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Can the Direction of current in an inductor in a closed circuit change instantaneously. The circuit is in steady state for t<0. The solution says I(0-) = -0.4A and I(0+) = 0.4A So, is the solution wrong or the direction really changed.

  • 1
    \$\begingroup\$ If you explain what has prompted this question and your level of understanding of circuit theory you will get a better answer. If you leave it as is you will get answers worth almost nothing to pages of calculus. Welcome to SE. See the help section on how to ask good questions. \$\endgroup\$
    – Transistor
    Jan 17 '16 at 13:39
  • \$\begingroup\$ Simple answer is no, the current trough the inductor is a continous function. The same applies to the voltage of the capacitor. \$\endgroup\$ Jan 17 '16 at 13:46

No, and it has nothing to do with whether the inductor is in a closed circuit or not.

The change in inductor current is voltage applied, times the time it is applied, divided by the inductance:

   dA = V S / H

Where A is amperes, V volts, S seconds, and H Henries.

To change current in a inductor quickly, S must be small, which means V must be large. To change current instantaneously requires infinite applied voltage.

  • \$\begingroup\$ What if it is connected to resistor and battery and after achieving steady state it is instantaneously connected to resistor alone. \$\endgroup\$
    – Monty
    Jan 17 '16 at 13:43
  • \$\begingroup\$ @Yash: Instantaneously, the current thru the inductor will be the same immediately after opening the switch to the battery as before. What voltages that causes has to do with whether the resistor is connected in series or in parallel with the inductor. \$\endgroup\$ Jan 17 '16 at 13:48
  • \$\begingroup\$ @Yash The current continues to flow unchanged at time t=0 , if there is no free path then the inusctur induces such back EMF that somhere the insulation breaks over. \$\endgroup\$ Jan 17 '16 at 13:49

No, neither the magnitude nor the direction of current through an inductor can't change instantaneously.

Remember that the relation between current and voltage for an inductor is given by: $$ V = L {di\over dt} $$

So, if the current through the inductor changed suddenly, the voltage across it would be infinitely large, which is not possible.

If some change a circuit (perhaps opening/closing a switch) caused a change in the calculated value of current through an inductor, then the circuit will go through a transient period, during which the current changes from the previous value to the new steady state value.


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