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How to build a stonger Electro-Magnet

  1. Jan 18, 2007 #1
    A friend of mine and I have been working on a science fair project but have run into a small problem. The Electro-Magnet that we have built is melting the wire that we have and we cannot buy any stronger wire because we do not have a place where we could buy any stonger wire. We were wondering if any one had an idea about how we could get about how we could get the magnet under control or if we should use a small battery. If we were to use a weaker battery tho we need to have the magnetc field to have at least the same strength. Thanks in advance to everyone that posts a responce.
  2. jcsd
  3. Jan 18, 2007 #2
    I personally used a power supply with my electromagnet with resistors. Batteries will often die to easily. As for increasing the strength of your electromagnet, just either add more current (amps) or add more turns of your wire. Make sure you place an iron solenoid. A good way to greatly increase the strength of your magnet is to get a horseshoe shaped solenoid.

    Hope this helps
  4. Jan 18, 2007 #3


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    DHS Science, what are you using as the core? You should use a material that is ferromagnetic, for example an iron core. The existing magnetic field (caused by the current in the solenoid) will align the magnetic domains of the iron core and thus the core will act like magnet increasing the strength of your overall magnetic field. Adding such a core could increase your magnetic field strength by hundreds or even thousands.
    Last edited: Jan 18, 2007
  5. Jan 18, 2007 #4
  6. Jan 19, 2007 #5
    http://en.wikipedia.org/wiki/Mumetal <--- use Mu-metal as the core instead of iron? wouldn't that increase B field?
  7. Jan 19, 2007 #6
    We are using a steel rod as a core and a 12 volt battery as our power supply at the moment but we would galdly take any suggestions on what we should use instead. If you could please try and explain what a "ferromagnet" is?

    Thank for your help.
  8. Jan 19, 2007 #7
    you mean ferromagnetism? simple. All it is is an object that can let magnetic fields pass through it easily.

    If you really want to know how strong your electromagnet is use this page to calculate the strength of an electromagnet. I use it sometimes. If you don't know how to calculate the strength of an electromagnet from this page then give me the specs of your magnet.
    amount of amps used
    number of wire turns
    length of electromagnet
    and so on....
  9. Jan 19, 2007 #8
    I will get those specs to you within the next couple of days because I will not be able to get into the school. But I have another question for you if that would be alright. I was wondering if what exactly you meant when you said that you used a "power supply." Did you mean that you used a power supply from a computer or not. Sorry about all of the questions.
  10. Jan 19, 2007 #9


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    No, Mumetal saturates at lower applied fields ("H") than iron, and it is hideously expensive. Stick to plain iron for your magnet core, mild steel (i.e., not alloyed or heat treated, the type used in framing nails) is fine too.
  11. Jan 19, 2007 #10


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    Your school might have a variable power supply that they could loan you. You could then adjust the voltage down from 12V to where your coil doesn't overheat (this would reduce the field proportionally, however). This web page shows some typical power supplies so you know what they look like:
    You'll need a supply that can source enough current and voltage, which means computer supplies won't cut it. One of your science classrooms might have a "multimeter" that you can use to measure the resistance R of your coil. You can then figure out the current I using Ohm's law
    I = V / R
    where V is the battery or supply voltage.
  12. Jan 19, 2007 #11
    What i ment was i used a power supply(cord) to an old laptop i had. It was rated around 19 volts and about 3 amps. I perfer this over batteries because short circuiting a battery will easily drain it. With a power supply you have constant power, which means your electromagnet never loses energy bacause the battery is weakening. You can probably get power cords where ever they sell computer supplies. Make sure you try stay low with voltage though.

    I want to make myself clear MAKE SURE YOU BUY RESISTORS LIKE THESE. With out resistors you will short out the power supply which is not good. Radioshack has a good section for resistors. I recommend you stick with the 100 or 50 ohm resistor. In practice the resistors will get hot, this is normal. This depends on how much current and amps you put through. For the resistors on that page they can at max take 10 watts of power, any more or they will overheat. For example say that i am using the 50 ohm resistor and with 19volts. So to get current i use
    V= voltage
    I= current(amps)
    R= resistance
    So for my example i put in 19volts=I*50ohms. Doing algebra I get .38 amps. After that i want the equation for watts. Which is
    V= voltage
    I= current(amps)
    W= watts
    So 19volts*.38amps=7.22watts. In this example the resistor will get hot but it should be fine since the max for these resistors is 10 watts.

    hope this helps:biggrin:
  13. Jan 22, 2007 #12
    Will that resistor be able to run for 24 hours straight tho or will that be to long for it?

    Thanks for all of the help and sorry again about all of the questions.
  14. Jan 22, 2007 #13


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    Yes it can. But just make sure that you purchase a resistor that meets the power requirements of your circuit. Use the formula that hover suggested to find power (voltage times current). For example, if you get a total power of 5W and you have resistor with a higher power rating, you should be good.
  15. Jan 22, 2007 #14
    Thats the point of physic forums, to ask questions:rolleyes: . Without questions this site would be useless.
  16. Jan 23, 2007 #15
    The only problem with using a resistor is that we cannot find one powerful enough for the application that we are using the magnet for. I have thought about using a breaker like what is in an electrical box but I am not for sure how much power the magnet will be pulling from the wall and I cannot afford to buy more supplies if the magnet does not work the first time.
  17. Jan 23, 2007 #16


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    Wow! How much power would you want to dissipate across the resistor? Mayb you could use 2 resistors with each one having a resistance of [itex]\frac {R_{TOTAL}}{2}[/itex], in that way each resistor would have half the total voltage across it and would dissipate half the power as one resistor. You would now have the same resistance that you are aiming for and the same current.

    Mayb you could provide use details has to what resistor value you plan on using and the voltage of the power supply. I'd advice not to take power directly from the wall outlet. Use a [step down] transformer or a power supply. Using a wall outlet directly will call for more power dissipation within the circuit
    Last edited: Jan 23, 2007
  18. Jan 23, 2007 #17
    This is certainly true. If you have 2 of the same resistors parralell in a circuit, it would decrease the amount of resistance by 2. Or if you really wanted you could add 3 or more in paralell, but remember that the amount of ohms of 1 resistor gets divided by how many resistors you have in parrallel. Example-

    100 ohms/ 2 resistors in parallel = 50 ohms
    100 ohms/4 resistors in parallel = 25 ohms

    Keep in mind thought that the number of ohms change when finding how many watts you are putting in a resistor. Example-

    50 ohms/ 3 resistors in parellel = 16.66 ohms..... so if you used 19 volts/16.66 ohms, you will get 1.14amps...so 19 volts * 1.14 amps= 21.66 watts. For the resistors i listed before this is double the amount of watts they are supose to take and will greatly overheat!!!
  19. Jan 24, 2007 #18
    The resistor that I am using is a 1-megaohm resistor that is 1/2 watt and has 5% tolerance. I have 5 of them in all, so would it be better to use like 2 or more of them or should I try and find one that can handle more watts and less ohms.

    So if i were to use more than one of the resistors will that take the number of ohms down or will that just take the number of ohms and seperate them evenly between the two resistors.

    The school that I go to does not have a power supply or a step down transformer.

    Also the power coming out of the wall has 15 amps so if that makes any difference.
    Last edited: Jan 24, 2007
  20. Jan 24, 2007 #19


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    How do you know how much current is coming out? The current you draw would depending on the resistance of the circuit. And 15A is alot! Unil you have decided what resistances you will have, only then can we know the power dissipation needed. These are values you should have before you purchase your components.

    So since you are using the wall outlet, you already know your voltage. I'm assuming that you have a numerical quantity of the magnetic field which you wish to achieve?

    It depends on how you connect the resistors, in series, the resistances would add. In parallel they would decrease (hover's example). Since in both of these cases, either the voltage drop or current of the resistor changes, the power dissipation could be "spread out" across the resistors.

    If you use 2 of your 1 mega ohm resistors is series, you would have a total resistance of 2 mega ohms.
    Knowing that the wall voltage is 120V, we get the total current in the circuit:
    I = 120/2000000 = 60 micro amps

    Each resistor would have 60V across it:
    P = IV
    P = 60 micro amps * 60 = 3.6 milli watts

    Your resistors will handle the power dissipation, but your current is small!
    Last edited: Jan 24, 2007
  21. Jan 24, 2007 #20
    I have to agree with ranger. Even though those 1-megohm resistors are well able to handle the outlet, they barely give you any current and current is "the thing" that makes an electromagnet an electromagnet. You shouldn't use an outlet anyway because the voltage is so high. Even if you got lower ohm resistors it would be a pain finding something that could handle the watts. Search around for a power cord for a laptop or something like i did. Something else to look out for are resistors that have a high wattage rating.
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