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Why do we use alloys for electric heaters?

  1. Jun 16, 2012 #1
    Good Morning,
    My TextBook says that -

    Electric heaters use alloy material because they have higher resistance, which helps in producing heat.

    What I say -

    But more resistance means less heat at constant voltage because
    1. H = I^2 R t and 2. P = V^2 / R[/U]

    (For eq. 1 Current, I, has double importance. Decreasing R would increase I, which in turn on getting squared, would produce significantly more heat)
    OR
    (Current I is more valuable in producing heat than resistance R, see eq. 1)

    So shouldn't we use metals of lesser resistance so as to produce more heat? I think using alloys would be much power efficient than metals, maybe thats why they use alloys.

    Note: Please refer https://www.physicsforums.com/showthread.php?t=5608

    Thanks in advance.
     
  2. jcsd
  3. Jun 16, 2012 #2
    With the alloy and its greater resistivity one needs less of a length of 'heating' wire, less weight, less bulk of enclosure, and less cost no doubt. Which one would you buy - the one with a copper heating element that is perhaps 6 feet square or the one 2 foot square made with a proper heating elementand the same wattage.

    Heat is measured in joules and power in watts which is just the rate of heat per second.
    You are mainly interested in the power output of the heater. If you are interested in the amount of energy usage for a length of time, just multiply the wattage by the amount of seconds the unit is on.

    If you want to change the wattage of the unit, then use less or more of length of the of the alloy wire - in most cases there is no need to change the resistance wiring material if you are the manufacturer .

    A heater hooked up to the power supply company would have with more wattage a lessor resistance than one with a lower power rating. The most wattage is limited by the household wiring and ciruit breaker usuaully 15 to 20 amps. Realize that the wiring has a resistance also, and the breaker is there to protect the wiring from overheating, the insulation from melting and subsequently your house from burning down. If you design your heater to have say a 1 ohm resistance with a supply of 120 volts, there would be 120 amps ( if you had no circuit breaker or fuse ) flowing in the circuit. the wiring and connections might have a 1 ohm resitance so the heat output could be split evenly between the wiring and the heater and you would be heating your walls and not the air whcih you wanted to do in the first place.
     
  4. Jun 16, 2012 #3

    tiny-tim

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    good evening, hasankamal007! :smile:

    P = I2R = V2R/(R + Rbattery)2

    when is that a maximum? :wink:
     
  5. Jun 16, 2012 #4
    This table http://www.allaboutcircuits.com/vol_1/chpt_12/6.html lists the resistivity temperature coefficient of many metals, including copper and Nichrome. When you plug copper wire in, there is a surge of current, the wire gets warm, and the current drops way down. Plug Nichrome in, and the current remains constant while it gets hot.
     
  6. Jun 16, 2012 #5

    haruspex

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    That argues for choosing R = Rbattery, which would mean rather a low resistance and half the energy being wasted cooking the battery.

    As 256bits indicated, the answer is that alloys are chosen not for high resistance but for high resistivity. If you have twice the resistivity you can make the element half the length to get the same resistance and the same power output.
     
  7. Jun 17, 2012 #6
    Thank you all for replying!
    So 256bits, we use alloys for handling money issues?
    But still why do we need resistance? We could use metals and have higher current which on getting squared, Would PRODUCE MORE HEAT. (Joules Law)
    It would be amazing if you could help me out.
    Hey tinytim,
    I actually know nothing about that equation! If you could tell me more.
     
  8. Jun 17, 2012 #7

    tiny-tim

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    yup … efficiency = useful power over total power = R/(R + Rbattery),

    so the higher R is, the less power is wasted :smile:
    P = I2R you already know

    I is the same throughout the circuit

    from Kirchhoff's law, V = IR + IRbattery, so I = V/(R + Rbattery),

    so P = V2R/(R + Rbattery)2 :wink:
     
  9. Jun 17, 2012 #8

    sophiecentaur

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    This is true. We could have a system with very low resistance heaters. But the connecting wires also have resistance and they would get hot and waste power if a lot of current were passed through them. So it is better to have higher resistance heating elements and connecting wires with as low resistance as possible. The choices, in practice, are based on what materials are available.
     
  10. Jun 17, 2012 #9
    Well yes, when designing an item the cost of materials and production has to be evaluated.
     
  11. Jun 17, 2012 #10
    Hey sophiecentaur, thanks a lot.
    That sentence simply clicked. That makes sense. I think that was it.
    Man you all really rock!
    So having resistant heater limits heating to the heater only. Then the wires wouldn't be the heaters. Right, sophiecentaur?
     
  12. Jun 17, 2012 #11

    sophiecentaur

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    Absolutely!
    Well done.
     
  13. Jun 17, 2012 #12

    krd

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    I don't think we could.

    Get hot and "waste power".......An electric heater "wasting power"?




    I think I'm going to have ask you to turn in your badge.
     
  14. Jun 17, 2012 #13

    tiny-tim

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    if it heats the walls that the wires pass through, yes!
     
  15. Jun 17, 2012 #14

    krd

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    An electric heater is just a large resistor. The electric energy "lost" to resistance, is lost in heat. Once electricity is turned into heat, it's very hard to turn it back into electricity.

    Alloy oxide ceramic mixes are used in electric heaters because they've just enough conductivity to conduct/

    The mix in a heaters conductor is as inefficient as possible at passing electricity - it's meant to stop it and turn it into heat.
     
  16. Jun 18, 2012 #15

    sophiecentaur

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    Perhaps you didn't read my post carefully enough.

    We can use heating elements with as low resistance as we choose. In order to make sure that the heat / energy / money goes where we want it to (i.e. the room or the bath water) the connecting wires need a total series resistance that is significantly lower than the resistance of those heaters. This is to avoid Wasting Power all the way from (and including) the generator to the heater. Can you think of a better way of stating the situation?

    It is common for large heating loads to be supplied, directly with high voltage AC, for this very reason - they have 'high resistance' compared with the supply cables for the very reason I mentioned.

    Do you still want my badge?
     
  17. Jun 18, 2012 #16

    krd

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    I can't really think of a better way of explaining it. Conducting electricity to the heating device low resistance is desirable in the conductor. The heating element itself high resistance is desirable, to turn the electricity into heat.

    The kind of wrong headed idea I would have had as a kid would be to make the heater more efficient by reducing the resistance. (My physics school teacher was really bad - he used to get us to cross out sections of our text books that were wrong according to him - the text books were not actually wrong at all. I am still trying to scrub some of his ideas from my brain.)

    Yes. AC is much more efficient means of transporting and using electricity. Although Thomas Edison tried to persevere with DC for a few years. Which was strange since he used to employ Tesla. He probably didn't listen.

    No, you can keep your badge, sheriff.
     
  18. Jun 18, 2012 #17

    sophiecentaur

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    Sigh of relief!

    The point about the AC bit was that AC is transformed down to manageable voltages usually - involving low resistances for a given power. It's sometimes kept at a high value, despite the inconvenience of dangerous voltages, so that the resistances of high loads can be kept relatively high. (That's just an alternative way of describing why they use HV for transmission - it's all about efficiency)

    This is a strange thread - the topic was introduced 'the other way round' to how this is usually discussed. It's a bit of a curve ball.

    Re. teachers: You just have to take the rough with the smooth. Some of the time they have a 'certain mission' of their own and you just have to get over it. They're often not 'as wrong' as you may think and they often don't have the time or energy to de-construct some of your questions correctly. They may say you're not right when it's what they understood of what you said that's not correct. And vice versa: I've had kids say "but you said . . . " when all I did was to agree to something I thought they said and then the rot set in. Try writing down what you mean in a coherent sentence (as on PF) and present it to your teacher. Light the blue touch paper and stand well clear. It may work. Remember - they are greatly outnumbered by all you oiks!!!
     
    Last edited: Jun 18, 2012
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