# Similar to PS AAMC IV #118

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#### chandler742

##### Senior Member
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15+ Year Member

Only 30 percent of the people got this question right so I'll give you a variation of this question that tests the same concept.

Let's say you have a "open" circuit composed of only one resistor and one capacitor(connected in series) with a DC voltage source(#118 has a AC source but it won't matter for this question).

What can we say the moment the circuit is closed?

A. Capacitor Voltage is maximum, Current in circuit is maximum
B. Capacitor Voltage is maximum, Current in circuit is minimum
C. Capacitor Voltage is minimum, Current in circuit is maximum
D. Capacitor Voltage is minimum, Current in circuit is minimum

What can we say when the circuit has been closed for a while?

Choose among the same choices.

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C is correct?

#### chandler742

##### Senior Member
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okay, but what about the 2nd part?

#### eurotrash

##### Member
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after a while it reverses, with max v and min i?

#### chandler742

##### Senior Member
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15+ Year Member
right. The purpose of the capacitor is to store charge. At the moment the switch is closed the battery starts to set up a Voltage(potential difference) on the capacitor(there is no voltage on the capacitor at the beginning). Current is max at the beginning.

The capacitor is charged exponentially, notice on #118(choices b and c) the square function implies linear charging(which is wrong).

After a while, the capacitor is charged and there is no current, however the voltage of the capacitor is maximum. So there is an inverse relationship between voltage and current.

good job