You are working as a demonstration assistant for a physics professor. He shows you the circuit in Figure P31.14, which he wants you to build for an upcoming class. The lightbulb is a household incandescent bulb that receives energy at the rate of 40.0 W when operating at 120 V. It has a resistance R1, which, for simplicity, we will assume is constant at all operating voltages. The battery in the circuit has an emf of 12.0 V. When the switch has been closed for a long time, the bulb glows dimly, since it is powered by only 12.0 V. When the switch is opened, however, the bulb flashes brightly and then gradually dims to darkness. Your professor wants you to determine two values: (a) the resistance R2 that is necessary for the bulb to initially flash, when the switch is opened, at the same brightness it would have if plugged into a 120-V socket; (b) the inductance L necessary to keep the current in the lightbulb above 50.0% of its value when the switch is opened, for a time interval of 2.00 s after it is opened. Assume a resistance-free inductor and that the resistance of the lightbulb does not vary with temperature.
You are working as a demonstration assistant for a physics
professor. He shows you the circuit in Figure P31.14, which
he wants you to build for an upcoming class. The lightbulb
is a household incandescent bulb that receives energy at the
rate of 40.0 W when operating at 120 V. It has a resistance
R1, which, for simplicity, we will assume is constant at all
operating voltages. The battery in the circuit has an emf of
12.0 V. When the switch has been closed for a long time, the
bulb glows dimly, since it is powered by only 12.0 V. When
the switch is opened, however, the bulb flashes brightly and
then gradually dims to darkness. Your professor wants you
to determine two values: (a) the resistance R2 that is necessary
for the bulb to initially flash, when the switch is opened,
at the same brightness it would have if plugged into a 120-V
socket; (b) the inductance L necessary to keep the current
in the lightbulb above 50.0% of its value when the switch
is opened, for a time interval of 2.00 s after it is opened.
Assume a resistance-free inductor and that the resistance of
the lightbulb does not vary with temperature.
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