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3.9.1Charge Cycle A voltage divider containing resistance and capacitance is connected in a circuit by means of a switch, as shown at the top of figure 3-9. Such a series arrangement is called an RC series circuit. In explaining the charge and discharge cycles of an RC series circuit, the time interval from time t0(time zero, when the switch is first closed) to time t1(time one, when the capacitor reaches full charge or discharge potential) will be used. (Note that switches S1 and S2 move at the same time and can never both be closed at the same time.)
NEETS MODULE 2-Alternating Current and Transformers UNCLASSIFIED 3-17UNCLASSIFIED Figure 3-9 Charge of an RC series circuit
NEETS MODULE 2-Alternating Current and Transformers UNCLASSIFIED 3-18UNCLASSIFIED When switch S1 of the circuit in figure 3-9 is closed at t0, the source voltage (ES) is instantly felt across the entire circuit. Graph (A) of the figure shows an instantaneous rise at time t0from zero to source voltage (ES= 6 volts). The total voltage can be measured across the circuit between points 1 and 2. Now look at graph (B) which represents the charging current in the capacitor (ic). At time t 0, charging current is MAXIMUM. As time elapses toward time t1, there is a continuous decrease in current flowing into the capacitor. The decreasing flow is caused by the voltage buildup across the capacitor. At time t1, current flowing in the capacitor stops. At this time, the capacitor has reached full charge and has stored maximum energy in its electrostatic field. Graph (C) represents the voltage drop (e) across the resistor (R). The value of eris determined by the amount of current flowing through the resistor on its way to the capacitor. At time t0the current flowing to the capacitor is maximum. Thus, the voltage drop across the resistor is maximum (E = IR). As time progresses toward time t1, the current flowing to the capacitor steadily decreases and causes the voltage developed across the resistor (R) to steadily decrease. When time t 1is reached, current flowing to the capacitor is stopped and the voltage developed across the resistor has decreased to zero. You should remember that capacitance opposes a change in voltage. This is shown by comparing graph (A) to graph (D). In graph (A) the voltage changed instantly from 0 volts to 6 volts across the circuit while the voltage developed across the capacitor in graph (D) took the entire time interval from time t0to time t1to reach 6 volts. The reason for this is that in the first instant at time t 0, maximum current flows through R and the entire circuit voltage is dropped across the resistor. The voltage impressed across the capacitor at t0is zero volts. As time progresses toward t1, the decreasing current causes progressively less voltage to be dropped across the resistor (R), and more voltage builds up across the capacitor (C). At time t1, the voltage felt across the capacitor is equal to the source voltage (6 volts), and the voltage dropped across the resistor (R) is equal to zero.