# Homework Help: Oscilloscope and RC Circuit

1. Mar 5, 2014

### tbdm

The biggest problem is that I have no knowledge about the math for oscilliscopes. This is a post lab but all we actually did was push a few buttons to record measurements, we did not go into any detail about the methods, just what an oscilliscope was supposed to measure.

1. The problem statement, all variables and given/known data
1) You place an AC voltage signal of amplitude 1.7 V and with a frequency of 1200.0 Hz, into the input of an oscilloscope.
(a) Find the distance displayed on the screen of the oscilloscope for a full wave, if the sweep speed is adjusted to 104 µsec/DIV.

(b) If the vertical gain is set to 0.34 V/DIV, how many divisions will the wave span from peak-to-peak of the wave?

2) Consider a series RC circuit with a R = 3.0 Ω and a capacitance of 14 µF being charged to a voltage of 8.0 V.
(a) If you wished the charging of this capacitor to have a vertical displacement of 4.0 DIV from 0 charge to fully charged, what value must you set the vertical gain on the oscilloscope to achieve this?

(b) What is the value of the horizontal sweep that you would need to use if you wished the characteristic time constant of this circuit to span 2.0 DIV on the oscilloscope screen?
2. Relevant equations
I'm afraid I don't know. I have been going off units to get the answers I have.

3. The attempt at a solution
I have 1a and 2a. For 1a I simply took 1/1200 Hz and divided it by the 104E-6 seconds/DIV to get 8.01 divisions. For 2a I divided the voltage by the divisions to get 2 V/DIV.

1b is looking for units of divisions and I tried 1.7 V divided by 0.34 V/DIV but the answer I got (5 divisions) was wrong.
2b is looking for units of microseconds/division and I'm afraid I'm not sure where to start, I think it has to do with the current but I'm not sure

2. Mar 5, 2014

### BvU

Well, this goes to show that post lab stress is the only thing that is found if you do a lab without some form of preparation and just carry out the instructions.
How much time did you have with the actual equipment and how did you use that time? Would you do it differently if given the chance to do it again? If so write down what you would like to do and politely ask for a chance to re-do the lab!

1a is probably right, but they ask what you measured on the screen. Can't be 8.01 divisions: you can never distinguish 8.01 from 7.98 or 8.01 from 8.03. Unless this was all LabView, including the AC generator.
1b same thing. To get the 'right' answer, ask yourself what amplitude 1.7 V means.

Is 2 also part of the lab ? Then why does it say 'consider' ? How far are you in class ? What did you find in 2a?
2b indeed has to do with the current. Your answers come from the relevant equations. Write down at least three you know and have an idea might be relevant here.

Google characteristic time constant if you really have no idea

3. Mar 5, 2014

### ehild

Correct.

5 divisions correspond to the amplitude of the wave but the peak - to peak value is the question.
twice the amplitude

ehild

4. Mar 5, 2014

### tbdm

This part wasn't actually done in lab, it was a set of postlab questions on webassign where we were supposed to think back to what happened in lab and apply it to these questions. I didn't have an oscilliscope to work with for it. We just started the RC circuits in lab today, I guess there's been a disconnect between lab and lecture now. For 1b I understand now that I was missing the amplitude part so I was only getting the number of divisions on the screen between the baseline and the amplitude, so doubling that would get the how many vertical divisions there would be on the screen in that circumstance. We went over the basics of the time constant and I was able to get question 2b (21 μs/division) though on this particular question I'm not entirely sure what it's supposed to mean in the overall purpose of the lab.

Unfortunately I don't think I'll be able to redo the lab as with the number of students taking the course the instructors don't really let anyone do that sort of thing. I will try to use your advice for the next lab though, I think it could help me understand this subject a little better. Thank you.

That makes sense. I was only looking at the top half of the wave, so it would have been only the top half of the screen if I was using the machine. Thank you for your help.

5. Mar 6, 2014

### BvU

Think of it as a voltmeter that has a dot going up and down instead of a needle (or a digital display). To see how the voltage changes in time, the dot moves to the right until it hits the right edge and then it jumps back to the left. For phenomena that repeat in time there is a smart gadget (the trigger) that makes every sweep start at the same point in the period, so it looks as if everything stands still, which initially can be very deceptive.

Since almost everything can be converted to a voltage, the oscilloscope idea is extremely useful in physics, seismology, medicine (the heart monitor you see in all tv series and movies!) and so on.

I apologize for the rant in my post #2. It says more about my sensitivities than about your situation. The advice is well meant, however, so I'm glad you pick it up.