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What is the purpose of resistance in a circut?

  1. Jul 19, 2005 #1
    What is the main purpose of a resistance in a circut.
  2. jcsd
  3. Jul 19, 2005 #2
    that depends on what the circuit is supposed to do. I old light switch dimmers the circuit was completed by varying the position in which the wire contacted the resistor. If the contact was placed near the start of the resistor the lights would be relatively bright, as the contact moves farther along the resistor, more of the electricity is dissipated into heat by the resistor to make the light switch dimmer. A toaster is another great example of a resistor. The metal coils inside have a high resistance, thus the electric energy is converted into heat energy via the resistance of the metal alloys. In these two example we make use of resistance in a positive way. A negative effect of resistance is static. Any cable line running into you house, or any speaker wire has internal resistance. Meaning the metal wires connecting devices have resistance (granted its usually quite small). If there is too much resistance in a wire your speakers or TV wont get a signal, thus you will hear hissing out of speakers or see white noise (not the movie) on your television screen.

    In general resistors decrease the electric energy running through a circuit and convert it into thermal energy
  4. Jul 19, 2005 #3
    Consider a Rheostat. This is an instument that allows you to vary resistance by a mechanical means. They are used on new Fuel injected vehicles. The are built into the throttle position sensors. The farther the throttle plate is moved the more resistance is created in the circuit. A current is passed through the rheostat and a voltage drop is measured via the cars computer. The larger the resistance the larger the voltage drop. And thus this varying voltage can be used by a computer along with simple code to determine the cars throttle postion and how much fuel should be added at that time.

    This is just one example. There are tons of uses. Resistance is just a term, lightbulbs would not shine if resistance didnt exist.
  5. Jul 19, 2005 #4


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    Another practical use of resistors is a voltage divider.
  6. Jul 19, 2005 #5
    They are also used as "shunt resistors" in amp meters. I believe thats the name anyways. And they are used in volt meters as well.
  7. Jul 19, 2005 #6

    Claude Bile

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    It depends on the circuit. In an anologue circuit, they are used to either create potential dividers or divert current (i.e. a shunt resistor). In a digital circuit they are used in conjunction with transistors to make various logic gates.

  8. Jul 19, 2005 #7


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    Sometimes the resistor is the purpose of the circuit: ie, in a light.
  9. Jul 20, 2005 #8
    They regulate voltages, divide currents, resistor can be in many machines the main purpose as russ mentioned..

    No circuit can be functional without resistances...
  10. Jul 20, 2005 #9
    There is no single purpose. The fundamental action is to dissipate power.

    Resistors are the only circuit elements which do anything in a DC circuit.

    In an AC circuit rheostats can dim lights, but so can variacs which are not
    resistors. Voltage dividers can also be made out of capacitors which would
    dissipate no power and be more efficient.

    Circuits can be functional without resistors. It's more accurate to say
    that any practical circuit will have to take resistance into account
    because (except for superconductors) every practical circuit element
    exhibits some of it.
    Last edited: Jul 20, 2005
  11. Jul 20, 2005 #10
    I didn't mean by functional that it wouldn't work without it, but i meant there would be no purpose for the circuit without a resistance, practically, most of the the machines, and electrical devices are regarded as resistances..
  12. Jul 20, 2005 #11
    Only if you want heat (or to dump energy out of your circuit) do you
    need a resistance. A motor for example is actually an inductance except
    for the fact that real wires have unavoidable resistance. But if motor
    makers could, they would in many cases prefer to make motors with
    zero resistance in the wires. Same with solenoids. They are modeled
    as resistances becuase they have unavoidable resistance but their
    machine qualities would work with zero resistance.
  13. Jul 20, 2005 #12
    Resistances in the wire, would produce a lot of un-needed heat, that would cause damage to the circuit, but inductance is a resistor current wise...
  14. Jul 20, 2005 #13
    I know what you mean Nomey. What you are trying to say is that an
    inductor can appear to act like a resistance to current. But this is
    more language than math. An inductor opposes a change in current
    while a resistor converts power into heat. The only way an inductor
    can look even a little resistive is if some of its energy is pulled out to
    do other work, other than heat.

    For example a motor with a load will look like a resistor. But it is not
    a resistor because the work is going out the shaft, and may or may
    not turn into heat. It could turn another generator that charges
    a capacitor.

    The general way this is described is the concept of "impedance" which
    can appear to be inductive, capacitive or resistive (or even negatively
    resistive for sources of energy).

    But electrical resistance is still not required for work to be performed.
    (Ideal motor sawing a log for example.)
  15. Jul 20, 2005 #14
    But it seems to me that the only function for a resistor to u is to convert power to heat, which is the most undesirable...
  16. Jul 20, 2005 #15


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    If you look at a typical piece of electronic hardware, you'll find dozens of resistors stuck all over the circuit board. I'd assume that none of these are used to create heat. So what are these resistors used for?
  17. Jul 20, 2005 #16


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    Bottom line is: It controls the flow of electrons.
    For example if you are sending 5Amps of current, then a resistor can be used to make this smaller-2Amps, 1Amp or even 1 milli-Amp as needed by any circuit.
  18. Jul 21, 2005 #17

    That is exactly my question to antiphon..
  19. Jul 21, 2005 #18
    Let me clarify.

    What I mean is that the physical function of a resistor is to make heat.
    In a particular circuit it could have many different engineering functions,
    the most common perhaps (in a DC circuit) being voltage division as was
    pointed out.

    My point was only that resistance itself was not absolutely necessary
    for a circuit to perform useful work. Of course any practical circuit
    will likely have resitors in it.

    I'll use the example I touched on earlier because it has all the elements
    of the idea.

    If you have an ideal (permanent magnet) DC motor running without a load,
    the coils have zero resistance. Yet the motor will draw no current after it
    reaches its operating speed. The back-emf of the coils is equal to the
    voltage applied to the motor so no current flows. It's somewhat like
    two batteries of equal voltage connected ++ and --.

    If you put a load on the motor shaft, there will be a current flowing
    into the motor and a voltage across the motor and will LOOK like a
    resistor. This was what Nomy's point was. What I wanted to clarify
    was that this arrangement was NOT a resistor, but it only looks like one
    electrically at the motor terminals.

    An actual resistor would have turned the electrical energy directly into heat while
    the ideal motor converts the electrical energy into mechanical energy which
    flows out the shaft of the motor. The work may then saw a log
    (heat+work), or it may lift a weight (potential energy) or light a
    lamp (heat).

    Resistors are very practical circuit elements and are often NOT used to
    produce heat. But that is what they fundamentally DO compared to other
    types of basic circuit elements.
  20. Jul 21, 2005 #19
    and i agree on that point, i fully understand inductance has a quite different role, but so it it appears to be...

    But in a practical not ideal motor, there'll be other circuits to regulate the voltage and current flaw, and hence there'll be resistances..But i udnerstand ur point.Thank for clarifying..
  21. Jul 23, 2005 #20
    One simple use of resistor to bring Voltage swing around the circuit for sharing the voltage across.
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