Science Buddies: "Ask an Expert"

I finished my experiments testing various electrolytes and electrode materials. I want to calculate the power (I*V) of each of my fruit and veggie battery . I am little confused about the correct way to calculate power. I have used I*V1, where V1 is closed circuit voltage, as there is no current flowing without external resistance and the closed circuit voltage does reflect internal resistance. In this case Power will be ((V1^2)/Rext ), where Rext is external resistance. Is this correct? Or should I have used I*V0, where V0 is open circuit voltage, in this case power comes out to be (V1*Vo/Rext)? Please confirm.

Also, I want to measure current needed to run a kit radio I built few years ago, which runs on 4 AA batteries, the batteries are connected in series. Any hints on how to do that. So, far the fuse in the DMM went out and we could not measure current. As you might have guessed, I want to calculate power needed to run my kit radio. I have new fuses, but have not tried yet.

Thanks,
Ani

Ani,

When using a DMM to measure the current, you need to plug the DMM leads into the proper jacks on the DMM which are marked for measuring current and you should always start off by setting the DMM to the highest range first when making your measurement. If there is little or no reading, then you can carefully switch the meter to the next lower range and observe if you now have a reading. You need to be careful not to exceed the maximum current for the range you select, otherwise it will blow the fuse in the DMM.

Make sure that the meter is connected in series with the battery and the radio kit such that it all makes one big loop and you have the leads of the DMM connected so that the current flows from the red lead (pos) to the black lead (common or neg).

Here's a link to an instruction manual for one such DMM. It gives a good explanation on how to correctly use the DMM to measure current

http://www.bkprecision.com/manual/388B_manua.pdf


Here's another web page which shows how to correctly set up the ciruit.

http://www.electronics-radio.com/articl ... urrent.php


Hope this helps

Janelle