Question

The switch in an effective LR series dc-circuit (consisting of an emf and a coil with inductance L = 0.5 H and unknown internal resistance) is closed at t = 0 sec. At t = r, the current is 9.5 A and is increasing at a rate of 1.1 A/s. What must the internal resistance of the coil be? A step-up transformer is designed to have an output voltage of 1800 V (rms) when the primary is connect across a 120 V (rms) source. (a) If there are 100 turns in the primary windings how many turns are required in the secondary? (b) If the load (i.e., secondary) resistance is 3600 Ohm, what is the current in the primary, assuming ideal conditions (i.e., 100% efficiency)? (c) What is the effective resistance that the secondary load presents to the primary circuit? (d) How much power is supplied to the load?

Answer 1

19)
L = 0.5 H
i = 9.5 A
di/dt = 1.1 A/s

V(t) = i*R + L*di/dt
V(t) = 9.5*R + 0.5*1.1
V(t) = 9.5*R + 0.55

At t = t
I(t) = Vs/R * ( 1 - e^-1)
9.5 = (9.5*R + 0.55)/R *  ( 1 - e^-1)
R = 0.1 Ω

Internal resistance, R = 0.1 Ω