current

Could I build a lightning detector?

rallain — Thu, 02 Sep 2010 09:50:54 +0000

Could I build a lightning detector?

A while ago, we had a ton of lightning. As a bonus, it always happened in the middle of the night. I love sleeping where it sounds like I am on the front line in WWI - no, I don't. But, while lying awake waiting for the next BOOM I thought of something. Instead of just counting the time between flash and boom, maybe I could make a lightning detector.

For mere mortals, the first step might be to google "lightning detector". I don't want to do that. What fun would that be? I did look up something. The current in a typical lightning strike is on the order of 30,000 amps. How could I detect this.

I can think of two things. Could I detect the magnetic field from this current more than 10 miles away? Or maybe I could detect the change in magnetic flux through a loop of wire. For the second method it depends on the time rate of change of the magnetic field and the electric potential it would generate around a loop.

Modeling Lightning

Let me pretend that lighting is a current that flows in a straight line (a huge wire in the sky). A single current creates magnetic fields that makes a circular vector field around the current. Here is a picture. (I will just use this lovely picture from wikipedia)

The magnitude of this magnetic field (for a long straight wire) changes with distance from the wire as:

Here μ₀ is a constant and r is the distance from the "wire" to the location of interest. So, if I am 10 miles from a lightning strike, what kind of magnetic field could I even detect? This would be a magnetic field of 4 x 10^-7 Tesla. Is that a large magnetic field? How about another example. What if I am 10 cm away from a wire with 1 amp of current? That would create a magnetic field of 2 x 10^-6 Tesla. So, maybe it would be possible to detect this magnetic field (I will have to play with some detectors to try this).

The other question to address is time. How long does this current last? If it is super quick, I might not be able to even detect the magnetic field. This is directly tied to the other detection method - detecting an electric potential that goes with a changing magnetic field. I am not going to go into all the details (for now), but if you have a changing magnetic field and a loop of wire then the change in potential around that loop would be:

Here:

N is the number of turns in the coil that makes up the loop
B is the magnetic field vector
n-hat is the unit vector that shows the orientation of the loop - it points perpendicular to the area of the loop
A is the area of the loop

Suppose the area does not change. Also suppose that the magnetic field makes an angle θ with respect to the n-hat vector. This means that:

I honestly have no idea how long this lightning takes to happen. Also, this would give me the average change in electric potential - there are could be a spike depending on how quickly the magnetic field changes. Let me just suppose that Δt is around 0.1 seconds (just a guess) and my loop is a circle with a radius of 10 cm with 100 loops. What would the change in potential in the loop due to the lightning strike be? (assume best orientation of coil to lightning)

Ok, that is pretty low. Granted, my value for the change in time might be way off. But if it is not, even increasing the area and the number of loops both by a factor of 10 would still give a potential of around 10^-3 volts.

There is another way to detect lightning - by the electromagnetic signal given off during the strike. If you google for "build a lightning detector", you will find lots of info on these. Ok, but there is a problem. These all detect lightning, but I want to know where the lightning happens. I guess you could build two EM style lightning. If I want to know WHERE, I will need two detectors no matter which way I detect the lightning.

Enough rambling. I guess I should set up some small scale experiments.

rallain Thu, 09/02/2010 - 05:50

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current

Light

magnetic

diy

lightning detector

magnetic field

Physics

Your delta-t is way too large. Use a smaller delta-t. Consider the time it takes for the electrons to make their way from the cloud to ground (or vice versa).

Consider using a ferrite bar to concentrate the magnetic lines of flux.

Also, consider using a Hall Effect magnetic field sensor. Allegro makes some nice ones (I like their A1321 device.).

1 mV is a very strong signal for RF purposes. You'll need to amplify it, but radio receivers are designed to work down into the uV range.

Another option may be to use optical sensors to detect the flash. These may be simpler to use than RF or magnetic field sensors.

Two sensors will allow you to use the timing difference to determine the azimuth to the strike (Note, though, that lightning bolts are not a point source, but are usually a 3 dimensional line source.). To get the distance, you'll have to add more sensors to allow you to get two azimuths so that you can cross the azimuth lines to determine the distance. Or, use another technique, such as the flash-sound technique, or use radar.

The real trick, though, is that most people want to determine when/where a bolt will strike at some point in the future so that they can take precautions. For this, look up "field mill":

http://en.wikipedia.org/wiki/Field_mill

Dave

I'd bet you already have a pretty efficient lightning detector, otherwise known as an AM radio...

My airplane has an instrument made by BF Goodrich (who bought the company "Ryan" who originated the instrument - I bring this up since that happens to be my last name) and now by L-3 Communications called a "Stormscope." Mine is the WX-10 model. It detects lightning, estimates distance, and plots it on an azimuthal display in the cockpit as either a supplement to or substitute for (in my case) weather radar (which detects precipitation with actual transmitted radar signals).

The device works by directly detecting the electric and magnetic fields generated by lightning strokes. There's a bunch of good data on both this instrument, prior art, and lightning in general in Paul Ryan's patent application at http://www.google.com/patents?id=3Tw7AAAAEBAJ&printsec=abstract&zoom=4&…

It has a range out to 200 nautical miles and I can zoom in to where 25 nautical miles is full screen. It has been, in my experience, very accurate in depicting areas of significant thunderstorm activity (a thing to be avoided in any airplane, let alone a light plane such as mine). You might wonder why I can't just look out the window. Well, if I'm not in clouds, that's certainly very effective but in instrument conditions, not so much.

@Rob,

That is awesome - thanks.

RA you are talking about physics as fun:
Of course this is. "i have listened to Whistlers"
i built some multi turn Goniometers in Aluminum tubing. The ELF was an interesting place "and still is" "lets capture and store those 30,000 amp strikes" tongue in cheek, as it is a whole different story. "thumbs up" for the illustration. "lets keep the radio airwaves clean" /rtg/ yvr.ca

Of course the cheapest one is to just stand there with an upraised metal rod in your hand.

Only works once though...

An analysis based upon induction of a magnetic field is a poor way to start. At distances of miles, the long wire magnetic field formula is very poor. At miles the magnetic phenomena is more like a magnetic dipole, whose strength drops as the inverse cube of the distance from the dipole.

A much better treatment would be based upon electro magentic radiation.

@Ed,

Very good point - if I am that far away, the lightning is clearly not a long infinite wire.

You can see it a lightning detector for DSLR camera :
http://rienquepourlesyeux.free.fr/Detecteur%20de%20foudre/Lightning%20d…
It work very fine. It is for info.

What is this #2: Answer

rallain — Tue, 31 Aug 2010 07:10:14 +0000

What is this #2: Answer

Reader Fruity was the first to correctly name this device - on just the 9th comment. Impressive. Honestly, when I found this thing I had no clue. I asked other physicists and none of us were sure. However, I did find the answer. Let me show you my secret.

I don't know how old this thing is, but it is old. When I open it, my nose and eyes get all tingly - probably from the mold. Here is that apparatus in the book (tome):

I can't show you the whole page because it has other stuff of awesomeness that I want to show you later. But, I can show you the description for that item.

I am still giving Fruity the credit for the answer "Ampere's Rule Apparatus", but clearly The Tome of Knowing says "Mounted Conductor (Ampere's Law Stand)". Basically, you run some current through this thing and you can place a magnetic compass at different locations to see the deflections. Here, I made a quick video of it.

rallain Tue, 08/31/2010 - 03:10

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"I am still giving Fruity the credit for the answer "Ampere's Rule Apparatus", but clearly The Tome of Knowing says "Mounted Conductor (Ampere's Law Stand)"."

Too funny.

Can't remember the exact words I googled, vintage, Welch, physics and apparatus come to mind. This link popped up and voila!

Out of curiosity, when was that Cenco sales catalog published?

@Fruity,

I don't have a publish date - let's just say old. Actually, I didn't look too hard for the date - maybe it already molded away.

I not trying to be snarky here: it was really nice to see you have the confidence to show a blown demo, and how calmly you dealt with it.

I forget what settings I use for a similar demo, but it is quite a bit more than 2 A. The wire used can take a lot. It helps to get the vector sum of the earth's field and the wire's field looking more like what you expect if the earth wasn't there.

@CCPhysicists,

1) I welcome and encourage all forms of snarkiness. How can I be snarky and not expect to get some back? Bring it on.

2) That power supply only went up to around 2 amps.

3) I really should have just restarted the whole video, but I was lazy.

Good way to blow up your laboratory power supply, especially those with poor or no current limiting. I am guessing that the resistance of the apparatus is very low and will appear as a short to the power supply. You may want to put a 10 ohm resistor in series to give the supply a easier load to drive. Make sure it has a high enough dissipation rating and be careful of the heat it will generate. If you don't have a suitable resistor, a light bulb in series will work as well. Just remember to take your voltage measurements for any calculations across the apparatus and not the power supply.

Electric eel at the aquarium

rallain — Mon, 31 May 2010 12:23:45 +0000

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.

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:

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?"

rallain Mon, 05/31/2010 - 08:23

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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.

Voltage labels are usually given in RMS values.

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.

>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?

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

>> 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.

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.

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?

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.

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?

>> 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).

is a eltric eel make power?

do eletric eels have tounges

What kind of electricity is lightning?

rallain — Sun, 15 Feb 2009 07:50:12 +0000

What kind of electricity is lightning?

Someone complained about a teacher using this question and the answer was disputed. After some time, the teacher claimed that the answer was that lightning is static electricity because she looked it up on the internet. So, I decided to look also, here is what I found searching for "what kind of electricity is lightning?" in google:

Electricity Facts - Dialogue for Kids (Idaho Public Television)

A couple of complaints about this site:

First, I am not fond of the term "Science Facts". What is a science fact? Something that is proven true? This really is not how science works.
Second, I am not fond of the term "electricity" either - what exactly is this?
The good: the site talks about the atom and talks about how Ben Franklin didn't INVENT electricity.
The site claims there are two kinds of electricity - static and current. I guess this ok since the definition of electricity is not too clear (to me).

Here is a table from that site:

Here they define current electricity as a steady flow of electrons. If they use this definition, then maybe lightning is static, but then what would household current be (which is AC and therefore not steady). I would define static electricity as non-moving electrons (static - doesn't that make sense). I would define current as moving electrons. In the case of lightning, it is definitely moving electrons.

Here are a couple of sites that seem to have it correct:

Here is a good site on why electricity is a poor word. (from the science hobbyist)
This site looks good also (they don't say lightning is static electricity) - Physics4Kids
This site (School for Champions - http://www.school-for-champions.com/science/static_sparks.htm) Does a pretty good job, it says at the end "static electricity causes lightning" - I am ok with that

Another google search for "electricity lightning" gives the following:
Weather Eye - http://weathereye.kgan.com/cadet/lightning/electricity.html - "Lightning is like static electricity except on a much bigger scale". Maybe I am just being picky (probably), but here it seems they are calling "getting shocked from rubbing your feet on the floor" static electricity. If this were the definition of static electricity, then they would be correct in calling lightning static electricity on a bigger scale.

In the end, if someone asked me "what kind of lightning is electricity?", I think would focus on what is lightning and just pretend like they didn't say "electricity".

rallain Sun, 02/15/2009 - 02:50

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considering you can measure the current of lightning, i'd say it ain't static.

Christmas tree lights - surprisingly cool

rallain — Fri, 05 Dec 2008 17:32:51 +0000

Christmas tree lights - surprisingly cool

Let me start off by saying I think I first read about this on HowStuffWorks.com (many years ago). So, why are christmas lights so cool? They are cool because they are a whole bunch of lights in series, but they still work if one of the bulbs gets burnt out. If you are not familiar with circuits, a series circuit is one in which all of the current goes through all of the items in that circuit (as compared to the case where the current gets split up). Here is an example of a series circuit with two bulbs.

In this case, the current comes out of the battery, goes through one filament and then the other. Both bulbs light. What happens if one of the filaments break? Then there would be no complete path for the current and thus no current. No light in either bulb.

If you remove one bulb in a string of christmas lights, they all go out. This indicates they are in series. However, if you break one of the bulbs and leave it in, the other lights stay on.

So how does this work?

Let me start with some more fundamental ideas. There is really a lot going on here, but I will kind of jump in the middle of it.

Key Idea 1: Voltage Loop rule

Also, this is called Kirchhoff's loop rule (just in case you go by that name). This says that the change in potential around a closed loop is zero volts. Or, you could say that if you add up all the changes in potential around any loop of a circuit, they must add to zero. Note that there are some cases where this is not true (in particular cases with changing magnetic field - but that I won't go there). Electric potential is really just the change in electric potential ENERGY per charge - so this whole thing comes from the work-energy equation.

Key Idea 2: Ohm's Law

This says that the change in potential across some element in a circuit is proportional to the current going through that element (like a resistor or a lightbulb). The proportionality constant is the resistance. This is called Ohm's Law.

I am mostly ready to talk about the lights now - but I will say that I should come back to this and explain in more fundamental terms. So, what happens when the lights are working normally? They are connected to an oscillating 120 volts powersource, but let's pretend it is a DC source for now. If I use the loop rule, then potential across all 50 lights will be 120 volts (they are like resistors). They all have the same resistance and the same current, so each one has 2.4 volts across it. Everybody is happy. But now, suppose one of the filaments in one of the bulbs breaks. Now there will be no current. However, the loop rule has to still work. This means that if there is 0 volts across all the unbroken lights (no current), then the 1 broken light will have to have 120 volts across it. Here is an example with just two bulbs:

Now with 120 volts across one of these broken christmas bulbs, something happens. Here is a close up view of a broken bulb.

This "extra wire" has some type of insulating coating on it so that it normally does nothing. When 120 volts is applied across the insulator something happens and makes it a conductor. I am not exactly sure what happens, but I guess it is something like a spark through the material that makes it either melt or become a conductor. Either way, after applying the 120 volts, the extra wire allows the current to once again flow through all the other lights.

rallain Fri, 12/05/2008 - 12:32

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Amazing, I did not know about this topic up to now. Thanks.