Lighting circuit Power

[BJ Conner]

Since it's quiz day try this.

Ole has to light the hen house. he only has 240 volts (no neutral wire so it's not a 120/240 volt system) available. He decides to wire two sockets in series and use 120 volt bulbs.

He uses a 75 watt and a 100 watt bulb, how much power does the circuit use??

 

[Rick Christopherson]

171 watts. Good question.

 

[gar]

100802-1028 EST

Rick:

My guess is that your answer is quite different than what actually would be measured. In fact the 75 W bulb might not last long.

I do not want to destroy my only 75 W bulb so I ran the experiment with 3 bulbs. For this problem the easiest way to get an answer is experimentally.

The series sequence is 100 W, 25 W, and 25 W across 246 V. The individual voltages were 7, 119, 119 volts. The series current was 0.21 A. Calculated input power 51 W.

The normal current to a 100 W bulb at 120 V is 0.83 A. As current drops the power dissipated drops, the filament temperature drops, and thus the filament resistance increases.

The above experiment self equalized at 0.21 A. Power dissipated in the 100 W bulb was 7 * 0.21 = 1.47 W and the resistance was 33 ohms. At room temperature it is in the range of 9.5 ohms with no current flow.

This is why Hewlett and Packard used a small tungsten filament lamp to stabilize the output amplitude in their original oscillator.

.

 

[mull982]

The normal current to a 100 W bulb at 120 V is 0.83 A. As current drops the power dissipated drops, the filament temperature drops, and thus the filament resistance increases.

.

Dont you mean to say that the filament resistance decreases with a drop in temperature?

I also come up with 171kW from a calculated value.

 

[steve66]

I also come up with 171kW from a calculated value.

Boy, I'd hate to have to pay the bill to run those two lights :grin:

Just kidding because I think you have an extra "k" in there.

 

[LarryFine]

He uses a 75 watt and a 100 watt bulb, how much power does the circuit use??

None after the 75w bulb burns out.

 

[gar]

100802-1534 EST

mull982:

You are correct. Yes, for tungsten the resistance drops with dropping temperature. It is easy to get words mixed up and that is why it is always good to question something that looks wrong.

Thanks.

.

 

[BJ Conner]

Mr Fine wins the prize

Mr Fine wins the prize

Zero is correct.
The prize was a dozen eggs but we can't see in the henhouse to get them.
I am assuming you have seen that before. The first time I missed it Moby Dick was a minnow.

 

[hillbilly1]

To get nit picky like we always do here on this forum, you didn't mention the voltage of the bulbs, what if the 75 watt was 130 volt, and the 100 was 120?:grin:

 

[hillbilly1]

I went back again and looked, you did say 120!:roll:

 

[LarryFine]

Zero is correct.
The prize was a dozen eggs but we can't see in the henhouse to get them.

A real electrician would install a 240/120v transformer and parallel the bulbs.

I am assuming you have seen that before.

Actually, I haven't. I'm just that good.

 

[Dennis Alwon]

Here's another teaser.

Three switches in a basement that control a light on 1st, 2nd and 3rd floors. Each switch controls a light on a particular floor but we do not know which switch controls which floor.

You cannot see the lights from the basement however you may make one trip up the stairs and you may only enter the room where the lights is on 2 floors- your pick.

You can turn the switches on and off as many times as you want but again only one trip up. How do you determine which switch controls the light on which floor.

You can carry as many tools as you want. Assume all lights function properly and are 100 watts and 120V.

So- How do you determine which switch does which floor?

 

[charlie b]

Zero is correct.

No it isn't. And Larry did not win the prize. I had thought of that same answer right away, and choose not to submit it. Here is the problem with this problem:

Suppose I change the situation. You have a 120 volt circuit in the henhouse, and you wire two fixtures in the usual (i.e., parallel) way. You put in the same types of bulbs from the original question (75 and 100 watts). So tell me, how much power will this circuit draw? 175, you say? Not so I says. The correct answer is zero watts, after both bulbs burn out.

However, in my version, and in the original version, there will be some amount of time during which no bulb has yet burned out. We don't know how long that will be in either case. In the interim, the circuit will draw some amount of power.

I have not done the math. But Rick's answer of 171 was confirmed by another member. So I say give the eggs to Rick!

 

[LarryFine]

So- How do you determine which switch does which floor?

Turn on two of the switches, wait 5 minutes, and turn one back off.

Now, you'll have three states of bulb: on, off and cool, and off but warm.


Next?

 

[charlie b]

I just sent Dennis the solution by PM. I didn't want to post it here until the other members had a chance to work it out. I still win, Larry! :grin:

 

[LarryFine]

No it isn't. And Larry did not win the prize.

So I say give the eggs to Rick!

Eggs? We don't need no stinkin' eggs!

 

[LarryFine]

I just sent Dennis the solution by PM. I didn't want to post it here until the other members had a chance to work it out. I still win, Larry! :grin:

Yeah, but I can prove when I posted my answer. We only have your word for it.

Besides, where is the "Contest not open to Moderators and their families." statement?

 

[Dennis Alwon]

I just sent Dennis the solution by PM. I didn't want to post it here until the other members had a chance to work it out. I still win, Larry! :grin:

Well I will say that you both win but Charlie at least gave the others a chance.

Actually Larry's post was one minute sooner than your post Charlie but his didn't take as long to type. :grin:

 

[gar]

100802-1852 EST

charlie b:

It won't be 171 W. It will be less. Maybe 150 W. As I previously said I do not want to damage my one 75 W bulb. So to provide a closer estimate I connected a 100 W 120 bulb in series with a 60 W 130 V bulb. With 246 V input the 60 W had 197 V and the current was 0.57 A. Power to the 60 W bulb was 112 W, total power was 140 W.

.

 

[LarryFine]

Well I will say that you both win but Charlie at least gave the others a chance.

My apologies. Answers PM'ed from now on.

Actually Larry's post was one minute sooner than your post Charlie but his didn't take as long to type. :grin:

Possible responses:

A) Hey, if I can use fewer words to make a statement . . .

2) Pretty good, considering I edged out a moderator, who may even get post notifications.

D) I still won!