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Homework Help: Multiple Circuit Questions

  1. Apr 20, 2008 #1


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    While doing revision, I've come across aspects on circuits which I cannot get my head around. May I have some help with these questions and ideas.

    1. What's a variable voltage and how do we draw it on a circuit? Does it have a different symbol to a voltmeter?
    2. Why does touching the leads together allow the voltmeter to be zeroed?
      Can I say that it discharges it. In the mark scheme, it says that it is short-circuited, is that the same as being discharged?
    3. When using ohmmeter, the resistance of a resistor should not be measured while it is in a circuit. Is it that this will cause the circuit to short-circuit because there are too many component to the circuit?

    I'm getting confused. Please guide me in the right direction and clear my confusion. Thanks in advance. :smile:
  2. jcsd
  3. Apr 20, 2008 #2
    When you say "variable voltage", perhaps you mean a time-varying voltage source, which is an active component that makes it become true that the voltage between its two terminals will be a certain function of time. The electrical outlet on your wall is an example of one. The symbol I'm a customed to is the plus and minus signs in a circle, with some expression written next to it to indicate the time-varying function. Or perhaps by the words "variable voltage" you mean the voltage across any two points in a circuit, not measured across a voltage source. You could apply a volt meter across those two points if you wanted to, but it wouldn't be meaningful to say that the voltage "is" a volt meter. A volt meter shows a measurement.

    A DC voltmeter placed across a time-varying voltage would measure an effective constant value. That may not be what you wanted.

    If you remove a voltmeter from the circuit and then short its terminals together, the short will guarantee that the voltage across the terminals is zero. If the meter displays a number other than zero, you can turn the screw to make it display zero.

    You have "calibrated" the meter, but I won't say that you "discharged", it because I'm not aware that a voltmeter stores a significant amount of charge that needs to be discharged.

    If you measure a resistor with an ohmmeter, the meter does it by measuring the voltage to current ratio of whatever is between those two terminals, in this case, just one resistor. If you tried to do it while the resistor is part of a circuit, the meter would read some voltage to current ratio of the entire network as seen at those two terminals.
    Last edited: Apr 20, 2008
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