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Thread: Resistance, Heat and Power

  1. #1 Resistance, Heat and Power 
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    Alright, you connect a conductor to a 240V supply. Assuming that the resistance of the copper wire is 0ohms, will a conductor with lower resistance produce more heat than a conductor with higher resistance?

    I am baffled because of this.
    P= V^2/R
    so it seems that if the copper wire has no resistance, the conductor with the lower resistance will produce more heat.

    However, it is well known that heating elements are supposed to have high resistance.. enlighten me please.


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  3. #2  
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    low resistance will cause higher currents and hence get hotter using V=IxR


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  4. #3  
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    thats what i thought.. but then why all heating elements have high resistance like nichrome?
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  5. #4  
    Forum Professor captaincaveman's Avatar
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    Quote Originally Posted by Liam90
    thats what i thought.. but then why all heating elements have high resistance like nichrome?

    not sure how highs high? you cant have the resistance too low because of current consumption. if you had no resisitance (as in a short) on mains the thing would just burn out

    so theres gotta be a comprimise between resistance and current draw to get it right
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  6. #5  
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    OK guys, here's the story,

    IF you have 0 ohms (almost impossible except superconductors) then the load will attempt to draw infinite current, usually a fuse will blow, if there are no fuses then the highest resistance part of the circuit will usually fail.

    You could burn out the wires, or stall the generator (unlikely unless it'a a prtable one). IF for example you short circuit a battery, the entire voltage will be lost through it's internal resistance (usually very low)

    There is a little thing called 'Maximum Power transfer theorum' - remember all sources of electrical have 'built in' resistance that's why when you overload a battery (say keep adding bulbs) they begin to grow dim, a flashlight battery has an IR (Internal Restance) of about 1 ohm.

    Your electric ketle will have a resistance of about 60 ohms when hot but will be different when measured cold. Please don't place low resistance conductors across any supply, eg if you drop a spanner across the terminal of a new fully charged auto battery, it will will explode and cover you in Sulphuric acid - which will melt you skin in about 5 seconds and leave you severly disfigured for life.
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  7. #6  
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    Quote Originally Posted by Megabrain
    OK guys, here's the story,

    IF you have 0 ohms (almost impossible except superconductors) then the load will attempt to draw infinite current, usually a fuse will blow, if there are no fuses then the highest resistance part of the circuit will usually fail.

    You could burn out the wires, or stall the generator (unlikely unless it'a a prtable one). IF for example you short circuit a battery, the entire voltage will be lost through it's internal resistance (usually very low)

    There is a little thing called 'Maximum Power transfer theorum' - remember all sources of electrical have 'built in' resistance that's why when you overload a battery (say keep adding bulbs) they begin to grow dim, a flashlight battery has an IR (Internal Restance) of about 1 ohm.

    Your electric ketle will have a resistance of about 60 ohms when hot but will be different when measured cold. Please don't place low resistance conductors across any supply, eg if you drop a spanner across the terminal of a new fully charged auto battery, it will will explode and cover you in Sulphuric acid - which will melt you skin in about 5 seconds and leave you severly disfigured for life.

    was i right then? in a simplistic way :wink:
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  8. #7  
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    It's actually a very complicated subject, even ohm's law (not part of the rent act as some think), v=ir is a simplification the actual law if I remember correctly goes something like this..

    'In a circuit where all physical and environmental conditions are constant the the current in a fixed resistance is directly proportional to the voltage applied'

    It all depends on how much current the source can provide, in a 1.5 volt battery if you put 1ohm across it, the voltage you measure will be around 0.75 volts (cos there's about an ohm inside) in the auto battery the internal resistance is about a few milliohms so it can provide 100amps or so for ten minutes.

    Interesting fact...

    When starting your car in the cold do not keep turning the key on/off - as the start current of a starter motor is several hundred amps, the running current is much less so turn the key and let it run - the more you stop it the faster you drain the bttery! - when it does slow down do not try it again until you have re-charged it, if it does not start within about 20 revs there is something wrong. In fact 20 is generous, 4 revs is good if you look after the motor.

    It's the amount of power dissipated that is the heat, it could be a low current and high voltage or vice-versa, eg a 15watt (240 volt) lamp gets hot but it's running current is low 0.0625 Amperes - a 15 watt 12volt lamp gets as hot but passes 1.25 amps.

    Power (watts) = voltage * current.
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