I'm currently struggling to solve this problem (problem below), I need to find U2(t) but I don't know how. I've only solved problems of this kind with closed circuit. Since the secondary loop is open I2 must be 0.

Our teacher labels voltage sources like current sources and my friend forgot to add the V. U1(t) is a voltage source.

While writing this I got a picture from a friend, is his method correct?

enter image description here

  • 2
    \$\begingroup\$ Where is Vab on your diagram? \$\endgroup\$
    – Andy aka
    Jul 7, 2016 at 14:31
  • \$\begingroup\$ I don't see how you can write U2 with a Z*I = U equation... The secondary is open, there is no current in the secondary loop... \$\endgroup\$
    – MathieuL
    Jul 7, 2016 at 14:52
  • \$\begingroup\$ @Andyaka sorry, it's the arrow pointing down that says u2(t), in the second loop \$\endgroup\$ Jul 7, 2016 at 15:58
  • \$\begingroup\$ @MathieuL I think there is some current that is coming from the first coil. I don't know. Something like this: i.imgur.com/jocAf1V.jpg \$\endgroup\$ Jul 7, 2016 at 16:02
  • \$\begingroup\$ @DenisN on the link you give, it clear that the load isn't in open circuit, there is a loop. However on this there is just a magnetic coupling and no load there U2. So it doesn't make sense to write down I2. You complicate your computations for nothing. Advice: write more clearly, I thought that I1 was I2 in your U2 equation. \$\endgroup\$
    – MathieuL
    Jul 7, 2016 at 16:11

1 Answer 1


Since the secondary loop is open I2 must be 0

Of course it is - there is no current only an induced voltage. Because there is no current, the capacitor on the secondary side is effectively short circuit. And, because there is no current in the secondary there is no part-current flowing in the primary that is due to secondary current.

This boils down to a simple transformer problem where you need to find the voltage on L (primary) then knowing the coupling factor (aka M) you can calculate the secondary voltage.

Can you take it from here?

  • \$\begingroup\$ I don't know how the voltage on L primary can help me find the voltage on L secondary. I came up with this i.imgur.com/lQATDEV.jpg \$\endgroup\$ Jul 7, 2016 at 18:46
  • \$\begingroup\$ @Andy aka "Because there is no current, the capacitor on the secondary side is effectively short circuit" is that correct? how come cap is shorted when its terminals are open? \$\endgroup\$
    – user1999
    May 28, 2018 at 0:59
  • \$\begingroup\$ @newage2000 a 1 ohm resistor is a 1 ohm resistor even if its termianls are not connected to anything. An uncharged capacitor is effectively a short circuit. \$\endgroup\$
    – Andy aka
    May 28, 2018 at 8:38
  • \$\begingroup\$ @newage2000 You tell me, if the capacitor is not charged, what is the voltage across it? \$\endgroup\$
    – user253751
    Aug 1, 2018 at 3:03

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