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Voltage, current, confused

  1. Aug 19, 2016 #1
    Hi,
    I'm learning the basics of EE, and I never fully understood voltage and current.
    So, in a very simple circuit, if we increase voltage, the rate of elétrons will increase due to the increase of the force that "moves them" right? Then in the end, voltage's only purpous is to manipulate currents right?
    ( I don't know if my concepts are good, so please correct me in any way you like )
    So why do eletronic devices have labled a maximum voltage? I can supply 1000v to muito mobile phone with a 4000ohm resistance and I'll get 0,25A of supply, and that won't hurt it right? Cuz in the end the amount of eletrons passing through are the only thing that matters
    I don't know, probably I'm wrong but please help me clarify
     
  2. jcsd
  3. Aug 19, 2016 #2

    Drakkith

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    No, you'll utterly destroy the phone, because the electronics are not designed to take a 1,000 volt input.

    Nope, the voltage matters too. Consider an extreme case of a lightning strike. The voltage between the cloud and the ground just before the strike runs around 3 million volts per meter, so high that it ionizes the air itself. This ionized air has a MUCH lower resistance than non-ionized air, which enables the current from the cloud to the ground to increase from near-zero amps to an average of about 30,000 amps.

    Similarly, applying 1,000 volts to your phone will almost certainly break down the resistive elements within the phone itself, creating shorts that increases the current well beyond 0.25 amps.

    See here: https://en.wikipedia.org/wiki/Dielectric_strength

    In addition to that, the electronics themselves cannot take anywhere close to 1,000 volts of input. They simply can't function with that kind of voltage because of the way they work. You'd need specially designed electronics to function with 1,000 volts input, and I believe those would be slower, bulkier, and more expensive.
     
  4. Aug 19, 2016 #3

    cnh1995

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    That voltage is the voltage which "directly" appears across the device.
     
  5. Aug 20, 2016 #4
    I don't know where did you get the values from. You mean if your device works with 5volts and 25mA and you decide to power it with 1kvolts and you use a resistor to "gather" the rest 995volts so you can have the usual 5volt 25mAmper supply to the phone? The electronic elements inside work with verry small currents-mAmps. When you first connect it to that 1kvolt power supply(the verry first second), there wont be current in the resistor to guarantee you these 5volts. The device will directly feel the 1000volts and will pass such a current that the phone will blow up.
     
  6. Aug 20, 2016 #5

    anorlunda

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    @Drakkith already told you that voltage matters too.

    I think that elementary DC circuits education could be improved. I would like to see it become standard to not only solve for voltage and current V=IR in every example, but to follow-up by calculating P=VI for every component in every example.

    The flow of power and the dissipation of energy should be the primary lesson to learn from each example, Ohm's law and the actual voltages and currents are secondary.

    Perhaps some PF members know of curricula developed along those lines.
     
  7. Aug 20, 2016 #6

    Drakkith

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    I did that in my Basic Electronic Circuit classes in the Air Force, but not in the basic DC circuits section in my initial undergrad E&M class this past spring. Very odd. Perhaps we didn't do it in my E&M class because it wasn't dedicated to circuits.
     
  8. Aug 20, 2016 #7
    It's not only the amount of electrons that matters it's also the speed that they are traveling at. The current is defined as the amount of charge crossing a point in the circuit for a given time: I=Q/t. The more charges there are and the faster they travel, the greater is the current. As you said the voltage is what drivens these charges. The potential difference between the battery terminals causes a current to flow until both terminals are at the same electrical potential - 0volts. When that happens there is no more potential difference and therefore no reason for the charges to flow. Now about the Resistance. Every conductor have some amount of free to move electrons and other electrons that are tightly bound to the atoms. The less the free electrons are, the less charges are there to support the current and it will be less. Thereby more resistance. When electrons flow through a conductor, their movement is not smooth. They bump into the tightly bound electrons and this impedes their movement, slowing them down. Again, the more free electrons and the less tightly bound electrons a material have, the less resistance It will posses. When flowing electrons collide into the tightly bound electrons, that will cause these tightly bound electrons and their atoms to viberate. This friction is what we call heat. Too much friction can supply the electrons enough kinetic energy to escape their atoms and this can destroy the bonding between the individual atoms. As the electrons move through this material faster and faster (the greater the current) this friction will increase and if it's too great, the conductor will burn. Because the movement of charges in resistors is slowed down, electrons loose energy and you must supply more voltage to get the same current as before. This we call Voltage drop - the loss of electrical energy. In resistors electrons lose electrical energy, giving it into heat, as it was said above, and you must supply more electrical energy to get the same current. And as you do so, you will lose more power to this resistor and it will heat up. Electrical power is simply the rate of doing work. Or more clearly said - Power is the amount of energy consumed per unit of time P=VQ/t=VxI.
    I hope this will help you clear out your confusion about Voltages and currents and such.
     
  9. Aug 20, 2016 #8

    NascentOxygen

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    You can do this sort of thing only to devices that act like pure resistances and obeying Ohm's Law. In essence, nothing in the real world behaves like an ideal resistance, not over an infinite range of voltages. A mobile phone's electrical usage looks nothing like a resistance, even a tungsten light bulb isn't a pure, fixed resistance.
     
  10. Aug 20, 2016 #9

    Drakkith

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    Perhaps counterintuitively, trying to think about current in terms of the speed of the charges is not helpful when working with circuits. More current does not mean that the charges are moving at a higher speed, nor do charges necessarily move at different speeds within conductors, semiconductors, and insulators.

    This is not a useful way of thinking about electrical resistance. For example, is the speed of the charges in a resistor lower than in the conductor just before and after the resistor? (keeping in mind that the current is the same through both)
     
  11. Aug 20, 2016 #10
    Thank you all for your answers! I have a better concept of voltage and current thanks to you all
     
  12. Aug 21, 2016 #11
    Well, thats what I am learned and it's fine with me. No, the speed of charges everywhere in the circuit is the same. The resistor will cause the electron drift velocity to decrease, as a result some electrons will build in in the resistor and through electric field they will impede the electrons behind them.
     
  13. Aug 21, 2016 #12

    Drakkith

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    You realize drift velocity is not the speed the charges are moving, right?
     
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