Simple 1.5V Electromagnet vs 9V Electromagnet

Discussion in 'General Electronics Chat' started by technically87, Dec 2, 2015.

  1. technically87

    Thread Starter New Member

    Dec 2, 2015
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    I am currently helping my daughter on her science project for school. We have a really basic design.

    We are using a 5 inch screw wrapped with 20 gauge magnetic wire. We tested how strong the magnetism was with a 1.5 V-AA battery and a 9V battery. Using a multimeter, The 1.5V battery measured ~1.4V and the 9V battery measured ~8.7V.

    What's really confusing is that the 1.5V battery seems to create a slightly stronger magnet than the 9V battery. Does anyone have any clue why this is happening?

    Also, I haven't been connecting the wire to the battery terminals with my bare hands. Should I be using alligator clips for a more secure and identical connection?
     
  2. #12

    Expert

    Nov 30, 2010
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    You have been measuring the amount the battery voltage sags, not anything relevant to magnetism. Magnetism is about amp turns. The AA battery can provide more amps than the 9V battery, and you applied both of them to the same number of turns.

    20 Ga wire is good for an amp. Make lots and lots of turns, then use alligator clips to attach a D cell in a D cell holder. That will snatch your paper clips!

    If you can use up about 150 feet of 20 gauge wire making turns, your coil might not melt if you leave it connected for more than a few seconds. Absurd amount of wire? You're right. That's why these experiments only last a few seconds.
     
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  3. technically87

    Thread Starter New Member

    Dec 2, 2015
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    I'm only using 10 feet. Made about 140 turns. I just finished reading about how a 9V battery will deliver less amperage because of the way its design. It has six 1.5V, very small battery which won't generate as much current as the 1-AA battery.

    Thank you
     
  4. #12

    Expert

    Nov 30, 2010
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    That's why you checked with the pros.;)

    If you want a good strong effect, go big or go home. Start with a bolt that Frankenstein would wear and a 50 foot roll of wire. You still can't just plug it in and leave, but now you're getting the idea.
     
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  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You are thinking these batteries are ideal batteries. In fact, they are just run-of-the-mill batteries and, because of that, you have to think about them as a battery with a resistor between the ideal battery deep inside and the positive terminal. To find out what size that internal resistance is, measure the voltage without a load. Then add a 100 ohm resistor and measure the voltage again.

    Then use the series resistance equation and then solve for the unknown resistor (you have open circuit voltage, voltage from the unknown resistor/100 ohm resistor node to ground and you know that one resistor is 100 ohms). You will see that the internal resistance of the 9V is much higher than the 1.5V battery.
     
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  6. WBahn

    Moderator

    Mar 31, 2012
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    I'm assuming that these measurements were taken on the batteries without the magnet hooked up. Measure them again with the magnet connected and you will probably find that the voltage on the AA battery is actually higher than the voltage on the 9V battery. As others have noted, this is due to the significantly higher internal resistance of the 9V battery.
     
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  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If your voltmeter has a 2 A current range, use that to measure the current in the circuit. Do this by connecting the meter in series with the magnet rather than in parallel with (across) the battery. Start with the 9 V battery and a large number of similar-shaped items, like nails, large paper clips, BB's, or small ball bearings. Swish the magnet around in the pile to grab onto as many as possible, lift them up until none are touching the surface, then remove them from the magnet and count them. Next, do the same thing with the AA battery. You should see a correlation between the current reading and the number of items the magnet can suspend. Next, you can add more AA batteries in parallel, observe that the relationship continues (although it might exceed the safe current range on your meter) and plot the number of batteries vs. the number of objects. Note that most electromagnets have many more turns of much thinner wire for a more efficient conversion of current to magnetic field strength. Your setup will demonstrate the basic principles, but is going to eat batteries. And while the coil might warm up, nothing is going to melt with a few AA batteries as your power source.

    The reason the unit of inductance is called the Henry is because Joseph Henry did exactly this (and much more) in the 1830's.
    https://en.wikipedia.org/wiki/Joseph_Henry

    ak
     
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