Basic DC electricity question

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If you connect two 6V lamps in parallel to a 12V supply, you're quite likely to see the two lamps doing an excellent imitation of fuses -- especially "if the power supply is big enough".
--
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Doug Miller (alphageek at milmac dot com)
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On Thu, 15 Feb 2007 01:35:28 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I have done that before. They usually do not burn out immediately, but glow brighter (and whiter) for awhile.
Of course that is not what I was talking about before. Maybe you missed that what I said was an ALTERNATIVE to using 12V.
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Mark Lloyd
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?
I was responding to your suggestion, visible above, that "if the power supply was big enough" he could connect two 6V lamps in parallel to a 12V supply.
That won't work for very long.
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Doug Miller (alphageek at milmac dot com)
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On Thu, 15 Feb 2007 02:52:43 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

That just might have worked IF I had forgotten what I said, and you had edited my quote to say that. "12V" does not appear in that quote at all.

OK, I left something out (saying to use 6V, or clarifying what "big enough" means). That's no excuse to stick in something that doesn't belong.
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Apparently you *did* forget what you said, or at least you forgot what you were responding to -- which was the girl's belief that, to light two 6v lamps, she should connect them to a 12v supply.
*I* said that would work fine if they were connected in series -- which is true, as each lamp would see 6v.
*You* said "or connect them in parallel if the power supply is big enough".

I didn't stick *anything* in there, and you know it.
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Doug Miller (alphageek at milmac dot com)
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On Thu, 15 Feb 2007 19:07:45 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I do. That's something that would have helped make it look like I was saying to use 12V.
I put in the word "OR" at the beginning of the line in question. Did you forget what "or" means? This was something that would work INSTEAD of "series and 12V". Why assume I meant something other than what I said?
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You *were* saying to use 12V. Look at the sentences. OP said daughter thought you needed to use a 12V supply for two 6V lamps. I said not a problem if you connect them in series. You said "or in parallel."
And that's not correct. Do that, and you'll burn them out.
The discussion to that point had concerned itself only with 12V supplies; thus, in the absence of any qualifying clauses specifying a different voltage, subsequent statements must be taken as referring to the 12V supply under discussion. If you *meant* "or in parallel, using a 6V supply" then you should have *said* "... using a 6V supply."

I know perfectly well what "or" means -- it means you were suggesting an alternative. The flow of context makes it very plain that the meaning of your suggested alternative was to connect them in parallel to a 12V supply, as opposed to my suggestion to connect them in series.

I *am* assuming you meant *exactly* what you said. The problem is that you apparently didn't say what you meant -- if you *meant* 6V, then you needed to *say* 6V.
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Doug Miller (alphageek at milmac dot com)
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Mark,
If I may interject here ... The discussion was about connecting 6V lamps to a 12V supply. As you said, you didn't mention anything about changing the supply to 6V. It's perfectly reasonable for Doug to assume you were still talking about a 12V supply. End of story.
Respectful regards,
Mark
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wrote:

The discussion was about 6V lamps, "12V" was a later addition, which I posted an ALTERNATIVE to. Sorry for not making things clearer.
I didn't say 6V because it was ALREADY 6V, as appropriate for a 6V lamp (perhaps I underestimated people's lack of knowledge). As I did mention, it would have been better to have included that information anyway. I apologize for the mistake I did make, just not for the one I didn't.

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No, it was not -- the 12V is in the ORIGINAL POST.

Wrong again. The ORIGINAL POST was complaining that the subject had been explained so poorly in school that the OP's daughter thought she needed a 12V supply to power two 6V lamps.
I said that was fine, if they were wired in series.
You said "or in parallel, if the power supply is big enough."

The only lack of knowledge apparent here is yours. It's not quite clear whether that lack is in the difference between series and parallel wiring, or in reading comprehension.

You still haven't figured out which mistake you really did make, and I'm still waiting for your apology for accusing me of having put things into your post.

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Doug Miller (alphageek at milmac dot com)
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Here's one you might enjoy, or at least get sone ideas from. http://blogs.gotdotnet.com/mhop/archive/2006/12/19/building-a-traffic-signal-with-christmas-lights.aspx
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I knew that stuff by fourth grade, but then I wasn't aliterate (can read but won't).

Cumulative resistance in the circuit. Which end, is irrelevant.
In a series circuit, current is the same through all devices. Voltage is divided according to resistance. If all lights are the same, it will be divided equally.
With a 12V supply and 2 (identical) lights in series, each light will be operating on 6V. Current for this circuit will be the same as one light on 6V.

How about using both series and parallel lighting circuits?
When I was in 3rd or 4th grade, a relative gave me a book of simple experiments. The section on electricity used both series and parallel.
BTW, I remember how hard it was to find the "#6 dry cells" that book called for. Most people hadn't heard of such things. These are large round 1.5v cells, bigger than the ones they make lantern batteries out of.

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Should they be learning to hook up

Sure, why not? Ohm's Law or not, its still a good exposure to basic circuits.

My friend graduated from MIT at sixteen. I'll bet he was doing Laplace Transform for circuit analysis and wave equations when he was at the grade 6 age.
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That's one way to do it.

So teach her ohm's law.

There is no "far end" of a circuit unless you have long small wires creating resistance.

"3v LED" suggests LEDs with built in resisters. Remember LEDs are polarized - if you connect them backwards, they won't light. 2 3v LEDs in series should work fine off of 6V. Do that twice for 4 bulbs.

A crystal radio? http://sci-toys.com/scitoys/scitoys/radio/homemade_radio.html
Bob
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Oops - I missed the 2 C batteries. Just parallel the 4 LEDs to the 2 cell in series.
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wrote:

And none at all with SERIES circuit.

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Jacobs ladder. Then when the class is over you can make it into a super deluxe Taser.
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Aha! LED's, and not filaments. Please DISREGARD my post elsewhere in this thread, where I ramble on about what happens when a filament is supplied with 6V instead of the designed-for 12V.
I'm puzzled, in your original post you refer to a "12 V light". Is that referring to these four LED's, wired in series? But above here you are saying they are actually powered by 3V, indicating that they are in parallel. Without a clear picture of what is going on, I'm not sure what advice to give you.

You could buy some resistors and red or yellow LED's from Radio Shack (if you're in the U.S.). They'll also sell battery holders, and some wires with alligator clips. You can experiment with calculating the LED current for different resistors, and seeing how the LED dims/ brightens when you try different resistors. For a 2V red LED driven by 3V, some resistors to try are:
(3V - 2V)/(0.030 A) = 33 ohms (3V - 2V)/(0.010 A) = 100 ohms (3V - 2V)/(0.003 A) = 330 ohms
(Even if the LED says it is rated for 20 mA, driving it at 30 mA for 5 or 10 seconds will be okay.)
Do you have a voltmeter? You could show that the voltages across the resistor and LED always add up to equal the voltage of the battery (which you should also measure, don't just assume it's 1.5)
I admit these suggestions aren't as exciting as a model boat that lights up and has a running motor, but it is a starting point to start a 6th grader on learning some of the basics of electricity.
Regards,
Mark
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I should also mention, there is a book by Forrest M. Mims called "Getting Started in Electronics". Many of the regulars in sci.electronics.basics recommend it for beginners, though I myself am not familiar with it. It's available at Amazon: (Amazon.com product link shortened)71500440/ref=pd_bbs_sr_1/105-7945383-4366032?ie=UTF8&s=books
Regards,
Mark
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Put a speed controller on the motor. A simple rheostat (variable resistor) demonstrates how motor speeds can be changed.
Get various resistors. Demonstrate how the different color bands correspond to ohm numbers - and then measure them with the meter. Radio Shack once sold little color wheels. Dial in the color and read te number.
Reading the values and then measuring with the meter should teach something about how the real world 'varies' from what is should be.
Meanwhile some numbers for that light bulb. A 12 volt light operating at 6 volts will output about 1/10 the lumens and consume 33% of watts as compared to 12 volt power consumption. Obviously the bulb is less efficient at those lower voltages. At 6 volts, the bulb will last about 8200 times longer.
A 12 volt bulb at 9 volts will output 40% the light at 12 volts and consume about 60% of the wattage. Unlike Pop', numbers can be provided with replies.
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