Basic Electrical Theory MAGNETIC CIRCUITSN = number of turns in a coil= rate at which the flux cuts across the conductor,DFDtWbsExample 1: Given: Flux = 4 Wb. The flux increases uniformly to 8 Wb in a period of 2seconds. Find induced voltage in a coil that has 12 turns, if the coil is stationaryin the magnetic field.Solution:V_{ind} =N^{}DFDtDF = 8Wb - 4Wb = 4WbDt = 2sthenDFDt4Wb2s2WbsV_{ind} = -12 (2) = -24 voltsExample 2: In Example 1, what is the induced voltage, if the flux remains 4 Wb after 2 s?Solution:V_{ind}12^{}020 VoltsNo voltage is induced in Example 2. This confirms the principle that relative motion must existbetween the conductor and the flux in order to induce a voltage.Lenz’sLawLenz’s Law determines the polarity of the induced voltage. Induced voltage has a polarity thatwill oppose the change causing the induction. When current flows due to the induced voltage,a magnetic field is set up around that conductor so that the conductor’s magnetic field reacts withthe external magnetic field. This produces the induced voltage to oppose the change in theexternal magnetic field. The negative sign in equation (1-20) is an indication that the emf is insuch a direction as to produce a current whose flux, if added to the original flux, would reducethe magnitude of the emf.Rev. 0 Page 43 ES-01

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