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Confused about voltage

  1. Jun 25, 2010 #1
    Hi

    I'm confused about voltage and it would seem every student before me has also been confused abotu voltage if you read all the internet forums :)

    From I = V/R if you increase the voltage you increase the current, so you get a greater amount of charge passing a point at a particular time. Does this mean the electrons travel faster or you get a greater quantity of electrons passing the point?

    If voltage is energy per unit charge then a voltage of nV means each coloumb of charge in the circuit has nJ of potential energy (i think). As someone with a tiny bit of physics knowledge, what are the implications of each unit of charge having more energy? How does it manifest? Does this mean you can do more work as there is more current?

    Presumably you need to input energy in the first place to create the voltage?

    What do you mean when you talk about the power of the lamp? Does this mean how much energy the lamp is using or generating? I would have thought the lamp was using energy but I don't understand how a lamp uses energy. Is it the energy required to create the voltage which pushes the electrons around the filament in the lamp?

    thanks
     
  2. jcsd
  3. Jun 25, 2010 #2
    Well, "electrons travel faster" implies "a greater quantity of electrons passing the point".
    Or did you mean more number of electrons contributing to the current.

    Ideally, you would assume that all (free) electrons contribute to the current.
    Drift velocity is proportional to the field, and hence the voltage. So, yes, electrons travel faster if voltage is increased.

    Yes you can get more work out of it as it is more energetic, for example, you can get a bulb to glow brighter or a fan to rotate faster.

    Yes.


    Power is the amount of energy the lamp uses every second (or in unit time)

    Energy is being used to send the electrons through the resistance of the filament.
     
  4. Jun 26, 2010 #3
    You get a greater quantity of electrons passing the point.

    Also, in one form of notation, P=I*E
    So, a 60watt light bulb in a 120volt circuit will consume .5amps of current.

    These types of relationships can be handy. For example, say that you have a 1000watt space heater and you plug it into a 120volt outlet with a 10 amp breaker. Using the formula, you can see that the max wattage the breaker can handle is 1200. Thus, for safety, you would likely not want to plug much else in that circuit, as you only have 200 watts left.
     
  5. Jun 29, 2010 #4
    Voltage is a measure of 'potential difference' that is, the difference between the potential energy of a unit charge at one place and at another.
    It's exactly the same concept as applied to potential energy when you lift a mass above the ground. Or move it from step to step up and down some stairs.

    To push a coulomb of charge up a potential of 1 volt requires 1 joule of work to be done.

    Conversely, if you drop a coulomb down a potential of 1 volt it will do 1 joule of work on the way. Usually the work is done against friction (resistance) and the result appears as heat (4.2 calories per Joule)

    Tip: Avoid the idea of electricity as electrons moving - often it's not electrons - it could be ions in an electrolyte or in a plasma - sometimes even the movement of 'holes' in a semiconductor - stick with the idea of a coulomb as a 'real' object.
     
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