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Old 03-01-2016, 10:07 AM
Machine Elf Machine Elf is offline
Join Date: Feb 2007
Location: Challenger Deep
Posts: 10,332
Originally Posted by Hari Seldon View Post
Yes and there is almost 0 voltage because the resistance of the copper plate is much lower than the internal resistance of the battery. Not recommended.
You mentioned the internal resistance of the battery, which is the key to what happened to the OP:

Originally Posted by greggamma
I hooked a 9 Volt battery using 2 alligator clips to a piece of copper plate. Here is what happened 1) My volt meter read nothing. 2) There was a pop sound 3)The battery heated up quickly...less than 15 seconds.
Now A) What can I do to prevent that? B) WHAT happened?
If you're drawing a circuit diagram, you can model the battery as a voltage source PLUS a resistor, arranged in series (the resistor represents the battery's internal resistance to current flow). Add another resistor in series to model the copper plate. So now your circuit diagram looks like just a voltage source with two resistors in series, (a voltage divider). As Hari Seldon notes, the copper plate's resistance will be much lower than the internal resistance of the battery. This means most of the battery's voltage will be dissipated across its own internal resistance; this is why the meter reads very close to zero volts (you're measuring +9 volts from the battery's chemical activity, together with -9 volts due to current flowing across its internal resistance), and why most of the power being produced by the battery is actually delivered to the battery. As heat, in that internal resistance. This configuration represents pretty much the lowest possible circuit impedance, so the battery is delivering as much current as it possibly can (I = V/Rtotal). So the battery is producing as much power as it can, and dumping all of that power into itself as heat.

You can't change the battery's internal resistance, so if you want to avoid roasting/exploding it, you need to not move so much current through it. To do that, increase the total circuit impedance. You can add resistors in series to make that happen. Note that power will be dissipated in those resistors, so they either need have high resistance (so as to make the current through them small), or they need to be rated for power dissipation (instead of little rice-sized resistors, you'll have a rectangular block of ceramic stuff maybe the size of your pinky). If you get low-value resistors, you'll still be moving a lot of current through your battery, and it will still get warm.

Last edited by Machine Elf; 03-01-2016 at 10:08 AM.