Review of RC circuits:

Does Ohm’s law apply to RC circuits?

No because there is no real “R”

To charge a capacitator with a battery, you must….

put a resitor in series with it

• Vb=VR+VC (Kirchoff’s voltage law)

• Ib=IR=IC (Kirchoff’s current law)

• at time 0 (moment switch closes) there is no charge yet on the capacitators, so all of the voltage drop is across the resistor

Charges built up on the capacitator while it is charging… how?

• as soon as a charge is on teh capacitator, there has to be voltage across it. So, therefore the voltage across the resistor must be going down (Vb=VR+VC)

• therefore, the current across the resistor is going down (V=IR)

• and, current in the entire circuit is going down (Kirchoff’s current law)

A charged capacitator acts like an….

Open circuit

• when the capacitator is fully charges (called time “final”), it can accept no more charges and no more current will flow in this circuit

• therefore, no voltage change across the resistor (V=IR)

• and all of the voltage drop is acress the plates of the capacitor (Vb=VR+VC)

What causes the change in the voltage drop in a charged capacitor?

Q=CV

what is T (tau)?

the time constant

What doe tau represent and how is it calculated?

• describes the time couse fo the change in exponential graphs

• calculated by R*C

• units are in seconds (Ohms =Volts/ampere; Farad= coulomb/volt)

• 1t of time is the amound of time it takes to reach about 2/3 of final values

What happens when there is charging a capacitor with a constant current source?

• at time final→ the capacitor is fully charged and can accept no mreo charges

• therefore, no current flows in the capacitive arm of the the circuit

• all the current is flowing throught the resistor

• therefore, VR=ICCS*R