Physics · Electromagnetic Induction · NEET
Like resistors. For inductors in series you ADD them: L = L1 + L2. For inductors in parallel you add the reciprocals: 1/L = 1/L1 + 1/L2. Capacitors are the opposite (add in parallel, reciprocals in series). So remember: L follows the R rule, C is the flipped rule.
In series the SAME current I flows through both coils, so the same dI/dt passes through each. The total back-emf across the pair is the sum of the two back-emfs: E = L1(dI/dt) + L2(dI/dt) = (L1+L2)(dI/dt). Comparing with E = L(dI/dt) gives L = L1 + L2. This is why the equivalent is bigger than either coil.
In parallel both coils have the SAME emf E across them, but the total current splits, so I = I1 + I2. Since dI/dt = dI1/dt + dI2/dt and each branch obeys E = L(dI/dt), you get 1/L = 1/L1 + 1/L2. Adding reciprocals always gives an L smaller than the smallest branch — same as parallel resistors.
Use 1/L_eq = 1/L + 1/L = 2/L, so L_eq = L/2. Two equal inductors in parallel give half the value. In series the same two give 2L. So series-to-parallel ratio for two equal coils is 2L : L/2 = 4:1.
Yes. L = L1 + L2 and 1/L = 1/L1 + 1/L2 are true only when the coils are far apart or the question says 'neglect mutual inductance' — which NEET usually states. If the coils share flux, you must add or subtract 2M (series aiding: L = L1 + L2 + 2M; series opposing: L = L1 + L2 − 2M). NEET keeps it simple by telling you to ignore M.
Two identical inductors are connected in two different configurations P (series) and Q (parallel), where a time-varying current I(t) flows. The induced emf across a-b for series is E_P and for parallel is E_Q. The ratio E_P/E_Q is (neglect mutual inductance):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
L = L1 + L2 + L3 + ... The equivalent inductance is just the sum, provided mutual inductance is neglected. The result is always larger than the biggest single inductor.
1/L = 1/L1 + 1/L2 + 1/L3 + ... For two coils this simplifies to L = (L1·L2)/(L1+L2). The result is always smaller than the smallest single inductor.
Yes, identical. Series inductors add like series resistors; parallel inductors add reciprocals like parallel resistors. Only capacitors use the flipped version of these rules.
When the problem states 'neglect mutual inductance' or when coils are placed far apart so their magnetic fields do not link. Almost all NEET numericals on this topic tell you to ignore M, so the simple series and parallel formulas apply.
It is a small but scoring sub-topic of Electromagnetic Induction. It appears as a quick one-step calculation or a series-vs-parallel ratio question, as in the ReNEET 2026 paper. Learn the two formulas and read the circuit junction points carefully.
