In the diagram is shown an RL circuit with a switch. ? = 115.0 V , R 1 = 70.0 ?
ID: 2302904 • Letter: I
Question
In the diagram is shown an RL circuit with a switch. ? = 115.0 V, R1 = 70.0 ?, R2 = 140.0 ? and L = 50.0 H. Find the values of i1, the current through resistor R1 and i2, the current through resistor R2, the current through the switch, the potential difference across R2, the potential difference across L and the rate of change of the current di2/dt in the time just after the closing of the switch.
(a) What is i1 just after the switch is closed?
(b) What is the value of the current in the switch just after the switch is closed?
(c) What is the the potential difference across L just after the switch is closed?
(d) What is the rate of change of the current di2/dt in the time just after the closing of the switch?
(e) What is i1 a long time after the switch is closed?
(f) What is i2 a long time after the switch is closed?
(g) What is the value of the current in the switch a long time after the switch is closed?
(h) What is the the potential difference across R2 a long time after the switch is closed?
Explanation / Answer
a) initially inductor acts open ckt
Rnet = R1 = 70 ohms
I1 = V/Rnet = 115/70 = 1.643 A
b) Iswitch = I1 = 1.643 A
c)
VL = V1 = I1*R1 = 1.643*70 = 115 volts
d) VL = L*di2/dt
==> di2/dt = VL/L = 115/50 = 2.3 A/s
e)
I1 = V/R1 = 115/70 = 1.643 A
f)
I2 = V/R2 = 115/140 = 0.821 A
g)
after a long time inductor acts as short ckt.
Iswitch = I1+I2 = 2.464 A
h) V2 = V = 115 volts (there is no potential drop across inductor)