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### Author Topic: Challenge from my professor  (Read 1127 times)

#### Fatdo13

• Newbie
• Posts: 1
##### Challenge from my professor
« on: July 16, 2018, 03:45:24 AM »
So my lecturer for my electrical class at uni occasionally hands out challenges “to stimulate the mind and get you students to think outside the box”. A few weeks ago he gave out a challenge I found particularly interesting:
Find a way to set up a basic electrical circuit that allows you to power a single light bulb. Then, with the same circuit, replace the single light bulb with 10 others, and power them simultaneously.

Rules:
You cannot use a variable resistor.
Each light bulb must be identical.
The power source must give out consistent power.

This is my first semester at uni, and I feel like my knowledge is limited here. The way I see it is that if you can power 1 light bulb, you don’t have enough power to light up 10. But if you can light up 10, then that power would pop 1 light bulb. For that reason surely you can’t just use a battery. I enjoy a brain teaser but this seems impossible. Without a variable resistor or power variance, how can you power two circuits with different loads?
I assume the 10 bulbs must be in parallel but that’s all I got. I feel like the key is in the motor (can’t be a battery right?). Is there a special alternator that I can use to accomplish this? Or is this guy a smart but crazy old man? Any help would be much appreciated.

Note:
I had a classmate come up with something. He had handmade solenoids (weighing 60grams) set up in a formation similar to what you’d find in a computer cooling fan. Using that as an alternator (he had a small motor powering it with a 12V battery), plain copper wires and LEDs from jaycar he was able to answer the challenge.
He showed us using a multimeter voltages of 1.5V for a single LED and 1.3V for 10 LEDs, while the current was sitting at 0.11A for both setups. All lights illuminated, although the set of 10 in parallel were dimmer. Now, when he attempted using handmade coils with more windings (weighing 120grams so it was significantly larger), he achieved up to 5V and 0.1A. But he said that popped the single LED and failed to illuminate the 10 LEDs. He claimed the large coils were able to illuminate up to 8 LEDs, anything more and no light. Could I please get a possible explanation for this? Shouldn’t there be enough power to illuminate the 10 LEDs considering the smaller one was able to, AND there was enough power to pop a single LED whereas the other did not?

FYI talks of an interaction with Lenz's Law and Back EMF are taking place. Every time I research about bypassing Lenz's Law or anything related to energy efficiency, this website is always on the list, so I thought someone on here could shed some light and help me out in understanding what's happening here. Again, any help would be much appreciated. Thanks