30. 0.2 seconds
31. A. Voltage
The L/R time constant is very important in radar and television circuits. To better understand L/R time
constants, observe the d-c circuit below which contains a battery, a switch, an inductor, and a resistor.
When S1 is closed, current does not immediately flow, because of the counter emf produced by the coil as
the magnetic field expands about it. Instead, current "builds-up" at a specific rate, depending on the value
of resistance and inductance. The current reaches its maximum value only when the magnetic field about
the inductor has become stationary. The current "build-up" is at an exponential rate, which is not linear.
When the current is turned off, the magnetic field collapses around the wire and tries to keep current flowing
after the switch is opened. Current decreases to zero at an exponential rate.
The exponential rate is plotted as a curve on the following chart, which is known as a Universal Time