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Quick questions for tomorrows E&M lab practical exam

  1. Apr 25, 2005 #1
    There are two things related to using oscilloscopes that I am a bit fuzzy on for a practical lab exam I will take tomorrow morning:

    1) Use the oscilloscope to determine the frequency of the square wave between terminals B and C of the test box. ( We will press and hold down the button on this box to activate it).

    I'm guessing the box has some kind of square wave function generator built in. So, it sounds to me like I wire this up to one channel of the oscilloscope and adjust the controls until I see the square wave on screen. Then, I take one full cycle of the square wave and count the divisions on screen. Then, multiply this by the time/div setting that is on the scope, and then calculate the frequency by 1/(time)? Does that sound correct?

    The next problem:
    2) First, before connecting the function generator to the circuit, set its output to a sine wave that is 8 volts peak to peak (peak to trough). Next construct the circuit shown (it's a circuit with a capacitor, inductor, 1000 ohm resistor, and the function generator all in series). Adjust the frequency of the function generator so that the current in the circuit is a maximum. Use the oscilloscope to measure the circuits maximum current at this frequency.

    My questions for this problem are:
    a) Should the function generator have an explicit voltage scale on it that I will be able to dial up the 8 volts peak to peak voltage? I dont remember the ones we've been using this semester having that capability.
    b) To measure current, do I measure the amplitude and then multiply by the volts/div setting, then use ohms law to get the current? I = V/R I am really not sure about this one.
    c) Just to make sure, I put the leads of the oscilloscope across the resistor, right?

    Thanks very much for any help on this. Sorry to be so long-winded!

    Last edited: Apr 25, 2005
  2. jcsd
  3. Apr 26, 2005 #2
    the voltage from the function generator may be ajusted using oscilloscope. There is one point- the input impedance of the oscilloscope should be set to high impedance (usually 1 MOhm)-but whether do you have this option or not depends on the oscilloscope.
    Other sounds correct.
  4. Apr 26, 2005 #3

    1) That looks right.

    2a) If not....well....won't you have something sitting there that can measure the peak to peak voltage of an AC signal? Hint: It has knobs and a screen! :wink:

    2b) That's right. An oscilloscope always measures the voltage across its inputs (well at least that's almost always true, and it is here). So you have to find a way to get the current from that voltage reading. i = v/R is pretty easy.

    2c) Well, if you put them across the resistor, the oscilloscope will show you the voltage across the resistor. That's what you want, right?

    Go to bed; you'll do fine!
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