is tau = p x E, so for a magnet tau = m x B (magnitude tau = mB sin theta). Potential energy of an electric dipole is U = -p . E, so for a magnet U = -m . B (that is U = -mB cos theta). Same forms, just swap p for m and E for B.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Replace the electric dipole moment p with the magnetic dipole moment m, and replace the constant 1/(4πε0) with μ0/4π. Do these two swaps in any electric-dipole formula to get the magnetic-dipole formula.
Yes, but only as a result of the two swaps. The electric field E maps to the magnetic field B, p maps to m, and 1/(4πε0) maps to μ0/4π. You do not change E to B on its own; you rewrite the whole formula.
Yes. Just like an electric dipole, a magnetic dipole has axial field twice its equatorial field at the same distance: B_axial = 2 x B_equatorial. The analog preserves this ratio.
Because it lets you answer magnetic-dipole questions using electrostatics you already know, saving derivation time. NEET often asks you to identify the correct magnetic formula by matching it to the electric one, or spots the trap of putting μ0 in the wrong place.
It is U = -m . B, which equals -mB cos theta. The potential energy is lowest (most stable) when m is aligned with B at theta = 0, exactly as an electric dipole is most stable when aligned with E.
