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Need help !

  1. Jan 28, 2004 #1
    need help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


    I am preparing for an exam and I really need help in understanding these concepts.

    The difference between conventional current and electron flow. Also explain why were these two conventions developed?

    How is direct current different from alternating current,and why is the alternating current system chosen in Canada?

    Thanks in advance :frown:
    Need help fast
  2. jcsd
  3. Jan 28, 2004 #2


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    Re: need help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    I'm assuming that, by conventional current, you mean the abstract idea that protons are positive and electrons are negative, and that this conventional current flows from high (positive charge) to low (negative charge).

    The difference is very simply stated (but don't be discouraged that it didn't occur to you, because it's just one of those things that you have to be told): Electrons flow in the opposite direction of conventional current. Why? Because the electron wasn't well enough understood at the time the convention was established.

    One thing to note: the convention does not affect the physics of circuits, only the way you do calculations. It does come to issue when dealing with magnetism, which is how it was determined that the negative charges are actually the ones flowing (see the hall effect).

    Direct current does not alternate (in polarity). Meaning, the direction that you draw the arrows on a circuit to represent the current do not change for direct current. Alternating current alternates (in polarity). Meaning, the side at which current enters a circuit element and the side at which current leaves a circuit element alternate periodically.

    DC and AC also mean something more general than just referring to current. They are used in the most general sense to characterize any electrical signal, not just current (i.e. DC can mean a steady voltage level whereas AC would mean a voltage that varies periodically/sinusoidally).

    It is chosen everywhere in the world. It is chosen for the sake of power distribution. Loss occurs in transmission lines do to current. So, if you transmit the same amount of power at a lower current but higher voltage, then you don't lose as much power. The most effecient way to make the conversion is using transformers, but transformers only work with AC.
  4. Jan 28, 2004 #3
    I believe DC Currents are much Cheaper than AC

    So far the Q concerned Mostly AC is used coz of shocks

    DC shocks are more harmful than AC
  5. Jan 28, 2004 #4

    Would you care to expand on this please? It is not something I have heard of. :smile:
  6. Jan 29, 2004 #5
    What I remember hearing in High School, is that the higher the frequency of the current, the more the current is confined on the surface of the thing it passes through (eg, you).

    We did this experiment with a high-frequency Tesla Coil, and although it had something of a 10,000V voltage, it was still safe to touch. Something that I wouldn't recommend with a DC. The teacher mentioned something about most classroom experiments with Tesla Coils being banned in most European countries (but not here, yay! ). I believe this came to be after the death of a student in France.
  7. Jan 29, 2004 #6
    Yes exactly what Dimitri said in addition AC moves in sinusodial waves wheras this is not true for a DC current.

    So there are possibility that u will get uneven and less deprived shocks in different cycles not true for DC,
  8. Jan 29, 2004 #7


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    The way I have always thought of it is 60Hz lets go of you every 1/120 sec (every zero crossing). DC NEVER lets go.
  9. Jan 29, 2004 #8
    Now that makes sense!

    But what about 'cheaper'? Surely all big generators produce AC anyway?
  10. Jan 29, 2004 #9
    Ya AC lets u go off

    Due to the fact that it crosses zero coz of its sinusodial nature and the nervous system can respond to it whereas it is impossible in case of DC.
  11. Jan 29, 2004 #10
    Right! In a DC current you always get the maximum intensity, but due to the periodic nature of the AC you feel the maximum divide by SQRT(2), don't you?
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