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Questions about batteries and magnets and EM induction

  1. Dec 7, 2016 #1
    I wanted to ask a few questions regarding magnets and batteries, and EM induction.
    First off, could I use batteries to replace a low voltage dc power supply, like say 2 V or so by connecting the ends of the battery in series to a circuit? As an extension, could I connect two batteries in series to obtain double the voltage? Is it ok to use two batteries in series with different voltage ratings, like say, an AAA and an AA?

    If I could, what would be the voltage I could obtain from one battery? As in, say I use one AA battery, which is rated as 1.5 V. Do I obtain a full 1.5 V with one battery, or is it 0.75 V?

    Another question is whether I could power an electromagnet with a voltage way lower than what is indicated. I have a 230 -255 rated ac-V magnet, which I got by halving and removing the upper half of a microwave transformer. I replaced the primary coil in it. It looks like this:

    Could I power it with a voltage in the range of 2-10 V dc? What would be the effect on its magnetic field? Would it be strong enough to induce a current in the mA range?
    Also, with such an electromagnet, what is the relationship between the current and the magnetic field strength?

    Finally, suppose I use an electromagnet to induce a current in a loop of wire. How could I connect a voltmeter to measure the induced emf? In series with the circuit, or in parallel to the circuit?

    Thanks for answering the questions. I know there are a lot.
  2. jcsd
  3. Dec 7, 2016 #2


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    Wow, lot's of questions. Let me try to answer some of them.

    Yes, you can put two or more batteries in series to increase the voltage. Two AA batteries in series will give you 3V, three in series will give you 4.5V, etc. Note that AA and AAA batteries are both 1.5V. It's not a good idea to put different batteries like AA and AAA in series because they will discharge at different rates.

    You get the full 1.5V from a AA battery.

    You could do this, but with a much lower voltage you will get a much lower current and hence a much lower magnetic field. Try using a multimeter to measure the resistance of the coil. Then you can calculate how much current will flow from Ohm's law.

    Note that induction requires a changing magnetic field. A constant current and a constant magnetic field will not induce any current.

    The magnetic field is proportional to the current times the number of turns, but to calculate the proportionality constant you need to know the details of the electromagnet.

    You normally measure voltage by putting the meter in parallel with the circuit, and measure current by putting the meter in series with the circuit.
  4. Dec 7, 2016 #3
    Thanks for answering all the questions.
    I plan on changing the area of the circuit in which I am inducing a current, while keeping the magnetic field constant. And I also plan on keeping the area constant and changing the magnetic field by using a rheostat to change the resistance of the circuit powering the magnet, and thus the current through it. In fact that is why I wanted to know whether the magnetic field was proportional to the current, so that I would know how it changed with the resistance of the circuit. I now know that the changes are themselves proportional to each other, and that was all I needed. I'm actually trying to compare the effects of changing the area of a circuit compared to changing the magnetic field strength.

    I'll also try measuring the magnet's resistance using the multimeter.

    About measuring the voltage, I'm still not sure I understand. I know that normally you connect the voltmeter in parallel across a component to measure the voltage difference, and that you could connect the meter in parallel to the power supply to measure the voltage of the whole circuit, but in this case, my circuit is just a closed loop of wire, powered through an induced voltage. Would I still be able to measure the voltage if I connected the voltmeter in parallel to that, even if it was connected parallel to any part of the circuit anywhere?
  5. Dec 7, 2016 #4


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    I see your question. You're right, in this case you want to break the loop and put the voltmeter across the broken loop to measure the induced voltage, like in the attached picture.

    Attached Files:

    • VM.pdf
      File size:
      13.9 KB
  6. Dec 7, 2016 #5
    Alright. Thank you for answering
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