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I understand that resisters slow current down. I have not read yet

  1. Mar 25, 2012 #1
    I understand that resisters slow current down. I have not read yet why anyone would want to do that. I'm guessing that if the current gets too strong the wires will heat up. But I have not yet found an equation that relates current to temperature. If anyone knows of such an equation I would like to know. Also, is that the purpose of resisters? People expressly want to slow current down?
     
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  3. Mar 25, 2012 #2

    phinds

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    Re: resisters

    It's not a matter of slowing it down, it's a matter of controlling it.
     
  4. Mar 25, 2012 #3
    Re: resisters

    what's to control? It's just electrons on a conductor. What's the worse that could happen?
     
  5. Mar 25, 2012 #4
    Re: resisters

    A simple example of using resistors:
    You want to measure a voltage with an analogue voltmeter. The meter has a coil. If you connect the terminals that you want the measure its voltage, according to Ohm's law, you have a current in the meter coil. This current generates a magnetic field which applies torque on the needle and deflects it. Now, suppose we want to measure a voltage 100 times larger. This causes a current 100 times larger. Such a current may break the coil and even if not so, the pointer will go to its maximum and we can't read the measured voltage. One method is to control this current ( or voltage) with the help of resistors, and of course we need to consider the current/voltage attenuation factor to calculate the measured voltage from the deflection of the pointer.
     
  6. Mar 25, 2012 #5
    Re: resisters

    شكرا اكتير. انا ما بعرف اذا تحكي عربي

    thanks i appreciate your help.
     
  7. Mar 25, 2012 #6

    phinds

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    Re: resisters

    Do you have the faintest concept of what an electrical circuit is? How electronic devices (radios, computers, etc) work?
     
  8. Mar 25, 2012 #7

    jim hardy

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    Re: resisters

    If you're going to use an analogy of particles, like the popular "Water in pipes",

    there's a couple of beginning rules you must keep in mind. ( I would have said ground rules but that term is misused too.)

    1. The particles drift along VERY slowly.

    2. What moves quickly is the force between them. Like stuffing peas into a pea shooter, the delay between one going in and one coming out is small - but the pea shooter is full, my friend, and the individual peas move slowly. The pea that comes out one end is NOT the same pea you just pushed in the other end.

    3. When we discuss current :: As you probably know current is the number of charges passing a given point per second. Do not think they are moving fast because they are not. They drift by slowly in a wide column, gazillions abreast.

    4. When we discuss voltage :: the particles do not possess kinetic energy akin to temperature, which is motion related.. they possess potential energy which is more akin to pressure.
    A resistor lets them give up that energy as heat and they exit it with lower potential energy.
    An electric motor lets them give up that energy as mechanical work, just as a hydraulic motor lets a fluid lower its pressure not its velocity.

    Here's a guy who's pretty good at explaining the basics for hobbyists and for students contemplating entering the field. He is an interesting character, peruse his hobby pages.

    http://amasci.com/ele-edu.html

    old jim
     
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