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Presented To: Sir Tariq Bhatti Presented By: Nadra Parveen (07-30)3rd Topic: Self Induction www.BZUpages.com Self Induction The phenomenon in which a changing current in a coil induces an emf in itself is called self induction www.BZUpages.com Explanation: Consider a coil is connected in series with a battery and a rheostat as shown in fig. www.BZUpages.com Cont… Magnetic flux is produced through the coil due to current in it. If the current is changed by changing the resistance of the rheostat. The magnetic flux through the coil changes that causes an induced emf in the causes an induced emf in the coil. www.BZUpages.com Such an emf is called as self induced emf. Let the flux passing through one loop of the coil be Φ , then the total flux passing through the coil of N turns will be NΦ. As this flux is proportional to the magnetic field produced by the current I , then N Φα I www.BZUpages.com or or NΦ = LI …..(1) L = NΦ ⁄ I …..(2) where L is constant of proportionality called the self inductance of the coil. www.BZUpages.com It depends upon the following factors. Number of turns of the coil. Area of cross-section of the coil. Core material upon which the coil is wound. According to faraday’s law, emf induced in the coil is given by ε L = A ΔΦ/ Δt www.BZUpages.com ε L = - Δ (AΦ)/ Δt Putting NΦ =LI from equation(1) into equation(3). ε L = - Δ (LI)/ Δt ε L = - Δ LI/ Δt ……..(3) ………(4) www.BZUpages.com The negative sign shows that the self induced emf must oppose the change that produced it. The self induced emf is also called back emf. Self Inductance: In equation(4), negative sign is ignored, then L = ε/(ΔI/ Δt ) www.BZUpages.com Its defined as ”the ratio of average emf in the coil to the time rate of change of current in the same coil.” Unit of Self Inductance: The SI unit for self inductance is VS/A which is called Henry. www.BZUpages.com