1.

When a coil joined to a cell, the current through the coil grows with a time constant tau. After what time, the current will reach 10% of its steady- state value?

Answer»

Solution :The GROWTH of current in L-R CIRCUIT at time .t. is given by
`i=i_(0)(1-e^(-(t)/(tau))), (i_(0))/(10)=i_(0)(1-e^(-(t)/(tau))), (9)/(10)=e^(-(t)/(tau))`
`(10)/(9)=e^((t)/(tau))`, Applying log on both SIDES
`LN((10)/(9))=(t)/(tau), t=tau ln((10)/(9))`


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