Question

When a switch is closed, completing an lr series circuit, the time needed for the current to reach one half its maximum value is _____ time constants?

Answer 1

The answer is 0.693 .  When a switch is closed, completing an LR series circuit, the time needed for the current to reach one half its maximum value is 0.693   

e^(-t/T) = 0.5 
-t/T = ln(0.5) = -0.693