ssk wrote:Hi Guys, we need help. My 8 year old is doing a science fair project with a friend. We made veggie/fruit batteries using a copper penny as the positive pole and an aluminum pop top as the negative pole. We tested with on a multimeter. (easy I thought... acid = high reading) BUT we tested a lemon, apple, potato, tomato and an orange. The potato made the arrow go off the meter. We couldn't get the meter to move except on the OHM meter so that was what we looked at.
What have we done?
Probably one of the most interesting things about batteries is the way that different materials and the way in which they are used can affect the characteristics of the battery. This means they can affect the output voltage and the amount of current that the cell can deliver. They can also affect something called the "internal resistance" of the battery. A battery cell made with a potato might provide a different amount of current than a battery cell made with a lemon or an onion. Battery cells made with different electrode materials, like copper, nickel, or zinc might produce different voltages. Batteries with different electrode shapes or surface areas might have different internal resistances.
ssk wrote:Thanks so much for your responses and patience. We have since redone the experiment using a zinc coated nail and a penny. And have switched to the (sorry, I barely know what I'm looking at) the scale that reads DC mA on the 0.5 setting. The other scale, as you have mentioned, OHM X1K, I'm thinking is the resistance... and the potato reading off the scale meant it went off the scale to the zero side. There is one other scale that reads DC V with a lowest setting of 5 which does not move in our experiment. So, now, the results are potato 100, tomato 50, apple 50, lemon 150, and orange 25 (could be because the orange is not juicy anymore. We appreciate your input! So fun for us to find a science community!
Craig_Bridge wrote:I'm going to provide a little circuit analysis here to help you appreciate some measurement issues.
When you used the Ohms scale, you were doing something you should never do to a multi-meter. Measuring resistance in the presense of an external power source is potentially damaging to the meter. Fortunately, the amount of power available from a single veggie cell was probably small enough so that it didn't damage the meter, just caused erroneous results.
When you used the DC mA scale, you were effectively shorting out the battery and determining what is called the short circuit current capabilities of the battery.
Now we need to ask the question of how many ohms/volt is the internal resistance of your multi-meter. Many inexpensive analog meters are 1000 ohms/volt which means the 5v scale is effectively a 5000 ohm load to which you have to add the internal resistance of the battery to determine how much current you need to get a reading. Based on your short circuit current readings, the internal resistance of batteries are higher than your meter. Your battery won't supply enough current to provide an accurate voltage reading with your meter.
Most DVM (digital volt meters) have 200Kohms/volt or greater so they are better choices for this experiment.
When would you use the DC mA setting?
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