i reaaaaally need some help here!

[harry18]

ok, electronics exam tomorrow, and I could do with some help on a few questions, some of this stuff ill Google, but some more input would be great.

1) what is the concept of a rotating phasor?

2) the DMM and CRO are both versatile measuring instruments. identify which instruments you would select in order to check the following signals, give reasons for your choice and comment on the value and accuracy of the instrument.
I) the a.c mains voltage
ii) a computer clock signal
iii) an audio CD pre amp output

3) A metal film resistor has a resistance of 200 ohms, when its mean temperature is 20'C calculate its resistance when its mean temperature is at 80'C
assume the temperature coefficient of resistance alpha(0) of the wire is 0.00428 per 'C, the following formula may be useful: R1 = R0 (1+alpha(0)t)

thatÂ’s it, any advice what so ever would be reallllllly helpful, thanks

 

[Alvin.C]

Re:

Hey,

I wish I was a bit clearer on some physics stuff too, I have a Physics exam at the end of next week but this seems way more complicated to me...

Well I get the phasor part...

A phase of a wave in motion, shows where it is in the wave cycle at that particular point. It is a rotating arrow. A phasor is therefore a vector representing a wave.

The image illustrates the relationship between a phasor arrow (clock arrow) and the amplitude of the wave. The arrow rotates once completely (360 degrees) as the wave goes though one whole cycle (peak to trough). The angle of the arrow at each specific point, will determine what the amplitude of the wave will be at the particular point.

The resultant amplitude of the wave is simply all the rotating phasor arrows added together, from tip to tail.

So, when two waves are superimposed (placed on top of one another), the following will happen to these specific waves:

In this constructive interference example, waves 1 and 2 are said to be 'in phase' with each other, because there is 0 degrees phase difference between phasor arrows of both waves, resulting in a wave with the sum of the amplitude of both those waves (as illustrated as 'both waves combined' on the bottom-left)

In this destructive interference example, waves 1 and 2 are said to be in 'antiphase' or 'out of phase' with one another, because there is a 180 degrees phase difference between the phasor arrows of both waves (the phasor arrow's amplitude for a given point in wave 1, will be the exact opposite amplitude at the same point in wave 2, by 180 degrees). So therefore, the resultant amplitude of both superimposed waves will be zero, because the phasor arrows in waves 1 and 2 cancel each other out (as illustrated as 'both waves combined' on the bottom-right)

More here:
http://www.kwantlen.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/Phasors.html

And here with better animations...

http://www.acoustics.salford.ac.uk/feschools/waves/super.htm

 

[harry18]

cool, thanks that helps with the first part, anyone anyideas for the rest?

 

[hofulstrof]

2) the DMM and CRO are both versatile measuring instruments. identify which instruments you would select in order to check the following signals, give reasons for your choice and comment on the value and accuracy of the instrument.
I) the a.c mains voltage
ii) a computer clock signal
iii) an audio CD pre amp outpu

what the heck is a cro?

the the audio cdpreamp voltage can be measured with a dmm and test tone. becasue frequency and ac current are both a sin wave . play 50hz ( 50 cycles per second) test tone , set your dmm to measure ac voltage, put dmm leads on the preamp outputs. and turn the volume up.take your measuremnt. (ac volts^2) / (z) = peak power expressed in watts ( multilpy by .707 to get rms value)

 

[harry18]

cro, is a cathode ray oscillospe, the test was this morning, and none of those questions came up so its alllll irrelevent......i hate whan that happens