could any one help us with this i have no idea where to start Find the instantaneous value of a 50 Hz waveform with a maximum value of 200v. 0.001 sec after it has passed through a voltage zero. many thanks . liam

 

could any one help us with this i have no idea where to start Find the instantaneous value of a 50 Hz waveform with a maximum value of 200v. 0.001 sec after it has passed through a voltage zero. many thanks . liam

v=Asin(2πft)
where v is the instantaneous voltage
A is the peak (maximum) voltage
f is the frequency
t is the time

so v=200sin(2πx50x0.001) =1.097 Volts

You said when it has passed through zero: so it may be positive like + 1.098 or negative like - 1.097

 

Shouldn't this be done using radians? v(.001) would then be (+/-)61.8V.

 

Shouldn't this be done using radians? v(.001) would then be (+/-)61.8V.

No, he said 0.001 seconds not radians

 

Yes, I know .001 is time, but omega(2pif) is in radians.

 

Yes, I know .001 is time, but omega(2pif) is in radians.

no its in radians per second and time is in seconds so id you multiply them you get radians

 

Right. So you get 200sin(pi/10), or 200sin(18degrees). Both are 61.8V, right? Am I doing something wrong?

 

The dimensional analysis is:

t in seconds.
2*PI = radians/cycle
f = cycles/sec.
2*PI*f*t = radians/cycle * cycles/sec * sec = radians.

So, take the sine in radians. The 61.8 magnitude is correct.

 

I got 61.8 as well.

hgmjr

 

Ok. Thanks.

 

I got 61.8 as well.

hgmjr

Oh yes you are right i forgot to make radians in degrees thanks for the correction. Stupid i am !!

 

Thanks for the replies guys. but im still none the wiser as were you got the answer from . i understand that the answer is 61.8v. but where did this come from . when i tried working out the equation v=200sin(2πx50x0.001) =1.097 Volts i got - 0.107 ????

 

Okay, step by step
1. 2*PI radians/cycle * 50 cycles/second * 0.001 seconds = 0.314 radians

This is just stating that 0.001 seconds into a 50Hz sine wave is 0.314 radians in.

2. Take the sine of this, but remember to set up your calculator to calculate your trigonometric functions in radians. So, sin(0.314 radians) = 0.309

Just so you understand, what this says is that at 0.001 seconds into a 50Hz sine wave, the voltage is 0.309 times the peak.

3. Multiply by the peak voltage, 200V. 200V*0.309 = 61.8V.

Got it?

 

your a absolute star! thanks for the really easy explanation. ive managed to work it out