Can't get potato light to work

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shez24
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Joined: Fri Jul 08, 2022 12:53 am
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Can't get potato light to work

Post by shez24 »

Hi. My daughter is doing a science project and her experiment is the well known potato light. We have done everything as we should, hooked up 6 large potatoes with galvanised nails and copper, copper wire, crocodile clips and the voltage meter reading from the multimeter for all is 4.4v. The light bulb is an 1.5v LED. We can light up the 1.5v led lightbulb with a 1.5v battery but we can't light it up with the 4.5v reading we are getting from 6 potatoes. We tried boiling them but there was no difference. What are we doing wrong? Do we need more potatoes or a even smaller voltage led light bulb? I read somewhere the little Red LED light bulbs require less power to light up. Is it because the voltage from the potatoes is not stable?
bfinio
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Re: Can't get potato light to work

Post by bfinio »

Hi - if you haven't seen it already, we have a version of this project on our site: https://www.sciencebuddies.org/science- ... to-battery

where we sell a kit with parts that we've tested and confirmed to work. Not that you need to buy the kit if you already have your own parts, but here are a few things to consider:

- In order to light up an LED you need enough voltage but also enough current. You can check the current of your potato battery with a multimeter. We have a multimeter tutorial if you don't know how to do that: https://youtu.be/ts0EVc9vXcs
- Most regular LEDs require about 20 milliamps of current for full brightness. They should still visibly be on around 10mA but if you start getting lower than that, they may be too faint to see. You can search for "super bright" or "high efficiency" LEDs that are easier to light up with small amounts of current (this is what we include in our kit).
- A bunch of things can affect the current. Increasing the surface area of the electrodes won't change the voltage (which is determined by the chemical potential between the two types of metal), but will increase the current. The spacing between the electrodes, whether the potatoes are dried out, whether the electrodes are getting old and corroded. All of these things affect the "internal resistance" of the battery, and if that resistance gets too high, you can still measure a seemingly normal voltage, but get very little current. (the same is true for regular batteries - e.g. a "dead" AA battery will not measure 0 volts - you will still measure a voltage, but its internal resistance will be so high that you will no longer get any useful current out of it).
- The voltage required to light up an LED depends on the color. In general you are right that red is lower voltage and is a good color to use. Avoid white and blue because they are usually much higher (up to 4 volts).

Sorry if that was a lot of info, but this can be more complicated than it seems at first! To summarize I would:

1) Check the current of the battery with your multimeter
2) Try various things to increase the current (soak the potatoes in water, fresh potatoes, more surface area of the electrodes by pushing them in farther)
3) If that still doesn't work, try buying super-bright/high-efficiency red LEDs

Hope that helps, write back if you have more questions!

Ben
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