# why t >= 0 for 'I'(current) and t >= 0+ for 'V'(voltage)?

Discussion in 'Homework Help' started by rohanchhibba, Aug 7, 2017.

1. ### rohanchhibba Thread Starter New Member

Aug 7, 2017
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For a simple L-R circuit , I = I... (the exponential expression) . The text says that its defined for t >= 0 but when we talk about the voltage of the inductor,
it's defined for t >= 0+.
Moreover what difference does '+' make when t > '=' 0?
Please tell the reason for this.

2. ### Papabravo Expert

Feb 24, 2006
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2,591
If you have taken calculus you will appreciate the difference. If you haven't, then take it on faith, that in order for there to be any non-zero voltage across the inductor there must be a change in current even if the change is measured over an infinitesimally small time increment.

3. ### WBahn Moderator

Mar 31, 2012
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7,423
While the current through an inductor MUST be continuous (since energy is continuous and the energy stored in an inductor is a function of the current), the voltage across it does not. Usually in the situation you are describing something has changed in the circuit (the proverbial switch is open or closed at t=0). The voltage thus jumps instantaneously from what it was just before the switch was thrown to what it was just after the switch was thrown. The voltage across the inductor at the moment the switch is thrown is undefined.

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4. ### MrAl AAC Fanatic!

Jun 17, 2014
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1,255

Hello there,

What text is that?