Tuesday, October 26, 2010

Making brains sprain, and doing so joyfully...

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Challenged students today: Measuring resistance of a headlight bulb filament, it shows 0.4 Ohms. Given a 12 volt power supply, Ohms law states the resultant current draw will be 30 amps, yet many vehicles protect the headlight circuit with a 10 amp fuse.
HOW?

8 comments:

TenMile said...

Resistance.

Crucis said...

However, most bulb filaments are coils so the standard formula for resistance does not apply (except for ball-park measurements.) You need to use the formula for impedance.

Hat Trick said...

The resistance is thermally dependent. The resistance will increase as the filament heats up and limit the current. The filament heats up faster than the fuse element does when the circuit is initially closed.

Carteach0 said...

Well done Hat Trick. That is the answer I want my students to come up with. I cover basic electricity the day before I challenge them with this, including factors of resistance. I can honestly tell them the answer is in their theory notes, although it's a higher order thought process to make the connection.

I get PAID to do this to teenagers. I LOVE my job.

Hat Trick said...

You're diabolical. :-)

Even having a degree in electrical engineering I had to think about it for a second. I think as a high school senior who was already deep into electronics as a hobby it would have been quite a stretch for me to come up with it.

Carteach0 said...

I had four students do it today, independently of each other, after only a few days of electrical theory and hands on testing. The task work I give them is multi faceted, and designed to make them question their own measurements at times.... at least.... if they are paying attention.

Tomorrow I introduce...(drum roll please)...Voltage Drop testing. That will set them back a little.

Hat Trick said...

Maybe I'm selling my H.S. senior self a little short.

I do think you're doing a good job getting your students to think if you had four of them get it independently.

Voltage drop testing shouldn't be much of a hurdle as long as you remind them that ohm's law applies to each discrete resistance as well as the total resistance of a circuit.

JohnMXL said...

Voltage drop, especially in cranking circuits, is something that I still see techs struggle with.

Shape those young skulls full of mush - I want to retire in a few years and want to have capable techs to carry on!