|Don’t throw away that string of burned out lights when you can do some physics on them! Don’t have any? Ask your students to bring some in.|
Why use Christmas lights? They're dirt cheap or maybe even free. After Christmas you can find strings of 100 lights for a couple of bucks or you can find people too lazy to figure out why their lights don't work any more. I haven't bought light bulbs for physics labs in years! Just grab a string of Christmas lights and a pair of wire cutter/strippers and get to work. I also usually create lengths of wire from the strands as well.
How Christmas tree lights work:
(all these comments apply to the miniature light strings – the larger ‘night-light’ size work entirely differently – they are in a parallel circuit)
The total voltage drop of a string is 120 v, but I’m going to round that off because it doesn’t matter much. Resistors in series add.
A little bit of math leads you to:
Bulbs in series have equal current through all of them. (series, duh…) So V=IR or I=V/R, and a larger resistor will have larger voltage for equal current. Since the larger resistor has more voltage, it’s brighter!
One additional thing: all strings with the UL lab tag (some of them are counterfeit, from the land that puts lead paint on your kid’s toys) have a fuse in the circuit. It’s hidden inside the plug, generally; and is a pain to get at. When you buy a new string, you usually get a blinking bulb, a spare bulb or two, and a spare fuse (little bitty cartridge thing). When you exceed the recommendations by stringing about 6 strings in series, you can blow this fuse. I’ve only blown one in about 40 years, so don’t get your hopes up. When one bulb burns out in a string, the rest stay lit because the burned out one ‘melts’ together. Since that resistor is gone, the rest of the string will get a little bit brighter (and a little bit hotter and burn the next one out quicker)
Finding which bulb is loose: Generally, most people will shake the string to see if it relights, then zero in on the loose one. Determined people will test each bulb in the string for tightness and inspect to see if they stepped on one. Physics teachers will go to the electrical dept at Lowes and buy a ‘sniffer’, that will detect the e-m emissions from a wire, then go down the wire until the buzzing stops. About $10 or so. You can also remove bulbs and test the resistance or conductivity using a digital meter from Harbor Freight (about $3). If you’re hip to these, you can easily demo that to students.
Uses for dead strings:
Since a 100 string uses about 1.2 v each, a flashlight battery (1.5 v) will light one bulb up just about right. And 2 batteries will light up a 50 stringer bulb, etc.
More material and “how the blinker works” (bimettalic strip, just like your wall thermostat) at http://christmas.howstuffworks.com/christmas-lights.htm/printable
New LED lights: Coming thing, but pricey at $10-20 a string. (‘course, they’ll be half off AFTER Christmas). They are much more efficient than mini lights (less of that 90% heat given off). Wait a bit – next year I predict they’ll be $10, then $5…..In theory, they should be cheaper to manufacture, and they’ll never burn out. Cities across America are replacing their stoplights with LEDs and saving big bucks!