resistance

Discussion in 'General Electronics Chat' started by keekeez, May 15, 2008.

  1. keekeez

    Thread Starter New Member

    May 15, 2008
    1
    0
    if resistance= voltage/ current
    for example:
    resistance= 12/3
    resistance=4
    its always voltage higher than current.... can it be the opposite?
    for example:
    resistance= 3/12
    resistance=1/4
    resistance= 0.25
    :confused:
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, sorry - we can't effect changes in Ohm's Law :)
    E = Voltage
    I = current (Amperes)
    R = resistance (Ohms)
    P = power (Watts)
    I=E/R
    I=P/E
    R=E/I
    E=IR
    E=P/I
    P=EI
    And other variations.
     
  3. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    Resistance ideally is independent of voltage and current (unless we are talking semiconductors).
    Resistance is resistance;) to the current flow. Theoretically it can have any value from zero to infinity, which depends on the material/cross section and length of the element. This resistance will then determine the current that will flow through it (for a constant voltage source) or the voltage drop across it (for a constant current source).
    If value of resistance is less than unity then mathematical value of voltage will be less than mathematical value of current (which is of no importance as both are different quantities. For example we can not say 5 miles are less than 6 seconds.)

    For a resistance having zero value we can have infinite current for a small amount of voltage (this condition almost occurs in a short circuit where the resistance goes extremely low--do not try this at your home; try this at your physics lab under the guidance of your physics teacher )
     
    Last edited: May 15, 2008
  4. mrmount

    Active Member

    Dec 5, 2007
    59
    7
    R=V/I cannot be changed as Sgtwookie says.

    And yeah, current can be higher than the voltage (and R less than unity) . There is no restriction on that. One practical example is a short circuit as recca02 points out.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    If you are asking whether the magnitude of the current can be higher that the magnitude of the voltage, then yes. It takes a resistor of less than one Ohm to accomplish this.

    It is the value of the resistor, and the magnitude of the applied voltage which determines the current. The value of the resistor is determined at the resistor factory.;)
     
  6. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    Another way of looking at this is to realize you are talking about two different quantities so the magnitudes don't actually directly relate to one another. Is a volt bigger than an amp? Is it bigger than a meter? a Newton?
     
  7. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    This is the most important point. Think about it, 12V/3A = 12V/3000mA = 0.012kV/3A.

    The magnitude is irrelevant.

    Dave
     
  8. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    This is also why a FET is capable of being used to provide gain, but doesn't actually have gain per se. Gain is when you take the output/input, but they have to be the same type of quantity. Volts to volts, amps to amps, etc. A FET has transconductance (1/resistance).

    Contrast this to the typical model of a BJT. In that device, beta is defined as the collector current/base current. These are the same quantity type and therefore beta is called the current gain.
     
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