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Homework Help: When a source gives out voltage

  1. Sep 2, 2003 #1
    When a source gives out voltage, that voltage is dropped when the electricity goes through a load right? So what happens when there is no load? Where is the drop in potential voltage?

    Also, can someone please explain polarity to me?
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. Sep 3, 2003 #2
    The drop happens when the electrons is trying to pass from the positive terminal to the negative terminal. Since like charges repel, therefore the electrons loses energy while overcoming the repulsion force produced on it by the negative terminal
  4. Sep 3, 2003 #3
    Re: Electricity

    You might think in this case the resistance is zero, and the current in infinite. But every source has its own internal resistance, which is what limits the current. After all, you have charged particles moving thru a medium, even in a source...
  5. Sep 4, 2003 #4
    Thank you guys for replying. So if there is no load there is some resistance in the wire, then why does the copper wire always heats up?
  6. Sep 4, 2003 #5
    What is resistance?
  7. Sep 5, 2003 #6
    Resistance R = ( p * A ) / l
    where p = resistivity of the material. It is a measure of the resistance the material offers to the flow of charge through it.
    Last edited: Sep 5, 2003
  8. Sep 5, 2003 #7
    Yes, of course.

    It was meant to be a leading question: from the concept of resistance to the cause of heat; thence to the answer to the question as to why copper wire heats up.
  9. Sep 6, 2003 #8
    I believe that if you connect a short wire directly from one terminal of a battery to the other, it will run out of power in a short time. (I wouldn't try it without knowing what you're doing. You could easily shock yourself, and, I don't really know, but the battery could do some unexpected things.)
  10. Sep 7, 2003 #9
    A voltaic cell is more complex than a wire. Chemical cells produce electric current by virtue of a spontaneous redox reaction. Many if not most spontaneous reactions are exothermic.

    Aside from that, electrochemical cells and conductors both offer resistance to current.; the more resistance the more heat that is produced. The question that is still unanswered is the connection between resistance and heat.

    One is tempted think about “friction” between moving charged particles and their surroundings and everyone knows that friction causes heating (although come to think of it I’m not sure why). Of course the portrayal of current-carrying particles “rubbing against” particles of a relatively more stationary medium is a gross oversimplification.

    The key to answering the question as to why current-carrying wire heats up requires a more sophisticated understanding of how current flows.

    It’s an excellent question and one worth spending some time on.
  11. Sep 7, 2003 #10
    Molecular Theory explains heat
    An explanation of what heat is comes from the Molecular Theory of Matter, which states that matter is made up of tiny particles called molecules. These particles are in constant motion within the bounds of the material.

    Velocity of molecules
    Since the relationship between kinetic energy of an object and its velocity is:

    KE = ½ mv2,

    which means that the more energy an object has, the faster it is traveling (or vice versa).

    Thus, when you provide extra energy to an object, you cause its molecules to speed up. Those molecules, in turn, can cause other molecules to speed up. The sum effect of the speed or energy of these molecules is the phenomenon we call heat.

    Sources of heat energy
    Molecules can go into high-energy motion, causing heat, from various energy sources.

    Light from the sun radiating on an object can transfer energy to the object's molecules, causing them to move faster. In other words, the object heats up.
    Chemical reactions, such as something burning causes nearby molecules to accelerate and thus produces the effect of heat.
    Electrical resistance can heat a wire, causing its molecules to move faster.
    Friction and nuclear reactions can also produce heat.
  12. Sep 7, 2003 #11
    So far we have:
    1. Resistance is like “electrical friction.” Electrons travel through a wire bouncing off the particles of the wire like a golf ball ricocheting through a field of boulders.
    2. Electrons flowing through a wire somehow cause the particles of a wire to move faster. It seems as though electrons “rub against” and “collide with” the particles of the wire to make them jiggle.
    3. The faster movement of the particles of the wire is what we experience as heat.

    A couple of questions come to mind about the nature of electric current:
    1. In a simple DC circuit, does an electron actually enter one end of the wire, travel through the wire, and leave the other end?
    2. Do electrons actually “rub against” or “bounce off” the particles of the wire?
  13. Sep 7, 2003 #12
    The electron gets absorbed by the material because it is unable to pass through the material like a bottle neck and it can't go back where it came from because the charge won't let it. as it gets absorbed the net energy of the material increases and as the net energy increases the the molecules move faster as the molecules move faster they let off infared radiation which is heat! The electrons don't rub eachother the molecules do. The electron is gone but its residule energy is present. The electron gets destroyed as it's forced into the material and is transformed into infered radiation through the medium of the material Metal or other.