Basic theory

Discussion in 'Physics' started by energyhead, Jun 24, 2011.

  1. energyhead

    Thread Starter New Member

    Jun 23, 2011
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    Hi Im new and am just wanting to learn the basics so I can answer some specific questions of my own. I would appreciate anyones help. How do you create current or amperage from high voltage. Is it created by the resistance of wire/components ect.(opamps?) In therory a megavolt could be stored in a giant capacitor and have no amperage until the electrons flowed out into a circuit creating resistance right? If its true then why do car batteries have amp ratings or people say that a particular battery has 500 amps. Am I right in saying the battery doesnt store amperage just voltage? Unless the resistive design of the battery creates amperage or current before making it available at the terminals. Thanks anyone
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    The heart of your questions revolve around Ohm's Law. This one equation dictates the relationship between voltage, current, and resistance. To use it correctly you need some algebra, but not very much.

    V = I * R
    I = V / R
    R = V / I

    As you can see, they interact nicely.

    A car battery can surge 500A, but it isn't good for the battery. Amp/Hours is a signifigcant number too, as it is a measurement how long a battery may provide power.

    Which brings us to a second equation, which is almost as important. That is for power, which is expressed in wattage. It usually shows up as heat.

    W = V * I
    W = V² / R
     
  3. beenthere

    Retired Moderator

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  4. energyhead

    Thread Starter New Member

    Jun 23, 2011
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    Thankyou so much for the help. Ive got some reading to do. Cant resist another quick question though. Do those equations pertain to all electricity or are there different kinds of electricity with different physical laws and there repective equations for example "static electricity"?
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
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    All the electrical phenomena you see have to do with charge carriers (e.g., electrons and charged atoms), their physical separation in space, and their movement.

    Since this is the physics sub-forum, let's review terminology. "Ohm's Law" is actually a specific empirical relationship that Ohm found held true over limited ranges of voltage and current -- and the relationship is that some metallic conductors have a linear relationship between the voltage across them and the current through them. But it's approximate. If you don't believe this, just keep increasing the current until you do. :p

    What the EEs usually refer to as "Ohm's Law" is technically the definition of resistance and it comes from a more general form that relates the electric field vector, the current density vector, and the resistivity of a material, which can be a tensor. You also need to realize it's not a fundamental law of electromagnetism, primarily because it's an ensemble macroscopic average and it's usually only concerned with electrical conduction in matter.

    That said, it's still one of the most useful simple equations I know.
     
  6. WellGrounded

    Member

    Jun 19, 2011
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    energyhead,

    The Ohms Law formulas pertain to what is called a "pure resistive" load and they will work for both AC and DC voltages. Resistors tend to be static devices in the sense that when you first apply a voltage to them and just before you remove the voltage the current traveling through the circuit is almost the same.

    When you add other components such as capacitors or inductors the formulas will change based on the circuit that you have. That requires a little bit more knowledge and understanding. At this point just try to master the basics and the later knowledge will come easier.

    Electronics is about 75% knowing math formulas and 25% insight into the circuitry itself, especially after time when viewing variations of circuits you can gain an instant partial grasp as to what the circuit is trying to do.

    Good luck in the future.

    Danny
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Having personally worked with a very competent and creative engineer with zero formal training (he learned the basics repairing guitar amplifiers!) I would have to reverse those proportions.

    Geese, you had to watch this guy stabilize a switch mode power supply (his design) with no idea what "j" was. I also watched him create a new inverter topology that did not inject a ton of noise into the chassis.
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I would agree but the circuit insight percentage would be about 90% with an electrical savant. You can call it a 'dream state' or a vision but having the ability is like having a song in your head that you can modify until it just sounds right. When you build it I would guess it's like playing a musical instrument without formal training. But even an electrical savant like Tesla can make fundamental mistakes without formal training at some point. Too bad it's a rare talent, most people have to design advanced electronics with formal methods instead of music.
     
  9. energyhead

    Thread Starter New Member

    Jun 23, 2011
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    Thankyou for your responses. I will have to really think about these concepts. Some are easier to understand than others but they are intriguing. Im hooked. Speaking of intriguiging I saw a guy on youtube demonstrating his van de graff generator ...lots of voltage from a low amount of rpms. Is that voltage that could be made to power dc appliances? I might want to build one for fun maybe make a wind generator but I would need to know how to regulate the power so that it was constantly drawing the same maybe the generator could feed a battery or capacitor array. I dont want to invert it to ac initially.
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    That is what we call STATIC electricity.

    high voltage yes. not many amps though. In fact it will probably not be able to do more than 'shock' you. Not enough current flows to drive motors or power supplies. This is due to how the electricity is generated. Static generators work by pulling electrons off of solid materials. After the solid has lost some electrons and become depleted of free electrons, it is almost impossible to get more from the material. This is due to the nature of atomic structure and the large positive charge that develops which 'holds on' to the remaining electrons. Very few of the electrons in an atom are easy to strip away.

    Standard magnetic based generators create current 'flow'. THere is a constant supply of free electrons coming into the circuit to replace the ones the generator sends out.
     
  11. Wendy

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  12. energyhead

    Thread Starter New Member

    Jun 23, 2011
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    Thanks..where do the free electrons come from?
     
  13. energyhead

    Thread Starter New Member

    Jun 23, 2011
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    Sorry wrong thread.
     
  14. energyhead

    Thread Starter New Member

    Jun 23, 2011
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    Thanks.. where do the free electrons come from?
     
  15. beenthere

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    Apr 20, 2004
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