Capacitors in RC filtering

[quietrain]

in AC source, C and R in parallel, why does the capacitor discharge when the voltage across the resistor decrease ?

[PLAIN]http://upload.wikimedia.org/wikipedia/en/thumb/2/27/RC_Parallel_Filter_%28with_I_Labels%29.svg/250px-RC_Parallel_Filter_%28with_I_Labels%29.svg.png

the Vin is a AC source, and the Vout measured is across the resistor.

QUESTION 2

also, why can't i use the multimeter to measure the RMS vale of the ripple voltage across R in this set up?

why do i have to use the oscilloscope?

aren't they the same as long as the multimeter uses true rms calculations?

QUESTION 3

also, if i connect a resistor of about 1kohm in series to a forward bias diode, is it resonable to say that the contact potential of the diode is the voltage across the diode when the voltage of the load first starts to increase?

so since voltage of load increase, current flows through entire circuit, so the voltage across the diode will be the contact potential?

thanks!

 

[Vailanor]

Answers:1. The capacitor discharges when the voltage across the resistor decreases because the current flowing through the circuit is inversely proportional to the resistance of the circuit. As the voltage across the resistor decreases, the current through the circuit increases, which causes the capacitor to discharge. 2. You cannot use a multimeter to measure the RMS value of the ripple voltage across R because a multimeter is not designed to measure AC signals. An oscilloscope, on the other hand, can measure AC signals, and is more accurate in measuring the RMS value of a signal. 3. Yes, it is reasonable to say that the contact potential of the diode is the voltage across the diode when the voltage of the load first starts to increase. As the voltage of the load increases, current will flow through the entire circuit, thus causing the voltage across the diode to be the contact potential.