Friday, October 24, 2008

Ok, riddle me this.....

I hit up a few students today with this problem, as it came up during an electrical testing task (as designed.....).

In testing the filament of a headlight bulb, we found .3 ohms of resistance.
With an assumed battery voltage of 12v, and using Ohms formula for amperage (I=V/R), we find the bulb has a current draw of 40 amps.

BUT.... we protect that headlight circuit with a 10 amp fuse, and run it for years like that.

How? Why?


Anonymous said...

Why? Because a hot filament has a much higher resistance than a cold filament. So the current decreases VERY quickly once current flow commences.

The rule of thumb is a 7 times increase, but sometimes the thumb is longer than others. So it depends on the filament.

Shrugged says: said...


Perfectly correct. The fun part... a former student stopped in to say Hi in the middle of this, and he had the answer down pat, but kept his mouth shut till he could tell me with no one around. :-)

My job is to make students brains sweat, as best I can, without causing permanent damage.

Shrugged says: said...

I should mention... in their class notes from the first lesson on basic electrical is this gem: A conductors resistance changes based on many factors, including ......and.... and.... and temperature... and....

That is the only clue I will give them, past "go check your notes".

I figure, if I can get them used to thinking their way through a problem, then I've given them something useful that will last a lifetime. Some of them will wind up far better at it than I, but that's Okay with me.

I'm regularly reminded by my closest and dearest friends that I'm a complete idiot at times.

aepilot_jim said...

Wait till you sit next to an 80 Amp fuse. It's on both breaker panels and not only will the entire cockpit go black but I suspect that knee will never be the same if it lets loose.

Shrugged says: said...

To the Anon commenter who pointed out my mistake in the post.... thank you and I appreciate the help. On the down side, I hit the wrong button and erased your comment :-(

Steve said...

Thanks for fixing that. I have a EET degree, and I was going bonkers wondering how in the world you got 42 amps for the original values.

I was like, okay, I've been out of school a while, but I still remember Ohm's Law.