15. /1 points SB5 32.PO1 My Notes Ask Your Teach A coil has an inductance of 6.0
ID: 2305891 • Letter: 1
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15. /1 points SB5 32.PO1 My Notes Ask Your Teach A coil has an inductance of 6.00 mH, and a current through it changes from 0.200 A to 1.50 A in a time of 0.300 s. Find the magnitude of the average induced emf in the coil during this time. mV 16. -1 points SB5 32.P42 My Notes Ask Your Teac An emf of 88.0 mV is induced in the windings of a coil when the current in a nearby coil is increasing at the rate of 1.4 A/s. What is the mutual inductance of the two coils? mH 17. -3 points SB5 33 P42 My Notes Ask Your Teacher A step-down transformer is used for recharging the batteries of portable devices such as tape players. The turns ratio inside the transformer is 15:1, and it is used with 120 V (rms) household service. A particular ideal transformer draws 0.250 A from the house outlet. (a) What is the voltage supplied to a tape player from the transformer? (b) What is the current? (c) How much power is delivered?Explanation / Answer
(i)
The induced emf in a coil is equal to the negative of the rate of change of magnetic flux.
The induced emf here is equal to -L(dI/dt) , negative sign here comes from the fact that the emf induced in the coil acts to oppose any change in the magnetic flux.
Magnitude of emf = 6 x 10-3x(1.3/0.3) = 0.026 mV
(ii)
When an emf is produced in a coil because of the change in current in a coupled coil , the effect is called mutual inductance.
emf = - M(dI/dt)
Thus magnitude of M = 88 x 10-3 / 1.4 = 0.0628 H = 62.8 mH
(iii)
(a) The input and output voltage of an ideal transformer is related to the number of turns by
Vs = Vp(ns/np) where s stands for primary coil and p stands for primary coil. Since it is a step down transformer np > ns
Vs = Vout = 120 x (1/15) = 8 V
(b) The input and output current is given by the relation
Is = Ip(np/ns)
Thus, Is = Iout = 0.250 x 15 = 3.75 A
(c) The power is given by
P = VoutxIout= 8 x 3.75 = 40. 0 W
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