Notices
Results 1 to 3 of 3

Thread: Appliances-Ohm's Law

  1. #1 Appliances-Ohm's Law 
    Forum Ph.D. Heinsbergrelatz's Avatar
    Join Date
    Aug 2009
    Location
    Singapore
    Posts
    994
    (this is not homework, just going over through some questions from past GCSE exam papers, cause my exams are in November, so revising purposes)
    -Why is it important that the wires connecting the heater to the mains supply have a low resistance?
    -is it because if there is high resistance, then more voltage, or current is required to pass through the resisting force, so thus creating overheat, or melting of the wires?
    -can anyone explain Ohm's law getting applied in this kind of situation?
    -does this have something to do with the temperature coefficient? -the amount of 'x' change in resistance can be calculated using the temperature coefficient of resistivity of the material?
    i appreciate your patience and time, and would thank all appropriate suggestions given


    Reply With Quote  
     

  2.  
     

  3. #2  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,822
    No, it doesn't have to do with the temperature coefficient. It is just the simple application of Ohm's law, V=IR and its variations, P=(I^2)R and P=(V^2)/R.
    It would be best to work out some examples for yourself, so you can see what is happening.

    If your heater is rated for 1000 watts at 220 volts then its resistance at operating temperature is (V^2)/P=48.4 ohms and the current it draws is V/R=4.55 amps.

    Now what happens if your extension cord has a resistance of 1 ohm? Then the series resistance is 49.4 ohms, and the current is 220/49.4=4.45 amps. The total power dissipated in the heater plus extension cord is V^2/R = 220^2/49.4=979.75 watts, with the power dissipated in the heater being I^R=(4.45^2)*48.4=958.4 watts.

    So what has happened is that less power is being used, and some of the power is going into heating up the extension cord.

    Try it with a resistance of 2 ohms in the wire. As you increase the resistance of the cord, more power goes into the cord, less into the heater, and less total power is used.

    The other consideration is, why is the resistance of the wires going up? It could be because the wire is too thin. In this case the current rating of the wire could be exceeded and it would overheat and start a fire. Or, it could be because the wire is too long. In this case it would not create a fire hazard but would still cause voltage drop at the heater, such that you would not get the rated output of the heater.


    Reply With Quote  
     

  4. #3  
    Forum Ph.D. Heinsbergrelatz's Avatar
    Join Date
    Aug 2009
    Location
    Singapore
    Posts
    994
    thank you for the information, i understand this concept now
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •