In the spirit of the season: The physics of Christmas tree lights!

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)
  1. All the strings plug into standard 120 v  AC  US electricity
  2. The bulbs are in series, known as ‘when one goes out, they all go out’  more on this later, because sometimes it’s not quite true.
  3. Lights are resistors that heat up enough from the electrical flow to give off light (explained in the Secret Life of Machines).  Most of the electricity goes to heat – about 90%!
Ohm’s law stuff:
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:
  • A string of 100 lights is 100 equal resistors,  so the Voltage drop in each bulb is 120/100 = 1.2 v (unless it's really two strands of 50 acting like one strand)
  • A string of 50 lights is 50 equal resistors,  so the Voltage drop in each bulb is 120/50 = 2.4 v
  • A string of 35 lights is 35 equal resistors,  so the Voltage drop in each bulb is 120/35= 3.4 v, about
All the bulbs have roughly equal brightness, so the larger strings will have smaller resistors as bulbs  P=IV  = I^2R , so to get equal power the current is higher and the resistance is smaller. You’ve probably learned that you have to be careful replacing bulbs – sometimes the new one is brighter/dimmer than the original.  Since the 100 stringers have lower resistance bulbs than the 50 stringers, you have to match them up. A 100 stringer in a 50 string will be dimmer,  a 50 stringer in a 100 string will be brighter.  The voltage drop is proportional to the resistance:

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.
  1. You now have enough knowledge to make your own lights for class use.
  2. Save some bucks, recycle those old power packs from cell phones (usually 3 v), cordless phones (12 v), etc.   A 12 v  power pack will light up a short string of 8-10 ‘100 string bulbs’ at 1.2 v apiece, 4 to 5 of the “50 string bulbs”, etc.   They last forever – I used them behind a stained glass window 24/7 and they usually burned out in about a year. (Let’s see – at a quarter watt apiece and a dime a kilowatthour, you can run one bulb for a year for about 20 cents!)   Now you can make your own string of lights around your license plate – just put them into a clear plastic tube for protection from the elements.
  3. You can easily and cheaply show students that
    1. More voltage makes for a brighter bulb.
    2. 2 bulbs in series will make them both dimmer, since the resistance is bigger
    3. You can pass out enough to do series /parallel labs with batteries
    4. How a variable resistor works (light dimmer).   Make your own resistor by making a heavy pencil mark (wide, too) on a piece of paper, then moving a wire connected to a light bulb and battery – more length of graphite, more resistance, dimmer bulb. Circuit consists of a battery connected to a bulb, wire from other end of battery and other end of bulb goes to the graphite line.
  4. Want to do a battery activity?  A couple strips of metal and a lemon will make enough voltage to light your bulb.
Oh – the crack about sometimes they’re not all in series?   The ‘end to end’ strings of 100 have 3 wires.  2 wires carry the 120 v along to the end plug for the next string, and the 3rd wire has all the bulbs in series.  Sometimes the strings of 100 are 2 strings of 50 with a connector in the middle, so only half the bulbs will go out when a bulb is removed…  Take a look at one and see!

More material and “how the blinker works” (bimettalic strip, just like your wall thermostat) at

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!