Lenz’s Law

Basic Electrical Theory MAGNETIC CIRCUITS N = number of turns in a coil = rate at which the flux cuts across the conductor, DF Dt Wb s Example 1: Given: Flux = 4 Wb. The flux increases uniformly to 8 Wb in a period of 2 seconds. Find induced voltage in a coil that has 12 turns, if the coil is stationary in the magnetic field. Solution: V_ind = N^æ ç è ö ÷ ø DF Dt DF = 8Wb - 4Wb = 4Wb Dt = 2s then DF Dt 4Wb 2s 2Wb s V_ind = -12 (2) = -24 volts Example 2: In Example 1, what is the induced voltage, if the flux remains 4 Wb after 2 s? Solution: V_ind 12^æ ç è ö ÷ ø 0 2 0 Volts No voltage is induced in Example 2. This confirms the principle that relative motion must exist between the conductor and the flux in order to induce a voltage. Lenz’s Law Lenz’s Law determines the polarity of the induced voltage. Induced voltage has a polarity that will 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 with the external magnetic field. This produces the induced voltage to oppose the change in the external magnetic field. The negative sign in equation (1-20) is an indication that the emf is in such a direction as to produce a current whose flux, if added to the original flux, would reduce the magnitude of the emf. Rev. 0 Page 43 ES-01