Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How is this magnet too strong?

  1. Mar 26, 2007 #1
    I have a 12 volt car battery hooked up directly to a 6 gauge wire that is wrapped around an iron rod. For some reason the wire can only last around 30 seconds and the wire completely melts and no longer works.

    How is it that a 12 volt battery has enough amps to melt a 6 gauge wire?
  2. jcsd
  3. Mar 26, 2007 #2


    User Avatar

    Staff: Mentor

    Lordy, please be careful doing stuff like that, especially if you don't understand why the wire is melting. When you put such a heavy load on a car battery like that, you could cause all kinds of problems with the battery, including having it explode on you. Acid everywhere, maybe some fire -- you know, dangerous stuff.

    Please consider using a high-current lab power supply instead of a car battery. At least with that, you can eliminate the explosion, acid and fire aspects. Then you are just left with looking in wire tables for the maximum rated current for the particular wire you are using. Use the resistivity numbers for the wire (in Ohms per kilometer or whatever) to calculate the resistance of some length of the wire, and from that you can figure out how many turns of wire to use to match the output impedance of the voltage source. Please be careful when dealing with these kind of powers!
  4. Mar 26, 2007 #3


    User Avatar
    Science Advisor
    Gold Member

    If you want to make an electromagnet you should be using many turns of small gauge enamel-insulated magnet wire. There are lots of resources on the web and in college libraries on how to do this and what the tradeoffs are. There is no substitute in engineering and physics for gaining knowledge, and you need some!
  5. Mar 30, 2007 #4
    What you did was very dangerous!
  6. Mar 30, 2007 #5
    The battery won't last long after such extreme current consumption. You pulled easily more than 100 amps in 30 seconds.

    Power = I^2 * R, assuming the resistance of the wire is low let's say 0.5 Ohms, power used then = 5000 WATTS or Joules/S

    In 30 seconds you unloaded 5000 W * 30 S = 150,000 JOULES of energy.

    That is more than ample to burn alot of things.

    A fully charged car battery can hold 4.3 Mega Joules


  7. Mar 31, 2007 #6


    User Avatar
    Gold Member

    An example of the potential hazard from personal experience:
    When I first kicked the compression ratio of my 440 up to 12.5:1, the stock 350 amp battery wouldn't even turn it over. I had to put a 690 amp tractor battery in the trunk and wire it in parallel with the stock one (using 2# welding cable). Upon the first starting attempt, it fused the field windings in the starter. :surprised
    I had to have a 5hp one custom-made with an extra 25 turns to handle it. :grumpy:
    On another occassion (with a 'normal' battery), a friend accidentally shorted the terminals with a 3/8" open-end wrench. It melted almost completely through before he could knock it loose, and you wouldn't believe the fireworks!
  8. Apr 1, 2007 #7


    User Avatar

    Staff: Mentor

    I don't think any newbies wonder why his hanbdle is DANGER! :biggrin: :biggrin:
  9. Apr 25, 2007 #8
    Wow man, u did some dangerous stuff(not to sound like an angry mother) that could have potentially hurt you. you should get professional help on this from a professor or someone good with that sort of thing. i may not know alot about this stuff but one things for sure....

    when wire starts melting, god knows their's too much power
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook