PHYS345 Electricity and Electronics

"What's wrong with the carbide lantern?"

Before Pat could utter a reply to Kim's question, darkness suddenly descended.

"Great! I've been begging my folks for a new lamp since the last time we went caving. Too late now! Let's fire up one of our candles."

It took a bit of rummaging through the pack to find one in the shroud of darkness. Finally ready, Kim struck a match but it immediately flickered out. This happened again with the second match, and again with the third. The reason suddenly occurred to Pat.

"You've gotta be kidding me -- I think the air is bad down here! The guidebook didn't say anything about that, did it?"

"Not that I remember. You know, my breath does seem a little short right now that you mention it. Think we made a wrong turn?"

"Don't know, but we need to get out of here right now. Shine your flashlight over here while I take another look at the map."

A minute passed while Kim rifled through the pack again.

"Bad news! I must have left the flashlight lying on the ground where we ate our lunch."

"You'd better be joking! We walked for four hours since we ate. We can't make it back there without a light."

"I'm serious. I remember taking it out to get the sandwiches. Guess I forgot to put it back."

"Never mind, I keep a small flashlight in my jacket. Heh, where is it? Damn him, my little brother is always stealing my #$%& stuff!"

"Calm down! Acting out won’t help anything!"

"OK. OK. You’re right. What do we have for a light?"

"Well, I do have a spare bulb for my lantern in my pocket. But I didn't bring a spare battery; they're so heavy."

"Do we have anything else with batteries in it? I've got my pager, but I think it only has one AAA cell."

"Weird, I've got my pager too. Why do we bring these along?"

"Good thing we did! But the bulb is for a 6 V battery. Do you think it will work with two AAA cells?"

"Even if it does, do you think it will burn long enough for us to get out? I'm starting to get worried..."

"Maybe we can use one at a time to make them last longer. It won't be as bright and we’ll have to walk slower, but maybe one at a time would be better. Heh, you had physics last term! You studied circuits. You had the lab. You got an A, right! So you must know about batteries. What should we do?"

The Exercise
Your lab exercise this week comes in two parts.

1. Your group will have available two AAA cells and the bulb described above. In the lab you will have available two multimeters, lengths of wire, and a choice of resistors. Make the measurements necessary to answer the questions in the preceding story. The thoroughness of your investigation will largely determine your grade for this part. You may wish to refer to the following online resources for supporting data.

2. Design a flashlight using materials that you might find in your pockets. Be creative - assume that you are not carrying a length of wire! Also, a switching mechanism would be a desirable feature. At next week’s class, your group will be given five minutes to assemble a working flashlight. And we may even turn out the lights!!!

Objectives
Continue developing skill in correct use of digital multimeter as voltmeter and ammeter.
Develop deeper understanding of simple flashlight circuits, their operation, and their limitations.

Additional Resources
Batteries:
Some Battery Characteristics
Primary Battery Types

As the Battery Goes, So Goes the.... from the NYTimes
Stretching battery life and saving money at Andy Baird's camera site

Speleology:
National Speleological Society (NSS)
Commander Cody Caving Club, local grotto of the NSS
The Virtual Cave, source of the images at the top of this page.
Important Caving Information, Safety, and Conservation, from the San Francisco Bay grotto of the NSS.