# Why doesn't a light bulb draw 100 amps?

## Summary:

Why doesn't a light bulb draw 100 amps?
I am asking this question in behalf of my father ( who does not have an account here to ask this question himself).

My father said he does not understand why a simple light bulb in an AC circuit does not draw far more amps than a simple light bulb draws. He thinks that a typical light bulb only draws something approximately like a tenth of an amp. His intuition would make him guess that a light bulb would draw far more amps than the light bulb needs, but a simple light bulb only draws something like a tenth of an amp. Why doesn't a light bulb draw 100 amps?

hutchphd
Homework Helper
The filament in a light bulb is actually a rather long piece of thin tungsten wire carefully coiled round. At room temperature the resistance of this wire is not large and the filament wants a lot of current but this makes it heat up. As it heats the electrical resistance increases a lot. Tungsten has a very high melting temperature and it reaches an equilibrium Resistance at a Temperature less than its melting temperature. For a 100W bulb this hot resistance is somewhere near 100 ohms and so it draws about 1 Amp. That's the physics. You can put in the exact numbers if you care.

mpresic3, etotheipi, fresh_42 and 1 other person
phinds
Gold Member
To put what hutchphd said into formal terms,

P(ower) = I(current)**2 x R(esistance),

so 100 Watt = I**2 x 100 ohm

and thus I**2 = 1 and thus

I = 1 (amp)

davenn and hutchphd
phinds
Gold Member
Summary:: Why doesn't a light bulb draw 100 amps?
It would if it were a 10,000,000,000 watt bulb with a 1,000,000 ohm filament (this is equivalent to stringing 10,000 100watt bulbs together and supplying 100,000,000 volts across the string).

Keith_McClary
Gold Member
If you look at a filament closely it looks like a helix, but under a microscope it is a "coiled coil".

Дагесян Саркис Арменакович, Wikipedia
For a 60-watt 120-volt lamp, the uncoiled length of the tungsten filament is usually 580 millimetres (22.8 in),[61] and the filament diameter is 0.046 millimetres (0.0018 in).
The cold resistance of tungsten-filament lamps is about 1/15 the resistance when operating. For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms).
I just measured the (cold) resistance of a 40W 120V bulb as 22 ohms.

mpresic3, Perfidium, hutchphd and 4 others
russ_watters
Mentor
My father said he does not understand why a simple light bulb in an AC circuit does not draw far more amps than a simple light bulb draws. He thinks that a typical light bulb only draws something approximately like a tenth of an amp. His intuition would make him guess that a light bulb would draw far more amps than the light bulb needs, but a simple light bulb only draws something like a tenth of an amp. Why doesn't a light bulb draw 100 amps?
Others gave numerical answers about what a light bulb actually does, but I'm just plain confused by your question. Why does a light bulb draw what a light bulb draws? Why is blue blue? Why is 1 1? Why does a duck quack like a duck? What is the meaning of is (is)? As asked, the question makes no sense. There must be a thought process behind it that you aren't (he isn't) telling us.

In order to adequately answer the question, I think you need to tell us why he thinks a light bulb should draw more than a light bulb draws. Why should 1 = 5?

sophiecentaur, NTL2009, DaveE and 1 other person
DaveE
Gold Member
Others gave numerical answers about what a light bulb actually does, but I'm just plain confused by your question. Why does a light bulb draw what a light bulb draws? Why is blue blue? Why is 1 1? Why does a duck quack like a duck? What is the meaning of is (is)? As asked, the question makes no sense. There must be a thought process behind it that you aren't (he isn't) telling us.

In order to adequately answer the question, I think you need to tell us why he thinks a light bulb should draw more than a light bulb draws. Why should 1 = 5?
Yup, this.
I spent several years working with a light bulb (OK, laser actually) that drew 65A at about 600Vdc. It all depends on the design of such things.

berkeman, Keith_McClary and russ_watters
Baluncore
Why doesn't a light bulb draw 100 amps?
The filament will not draw 100 amps if the filament bulb designer does not want it to draw 100 amps.

The operating voltage is very important in deciding the resistance of the filament needed to avoid it fusing immediately.
Say 230 volts, 100 watts, the operating current will be 100 W / 230 V = 0.435 amp.
The resistance of the hot operating filament will be 230 V / 0.435 A = 530 ohms.

But filament resistance is proportional to absolute temperature, (which helps regulate current).
The cold filament will be at about 293 K, while the hot operating filament will be near 2930 K.
That is a factor of ten change in absolute temperature, and also in filament resistance.

The cold filament will therefore have about one tenth of the hot filament resistance.
When turned off, the 530 ohm hot filament will come down to about 53 ohms.
At the instant the light is turned on, the filament current will be 240 V / 53 ohm = 4.5 amps.
Then as the filament rapidly heats, resistance increases by a factor of 10, and the current falls to the designed 0.435 amps, for 100 watts.

Now what about a 12 volt, 120 watt filament globe ?
Operating current will be 120 W / 12 V = 10 amps.
Cold start current will be 10 times hot = 100 amps.
Which shows that some filaments will draw 100 amps sometimes.
A 12V globe running on 100 amps continuously would radiate 1.2 kW.

It would if it were a 10,000,000,000 watt bulb with a 1,000,000 ohm filament (this is equivalent to stringing 10,000 100watt bulbs together and supplying 100,000,000 volts across the string).
100 amps times 115 volts is 11500 watts, so for 100 watt bulbs wired in parallel, 115 of them would draw 100 amps.

timmeister37 said:
Summary:: Why doesn't a light bulb draw 100 amps?
In the US, house outlets supply current at about 115 volts. The circuits are usually limited to 15-30 amps of current by fuses or circuit breakers that will blow (fuse) or trip (breaker) if more than their rated amperage is demanded by whatever is plugged into the outlet. Given that watts = volts times amps, 100 watts at 115v draws about 0.87 amps.

phinds
Gold Member
100 amps times 115 volts is 11500 watts, so for 100 watt bulbs wired in parallel, 115 of them would draw 100 amps.
True. I decided to use series as an example because I knew the numbers would be outrageous.

His intuition would make him guess that a light bulb would draw far more amps than the light bulb needs, but a simple light bulb only draws something like a tenth of an amp.
This question is really not clear, and for an unclear question any answer would just make thinks worse.
I think he should just challenge his own intuition and work some more on that question first. A good question is always half of an answer

FactChecker and NTL2009
.Scott
Homework Helper
My guess is that @timmeister37 father measured the resisstance of the bulb and found it to be fairly low - perhaps about 1 ohm. With "A/C" being 115, he would then expect it to draw about 100 amps. Of course, it does draw that much current - but only when it is cold and that only lasts milliseconds. Once the filament is heated, the resistance soars.

mpresic3, Delta2, Vanadium 50 and 3 others
Staff Emeritus
It's a pity the OP has decided not to participate in his thread.

The question asked is "Why?" and the answer is that nobody wants to buy a bulb that produces 1000x as much light as needed and costs a hundred dollars a day to operate. So it should be no surprise that the light bulbs in your house don't do that.

What people seem to be answering is "How?" (which might be what the OP wants to know). @.Scott answered it well: the resistance goes up as the filament heats up. This makes the bulb self-regulating, which was a key factor in its adoption back in the day. If the bulb gets a tiny bit hotter, the resistance goes up, the power drops, and the bulb cools. If the bulb gets a tiny bit colder, the resistance goes down, the power increases, and the bulb warms up. So the bulb stays at a constant T, which means a constant R and a constant P.

russ_watters, .Scott and phinds
.Scott
Homework Helper
If the bulb gets a tiny bit hotter, the resistance goes up, the power drops, and the bulb cools. If the bulb gets a tiny bit colder, the resistance goes down, the power increases, and the bulb warms up. So the bulb stays at a constant T, which means a constant R and a constant P.
Or, it would if it was driven with DC current instead of AC.
Incandescent (Tungsten) bulbs are referred to as "Lamp Loads" - in contrast to Ohmic or inductive loads.

Last edited:
The question could be driven by the erroneous assumption that the filament seems like a bare piece of wire thereby making a short-circuit and producing very high current flow. What the answerer explained was when cold it's relatively conductive, but is only like that for a short period of time. When hot, it's a resistor and that limits current. It may also explain why incandescent bulbs blow out just as your turn them on when the high-current surge goes through the failing filament.

sophiecentaur
Gold Member
2020 Award
seems like a bare piece of wire thereby making a short-circuit
"seems like" is not a description that suits any Physical discussion.A short circuit is only a conceptual idea which makes it possible to get reasonable answers out of circuit calculations.

No pieces of tungsten have anything like Zero Resistance. Resistance can be as low as you care to make it by using a short enough and fat enough piece. The last thing you can afford to be is 'intuitive' when it comes to Electricity (or any other Physics, for that matter). It's actually a bit disrespectful of the topic. No one managed to make Electricity do what they wanted until they learned that they absolutely have to do some formal sums first.

If anyone's Dad starts to get lairy about Science (and they sometimes do) they just need to be given an elementary text book. The only valid answers to that sort of opinion involve demanding the 'dad' character makes the effort of learn basics.

My Grandad embarrassed me deeply with his level of ignorance of Science (he expected to hear the Satelloon passing overhead) - but I was a teenager at the time. I forgave him, of course.

I am afraid I may be highjacking the thread here. The OP asked a question in place of his/her father who did not have an account. The question was answered (to a large degree) by the next poster, who did a good job. Actually, I did not know this answer, and I was satisfied with the answer at this point. I think the OP would be too. If more depth was needed, the OP would likely come back and ask for it.

There were several more replies, some fanciful about a million lightbulbs, etc. The number and tone of these replies (at least I felt) were over the top. Vanadium suggested the OP was not following the discussion, and I can see why this feeling would be justified. I do not think the physics forum should be a place where a simple question should be met treating it in a glib, or hostile manner. This brings me to the last note by centaur.

Sophiecentaur, your tone is more hostile than the rest, and I could not let it go by without comment.

I know I was told in school the lightbulb filament has very low resistance, what is wrong with the logic that hooking the lightbulb up to 110 V would deliver a lot of amps. "Seems like" is appropriate for any discussion. Hooking a battery directly with a piece of wire is likely to destroy the battery, and in general parlance this is known as a "short circuit". Forgive us, if the wire really had some resistance and the circuit was not "perfectly" short in the mathematical sense.

As far as the idea that, "No one managed to make Electricity do what they wanted until they learned that they absolutely have to do some formal sums first.", millions of people do just that.

Personally, when I was 8-9 years old, I had wires controlling electricity, lighting lights, ringing bells, motors running with cars racing at variable speeds along tracks, without doing formal sums first. I suspect others have too.

Maybe I missed something, but when a poster asks something on the forum with a reasonable question, he or she is entreating us to help with the answer. I know in many posts, the answer they are given is, read wikipedia, or read an elementary textbook. This is really unnecessary. I can envision many friends and relatives that would wonder about the same question as the OP and would have been satisfied early on with the answers without being told the answer requires days weeks or months of study.

Now I know that I personally once answered a post on this forum with impatience, and I let it show. I suspect SCentaur did the same. I think many earlier posts by SC were helpful and look forward to later ones.

hutchphd
@sophiecentaur, I find it amusing that you synecdochically call mathematics 'sums' ##-## @mpresic3, please read some more of the content posted by @sophiecentaur here on PF ##-## I think that if you were to do that, you might thereby be led to a less disdainful set of impressions.

hutchphd
I will do sysprog. As I said, I have occasionally expressed impatience at a post, myself at one time or another. I should not have judged Sophiecentaur, and I should have limited my criticism to the response alone. I hope Sophiecentaur will not judge me on commenting quickly and harshly.

That matter aside looking at other folds in the forum

Other replies I have seen also referred the poster to wikipedia or elementary textbooks for answers. This is sometimes appropriate, but sometimes it is unhelpful and unmotivating.

Today, Thanksgiving, I wanted to find out how long to cook a turkey. It was easier to ask my sister on the phone, than to read a cooking recipe. Hutchphd did a really good job explaining that the resistance of the filament increases as it heats up. And he added the tease to motivate further investigation

That's the physics. You can put in the exact numbers if you care.
[/QUOTE

sophiecentaur
hutchphd
Homework Helper
I appreciate the the kind words. Let me ad to the chorus who finds @sophiecentaur's remarks interesting and typically pedagogically astute

sophiecentaur and sysprog
sophiecentaur
Gold Member
2020 Award
Sophiecentaur, your tone is more hostile than the rest, and I could not let it go by without comment.
Well yes it was. But I was being "hostile" on the OP's behalf. His Dad was the one who needs some 'redirection' in his attitude.

Cheers, the rest of you for allowing me to be a bit over-grumpy at times.

sophiecentaur
Gold Member
2020 Award
Other replies I have seen also referred the poster to wikipedia or elementary textbooks for answers. This is sometimes appropriate, but sometimes it is unhelpful and unmotivating.
+1
There is nothing less helpful than a post that consists just of a link. People who ask question on PF come from a wide variety of backgrounds and some sort of conversation with posters is necessary if we want to come up with an appropriate (helpful) reply. A lazy question will always be revealed eventually.

mpresic3
Well yes it was. But I was being "hostile" on the OP's behalf. His Dad was the one who needs some 'redirection' in his attitude.

Cheers, the rest of you for allowing me to be a bit over-grumpy at times.
I understand what you are saying a bit. But we should not make too many assumptions about "fathers".
My father was born in 1920, and fought in WWII. When computers came out and Wikipedia, he was already very elderly. My father and uncles would ask me a question, (being a physicist), and it would have been purposeless to tell them to look elsewhere.
Being able to answer their questions was one of the reasons they were so supportive of educating me.
They explained when WWII ended, the GI bill was generous in allowing them further schooling, (and many of that generation went) but they were now in their mid to late 20's and wanted to get married and raise a family. We all have to make choices.
That maybe why I was sensitive to the thread