Re: a-level question help Please!?

On Nov 5, 3:11 am, paulreynolds...@xxxxxxxxx wrote:
Hi There

I'm writing an a-level paper in about four days and I'm having
problems finding info on a particular question in one of the past
papers. None of my text books have the info I need to resolve the
problem. The paper is available at the following link:-

http://www.cie.org.uk/docs/dynamic/15856.pdf

The mark scheme(answers for the paper) is available here:-

http://www.cie.org.uk/docs/dynamic/15810.pdf

The question I'm having difficulty with is Question:3b(i)1

It asks for the mean voltage of a pd across a load resistor in an RC
circuit. The circuit consists of a half-wave rectified ac source, the
load resistor and a capacitor in parallel with the resistor. The
resistor resistance is given, as well as a graph of of the pd/time for
the rectified pd, also, a graph for the smoothed rectified pd.

My difficulty is finding the mean voltage across the load resistance
when connected with the capacitor. My guessing would be the it's the
average value between the ripple max and min, but the exam paper's
corresponding answer *** does not agree.

Any help what so ever would be much appreciated.

The rectifier is half wave so it is easy to break the
charge and discharge parameters into two problems.

First derive the capacitor from the longer discharge
interval and the known load.

http://en.wikipedia.org/wiki/RC_time_constant

The source resistance appears insignificant because
there is no perceptable change in the position of
shape of the peak when the first capacitor is added.

This is the gist of (c) where the second capacitor
*does* produce a perceptable change to the
shape and position of the peak.

You might post an ascii sketch showing the
peak with both a large and small C for some
comments on your grasp of how the capacitor
changes things but I think you know.

More to the point of the answer ***:
Show how you'll find the mean for each of the two
curves and how you'll find the mean of the two.

Sue...

.