Inductive time constant, value of resistance

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clockworks204
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1. A current in an RL circuit rises to one half of its maximum value in 7.0s. a) what is the inductive time constant for the circuit? b) if L= 0.8 H, what is the value of the resistance?


2. My book says the (time constant) = L/R



3. I've tried plugging into the above equation, but I know there's something missing, and I don't know what it is.
 
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What is your equation for the the current in an R-L circuit?