Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

My 5th grade son is doing a science project to show that a weak electrical current applied to a lima bean will help it grow faster than a lima bean grown with no current. Following an experiment outlined on the web, he has two Styrofoam cups with garden soil and three lima beans each. One cup has two 12" 18 gauge copper wires connected on one end to a 9 volt battery with alligator clips and the other ends are inserted in the soil. He is supposed to water twice a day, connect to the battery for five minutes per day, and keep track of all inputs. Hopefully the beans sprout in ten days and grow for six before the results are due.

The first time he connected to a new alkaline battery for three minutes and the battery got very warm. Since then the battery has been connected as instructed but no longer gets warm.

I'm no electrical engineer, and the whole thing is just making me ask questions. Any comments or insights would be most appreciated.

Does current automatically flow just by attaching two copper wires to the battery terminals? The copper wires are not supposed to touch in the soil according to the instructions -- what if they did? Does it matter if the soil is moist? Can the amount of current that flows be measured without expensive equipment? Can it be theoretically calculated with any reasonable degree of certainty? How quickly will the battery be drained?

Sorry these are all rough newbie questions -- you can tell I'm out of my league!

Note added 2/18: Thank you for all the wonderful comments, really helping me to understand what's going on. Yes, the battery leads were touching, I'm sure that's what caused the battery to heat, fortunately no lasting harm done, lesson learned, and the battery still tests good on my battery tester. I'm going to try to get a digital multimeter as suggested in several responses to answer the question about soil resistance and find out roughly how much current is flowing past those beans.

share|improve this question
    
Now I have a Southwire Analog Multimeter model 10010S and a bunch more questions. I did read the multimeter instructions and a really useful tutorial (sciencebuddies.org/science-fair-projects/…), so I hope I am using it properly. – SSP Feb 18 at 19:46
    
Measuring resistance: 20kOhm if I stick the probes direct in dirt, 4kOhm when touching them to the wires coming out of the dirt, one on either side and no battery attached. Does that make sense? (Also, 7kOhm in sour cream, no reading in ice cream?) For current, with battery attached and gauge set to 250 mA, the needle goes off the scale? That's the highest range for measuring current...what gives? Shouldn't current be in the range 0.45 to 2.25 mA if either one of the resistance measures is correct? Multimeter also measures voltage; check that on the DCV10 scale and get a 9, no surprise. – SSP Feb 18 at 20:01

The soil between the copper electrodes will conduct electricity because it's wet, the soil has salts in it, and these salts will disassociate in the water into ions which can move about. The positive ions will move towards the copper rod attached to the negative side of the battery (the cathode), and the negative ions will move towards the copper rod attached to the positive side of the battery (the anode).

At the anode and cathode, if you run this experiment long enough, you'll begin to see the results of these ions reacting electrochemically with the copper. You'll probably notice one is more corroded than the other, if you always connect the battery the same way.

Any charged particle moving, be it mobile electrons in copper, or mobile ions in water, is an electric current. Electric current is measured in amperes (A), often with the metric prefix milli- (m). 1000 mA = 1 A.

You might measure how much current is moving a couple ways. The most straightforward and accurate is to get an ammeter (a place like Radio Shack should sell a multimeter that's cheap and sufficient) and hook it up in series with the battery, like this:

schematic

simulate this circuit – Schematic created using CircuitLab

It doesn't really matter on which side you connect the ammeter, or in which direction. If it's "backwards" then it will just indicate a negative current.

The magnitude of the current that will flow will be a function of the resistance of the soil. Less resistance means more current. Resistance is measured in ohms (Ω), often with a metric prefix like kilo- (k) or mega- (M).

The resistance will depend on (among other things):

  • how wet it is
  • the composition of the soil
  • how well the electrodes contact it

If you have a multimeter, you can also set it to measure resistance, disconnect the battery, and connect the meter probes where the battery would be. I just measured the resistance of the closest wet soil to me and found it to be 382kΩ. Your results will likely be very different.

Knowing the resistance, Ohm's law can be used to calculate the current that will flow, given some voltage applied to that resistance. Just divide the voltage by the resistance. In my case:

$$ {9\:\mathrm V \over 382\:\mathrm{k\Omega}} = 0.024\:\mathrm{mA} $$

Complicating matters is that the battery will add some resistance of its own (called the internal resistance), and this resistance will rise as the battery discharges, resulting in less current. When the battery is "dead", its internal resistance is so high that almost no current flows.

In any case, soil resistance would be a good thing to include in the data in itself, since it's independent of the battery's condition, and it's another variable you can analyze. You might even see how watering affects soil conductivity, and attempt to keep the conductivity of the test and the control beans the same.

Battery capacity is usually specified in mAh, or milliamp-hours. A 9V alkaline battery has a capacity anywhere between 90 mAh and 300 mAh, depending on how fast you discharge it. A faster discharge means less capacity.

1 mAh means the battery can supply one mA for 1 hour before being dead, or 2mA for half an hour, or 0.5 mA for 2 hours. This is a good opportunity to practice dimensional analysis. For example, if I measure the current to be 0.02 mA, I might estimate the battery will last for:

$$ \require{cancel} {300\:\mathrm{\cancel{mA}\cdot \cancel{hour}} \over 1} \cdot {1 \over 0.02\:\cancel{\mathrm{mA}}} \cdot { 1\:\text{day} \over 24\:\cancel{\text{hour}} } = 625 \:\text{days} $$

I can tell you that if the battery is getting hot, quite a lot of current is flowing, and the battery will be dead very soon. So either your soil resistance is much lower than mine, or there's some fault in your setup, like the wires in the soil are touching. Using a battery that starts fresh and is dead before the experiment is done will probably invalidate the results, so I'd suggest investigating this further.

share|improve this answer
    
Thank you for that thorough explanation -- very helpful! – SSP Feb 18 at 1:42

To answer your questions:

  1. If the copper wires touch, they will represent almost a direct short to the battery which will quickly drain the battery and it will heat up. Therefore, make sure that this does not happen.
  2. A moist soil will be more conductive (lower resistance) than a dry soil. Therefore, more current will flow from the battery.
  3. A simple multimeter, set to a current range, and placed in series with one of the copper wires, should be able to measure the current.
  4. It is difficult to calculate the current level since it depends on a number of factors including the size of your copper wires, the moisture content of the soil, the volume of the soil, impurities in the soil, etc.
  5. Because the current level is not known, it is difficult to predict how long the battery will last. If you can measure the current level, then a better determination of the battery life can be made.
share|improve this answer

If the battery is getting hot, it sounds like you're producing a good deal of current, and not a little bit, and the battery may be dying very fast.

To provide a constant, small current very accurately is probably above and beyond what you want to do. I have a suggestion, though, that may get you close.

Instead of hooking up the battery, directly in its place hook up a $4 digital multi meter that you can buy in a place like Harbor Freight, and pay attention to the number you get when measuring the resistance (you'd need to turn the knob to "ohms"). Let's say the number you get out is 200 ohms (I have no idea if that's even in the ballpark -- also the number may change with watering). Now, let's say you'd like to generate 10 milliamps of current. Since V=IR, and V=9 volts because of the battery, 9/0.01 yields 900 ohms. Since you've already got 200, you need to add a resistor of about 700 ohms in series with the dirt. You might be able to buy resistors in Radio Shack if you can find one that's still open.

All this assumes that there's a DC path for current, which there seems to be as the battery gets hot, and that it won't change over time.

If you can't make the measurement, you might just put a 500-700 ohm resistor in series anyway.

share|improve this answer

If the battery is hot, there's too much current going through the wire, as everyone has said. If there's still too much current, the battery could explode (pop, really) or leak acid: what a mess.

You need to reduce the current going through the soil. You do that by increasing the work electrons have to do to get through the soil.

First thing to try: move the wires in the soil further apart.

An Engineer would "add a resistor in series". You probably don't have a resistor lying around the house, but they're easy to make!

Make a 2-inch square, really dark, on a piece of paper, with a pencil-- graphite conducts, but not perfectly. The black square is your resistor. Tape wires to opposite sides of the square, making sure they're really firmly on the graphite. Maybe use a weight to squish the wires onto the graphite.

Link everything like this:

(+ terminal)---wire1---[black square]---wire2---[dirt]---wire3---(- terminal)

Which is what we mean by "adding a resistor in series".

Extra credit: vary the current going through the soil by moving the wires closer or farther apart, and by varying the size of the graphite square.

Then make a chart! First prize.

share|improve this answer
  1. What this experiment is really checking for is whether adding copper ions into the soil has effects on lima growth. If you really want to study the effect of electrical current alone, consider getting graphite electrodes.

  2. Unless you're watering your seeds with Dead Sea water, the battery can't realistically get hot. You almost certainly crossed those copper wires under the soil, creating a short. Your battery got hot the first time because of that.

  3. After being shorted for several good minutes, the battery should be considered dead (good thing it did't explode, btw). That's why it doesn't get hot anymore: dead don't sweat. You'll need to get a new one if you want to continue. Putting a resistor in series is a sound idea, if you want to make sure your second battery lasts longer.

share|improve this answer

Do you have anything that will tell you if that battery is still producing close to 9 volts? If you have a multimeter, it's probably suitable. I've also seen some small radios that required 9 volt batteries. If the battery is no longer producing close to 9 volts, treat it as dead and get a new 9 volt battery.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.