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# Electrical Engineering/Calculating the Amps in an electromagnet coil

Question
Hi Cleggsan,

Hisashiburi desu! I hope this message finds you genki.
I am sorry to trouble you with such a trifling matter, but I am getting a strange result from some calculations, and I was wondering if you might be able to tell me where I am going wrong.

I am trying to calculate the Amps in an electromagnet using a 9Volt battery.
The wire is a 0.5meter long stranded copper with 12 x 0.18mm diameter copper strands (total diameter 2.16mm)
I used a resistance calculator I found here ( http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html ) and found a resistance of 0.0027438312189042757 ohms for a length of wire 0.5meters long.
9 volts divided by 0.0027438312189042757ohms = 3280.08513716 amps.
I have clearly gone very wrong somewhere and I am embarrassed to say I can't work this out myself!
If you could point me in the right direction, I would be as grateful as always. I am sure I am making some Very basic error, so I am sorry to bother you.

Thank you very much and best regards,
Eddie

Looks okey to me.  A short piece of copper wire is a pretty good short circuit!  Of course the battery cannot produce that kind of current so you needn't worry about burning anything.

What is the gauge rating of the wire?  What is the resistance from the gauge charts?  Do they seem pretty much in agreement with your calcs?

http://www.powerstream.com/Wire_Size.htm

PS: The internal resistance of the battery is what prevents it from giving this huge amount of current.  The voltage drop across the internal resistance will provide very little left over to power the wire/short across the battery terminals.

Ohm's law is always dependable - if you are using the right numbers.

As an experiment, when powering the coil measure the voltage across the terminals of the battery and you will probably find it very, very small or "0" since the internal resistance of the battery is much higher than that of the coil; therefore the voltage of the 9v battery will be divided mostly across the internal resistance of the battery.

Internal resistance of the battery is a function of the charge level and the chemistry of the storage media inside the battery.

Does this help?  Omoshori.

Merry Christmas.

Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thanks Cleggsan! Merry Christmas to you!

Electrical Engineering

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#### cleggsan

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