In the figure a circuit has been shown of a flashing lamp of the type that is attached to barrels at highway construction sites. The fluorescent lamp L is connected in parallel across the capacitor C of an RC circuit. Current passes through the lamp only when the potential across it reaches the breakdown voltage ; in this event the capacitor discharges through the lamp and it flashes for a very short time. Suppose that two flashes per second are needed. We have to calculate the resistance R of the resistor for a lamp with breakdown voltage which is used with a 95-V battery, and a 0.15- capacitor.

Solution:             Click For PDF Version

In the figure circuit has been drawn of a flashing lamp.

Capacitance of the capacitor in the circuit is

EMF of the battery is

The breakdown voltage of the fluorescent lamp is

If two flashes per second are needed, the time for charging the 0.15- capacitor to 72 V using 95.0-V battery has to be 0.5 s. This requirement fixes the value of the resistance R in the circuit.

Equation for charging a capacitor of capacitance C in a RC circuit is

In our problem

If the lamp has to flash in , voltage across the capacitor has to build from to in 0.5 s. This requirement will determine the resistance R.

Therefore,