Dot Physics

Electric eel at the aquarium

We went to the aquarium, the kids like it. In the Amazon river section, they have an electric eel. Here is the sign next to the Eel.

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Sorry for the poor image quality. I took the picture with my crappy phone and didn’t even realize it was bad until later. I used some magic on it to make parts of it readable, but if that was not good enough, here is what it says:

The Electric Eel is the most powerful of all the electric fishes. It can discharge up to 650 volts: six times the power of a household current. A shock can fend off attackers or stun prey so the eel doesn’t risk getting hurt in a struggle.
A person might survive one blast from an Electric Eel, but not several.

Let me address this in three ways. First, what is wrong with this. Second, why is this a problem that it is wrong. Third, what would I have put on this board.

What is wrong with this?

The biggest problem seems to be the confusion between electric potential (measured in volts), power (measured in watts) and current (measured in amps). The narrative clearly says that the 650 VOLTS is six times the POWER of household CURRENT. Those are three different things.

If you look at the Wikipedia page on electric eels, it says the eel could produce 500 volts at 1 amp of current. For electric circuits, the following is the relationship between electric potential, electric current and power:

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This would make the eel capable of 500 watts in the attack.

What about a household circuit? First, these are alternating currents (AC), but let’s just pretend it is DC for simplicity. A normal household outlet is at 120 Volts and can produce currents of about 10 amps. This would be a maximum power of 1,200 watts. So, the eel is not 6 times the power of the household outlet.

Note: I would not recommend sticking your finger in either an electric eel nor in a household outlet.

Why does this bother me?

Really, this is a similar problem to the problem with ESPN Sport Science. Here is a great opportunity to help people learn something, or at the very least do no harm. Also, clearly there was some effort put into this production. I bet the sign alone cost at least $100. Yet it appears that no one bothered to contact a local high school physics teacher to look over this. If you can’t find someone, email me. I will be happy to look over your sign.

I could see if a blogger made this and it was wrong. Mistakes happen, it is no big deal – especially when it is just one person. But this case is different. How long will that sign be there?

My sign

Ok Mr. big shot blogger man. What would you put? Well, first I would state the important details.

An electric eel has multiple cells along its body that create a change in potential of over 500 volts. The typical current produced in an electric eel attack is around 1 amp.

Now, the problem is that most people of no experience with with 500 volts is like or 1 amp is like. So, I could add something like this:

500 volts would be the same potential as about 330 D-cell batteries connected together. A typical two-battery flashlight might use a current of about 1 amp. However, since the eel has a much larger potential difference, the effects can be severe.

Just my first thought. You could probably come up with a cool graphic to show a comparison between the eel and a flashlight. Oh – how about two electrodes that visitors can touch and get shocked. Next to it, there could be a sign that says “that hurt, didn’t it?”

Comments

  1. #1 Jmchez
    May 31, 2010

    Your sign would never be accepted; not pithy enough. In exhibits directed at children and laymen, you need examples that people are familiar with.

    While the comparison to 330 D cell batteries is a good start, a better analogy would be compare the power produced by the eel to that required to run an appliance. A small microwave oven would be my choice.

    You could also say that, if the eel kept at it, you could make half a glass of water (4 oz or 118 ml) go from room temperature (22C)to boiling in about a minute and a half. Of course, I don’t think that eels put out that much total energy.

  2. #2 Starwatcher
    May 31, 2010

    Voltage labels are usually given in RMS values.

  3. #3 Lassi Hippeläinen
    May 31, 2010

    You still miss the time dimension. The output is pulsed. A 20 kg fish can’t produce half a kilowatt continuously for a long time.

  4. #4 Sue VanHattum
    May 31, 2010

    >change in potential

    Too hard for me. (I aced the first semester of physics – mostly mechanics. I barely passed the second semester, because I got totally stuck on how weird electricity was.)

    The Wikipedia text looks good, but some simple comparison would be nice. How does it compare to an electric fence?

  5. #5 kevin
    May 31, 2010

    Cripes. Your details aren’t really helpful for the intended audience, and your comparisons stink. How does one visualize 330 D Cell batteries? And who the hell uses D cell batteries these days? I mean, I’ve got a knock-off mp3 player that takes one AAA battery. Otherwise, all my gadgets (laptops, etc.) come with built-in lithium batteries. And a 1-Amp flashlight? That would be a honking big LED flashlight.

    Obviously I’m being overly picky, but that’s because I agree with you that the original sign stinks and is a missed opportunity.

    My comparison would be something like “can generate enough electricity during an attack to power five conventional 100 W bulbs, but for a a quarter of a second”. Or more gruesome: “four eels attacking together are about as powerful as an early model electric chair, though only for a half a second (or whatever)”. If you like the numbers, preface it by just the facts: “An eel can produce about 500 Watts of electricity.”

    -kevin

  6. #6 spork
    May 31, 2010

    >> I bet the sign alone cost at least $100. Yet it appears that no one bothered to contact a local high school physics teacher to look over this.

    That pales compared to what I’m sure they pay you to teach physics, and you can’t be bothered to have a high school physics teacher look over your blogs.

  7. #7 Rhett Allain
    June 1, 2010

    Yes, I think this is a tough one. What kind of comparison would you like to make in a short space? What concepts do you want to try to support? I am not sure what the answer is.

  8. #8 spork
    June 1, 2010

    I’m sorry, but if that is a response to my post I honestly can’t follow it.

    >> What kind of comparison would you like to make in a short space?

    I was comparing the $100 you presumed they spent on the sign against your salary.

    >>What concepts do you want to try to support?

    I’d like to support the idea that a physics instructor should be open-minded, be willing to admit when he might be wrong, and not call people hoaxers on his public forum if he doesn’t want to become the poster boy on thier forum.

    >>I am not sure what the answer is.

    To which question – DDWFTTW?

  9. #9 coldbilly
    June 1, 2010

    How long will that sign be there? How long has it been there? I took my nieces to this same aquarium almost 20 years ago and I have a distant memory of reading this very same sign (or a sign very similar).

    I suspect that neither the museum nor the sign maker were trying to make any point other than that we should be impressed by the interesting electrical fish killing power (not IV) of the electric eel.

    By the way, I went to the WWII museum on Saturday. On the front page of a paper posted on a wall (I forget from where) describing the D-Day invasion were two other lead stories. The first described how a 26 year old single woman was murdered blocks from her home and the second described how a 14 year old formerly fat girl had starved herself to death because she was tired of being ridiculed for being fat. What does this have to do with physics? Nothing I’m happy to say.

  10. #10 rob
    June 1, 2010

    i had the same thoughts as Rhett after i read the sign. then i realized he was going to point out the issues with power, electric potential and current. it would be nice if the aquarium found out about this post and fixed up the sign.

    @spork:feeling grumpy eh?

  11. #11 spork
    June 1, 2010

    >> feeling grumpy eh?

    Yeah – I suppose it annoys me when an incompetent “physics instructor” accuses me of perpetrating a scam on his public forum just because he can’t understand high school physics (and doesn’t care to try).

  12. #12 jacy
    October 20, 2010

    is a eltric eel make power?

  13. #13 naana
    October 20, 2010

    do eletric eels have tounges