# Draw schematic: (3) 1.5 V Batteries (4.5 V), 3-way selector and three lamps . . .

Joined Sep 4, 2016
3
Was hoping someone could confirm my logic.

The problem is worded:
Using 1.5 V batteries, a switch, and three lamps, devise a circuit to apply 4.5 V across either one lamp, two lamps in series, or three lamps in series with a single control switch. Draw the schematic.
My solution is the following:

Without any details re: the specifications of the lamps, I can't calculate any resistances and therefore no measure of current. (I think?) Therefore, I am unable to calculate a fuse value.

Anyone with a more experienced perspective, have I fully answered the question? Given the minimal details, is my schematic acceptable?

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#### AlbertHall

Joined Jun 4, 2014
12,328
I think that would do. Except the question doesn't say anything about a fuse so I don't think you should include one.

#### WBahn

Joined Mar 31, 2012
29,871
I agree regarding the fuse, but it is a good sign that you thought to include one.

The question didn't say what kind of switch you could use, so are you sure that the one you did use is acceptable? My guess is that it is.

Joined Sep 4, 2016
3
Thank you for the quick replies.

The question came from the text book without the suggestion of a 3-way switch, but when the professor put it up on the screen, he offered the following symbols as what we needed to use for the schematic:

As for the fuse, early in the semester, he presented a circuit without one and asked what was wrong with the circuit. All elements of the circuit were correct. Voltage, Resistance and Current all worked out. The class was stumped. With dramatic frustration, he pointed out that the circuit was "unsafe" due to the absence of protection in the circuit. My thinking is that I'm erring on the side of caution by including it and if he docks points for it, I'll refer back to his earlier lesson and hope that justifies the inclusion.

Definitely a relief to get some feedback. Thank you, again.

Any additional advice, suggestions or ideas are very welcome. Specific to this question of just dealing with professors, in general.

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#### WBahn

Joined Mar 31, 2012
29,871
The point he was making about the fuse earlier was a reasonable point to make, as far as it goes. People, particularly students, tend to overlook issues like fusing and so it's nice to see an instructor emphasizing it. But hopefully he also established a context for that emphasis -- I doubt he would have maintained that a typical old-fashioned flashlight is unsafe because it doesn't have a fuse.

Joined Sep 4, 2016
3
A valid clarification. I'll certainly keep it in mind.

The professor has a rather thick Korean accent, and if that context *was* established, it was lost in translation. ;-)

Looking back in my notes at the circuit he was referencing, it had a 100 V source and a rheostat, which could feasibly be accidentally set to 0Ω and short the circuit. So, absolutely, a fuse would be recommended/required. A much more practical example for making the point.

Now I'm second guessing the fuse for the homework question. The point that it was not included in the list of available components has me leaning towards not including it. The validity of your clarification is pushing me even further to delete it. I have until Monday to make a final decision.

One course of reasoning I have for including it is not for safety in the physical sense, but more towards protecting the components to reduce the potential for blowing bulbs unnecessarily. I'm just not sure if that's a reasonable or even legitimate concern.

I'm sure it's understandable the regret (kick myself) I'm trying to avoid.

Thank you again for the reply and great insight. I appreciate you taking your time to help out.