ohms. At the instant that E is applied to the circuit, the current

is 0.1 mA and the applied voltage E appears across R. Since one

coulomb is the quantity of electrons required to pass a given point

in the circuit during one second of time in order to produce a

current flow of one ampere, the initial rate of charge must be 0.0001

coulomb per second (this is determined on the basis that the initial

current is 0.1 mA) or 0.0001 x 106 coulomb per usec. After 1 usec,

the voltage across C is equal to 0.0001 x 106 (charge in coulombs)

divided by 109 (capacitance in farads) or 0.1 volt. The voltage

across R, therefore, is equal to 0.9 volt and It is equal to 0.09 mA.

Since It now is equal to 0.09 mA, the rate of charge has decreased to

0.00009 coulomb per second or 0.00009 x 106 coulomb per usec. After

2 usec, Q is equal to 0.00001 x 106 + 0.00009 x 106 or 0.00019 x

106 coulomb. Ec then is equal to 0.00019 x 106/109 or 0.19 volt.

The rate of voltage change is now 0.09 volt per usec. ER, at this

time, is equal to 0.81 volt and the current is equal to 0.081 mA. In

a similar way, all values of It, ER, and Ec, and the charging rate

can be determined until the steadystate condition is reached.

PRACTICAL APPLICATIONS OF RC AND RL CIRCUITS

Section I. RELATIONSHIP BETWEEN TIME CONSTANTS AND

PULSE DURATIONS

14.

TIME CONSTANT CHARACTERISTICS.

a. In the series RC and RL circuits discussed previously, the

time required for the output current or voltages to reach a steady

state condition depended on the time constant of the circuit. Time

constants were used in describing pulse characteristics during the

rise time, duration period, or decay time of the output pulse. Time

constants are a factor in determining the amplitude of the output

pulse since it can prohibit the amplitude from reaching a value equal

to the applied voltage (Figure 21D). Time constants are also a

factor in determining the value to which the output pulse decays

since it can prohibit the pulse from decaying to zero before the next

input pulse is applied (Figure 21C).

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