# Problem: How can American 110V light bulbs be used on a 240V grid?

1. Aug 26, 2014

### pstir2

1. The problem statement, all variables and given/known data
In America power is supplied at 110 V. In Australia power is supplied at 240 V, however American lights are routinely used in Australia.
a) How is this possible?
b) Calculate the power that a pair of 1000 W 110 V light would dissipate when plugged into a 240 V system in series.
c) Calculate the power that a 1000 W 110 V light would be required to dissipate when plugged directly into a 240 V system.

2. Relevant equations
V=IR
P=VI
P=(V^2)/R
P=(I^2)V
Total Resistance = R1 + R2 + R3 + .... + Rn

3. The attempt at a solution

I'm not sure if this is correct or even going in the right direction.

A) i'm not sure. Im not great at the conceptual side of this subject.

B)
Resistance of one bulb
P=(V^2)/R
1000=(110^2)/R
R=12.1

Total Resistance = R1 + R2
=12.1+12.1
=24.2

Power from outlet
P=(V^2)/R
P=(240^2)/24.2
P=2380.17W

Power Dissipated = 2380.17-2000
=380.17W

The two lightbulbs in series would need to dissipate 380.17 Watts of power.

C)
P=VI
1000=110I
I=9.09A

P=VI
P=240x9.09
P=2181.81W

Dissipated Power = 2181.81-1000
= 1181.81W

The lightbulb would need to dissipate 1181.81 Watts of power.

Resistance of one bulb
P=(V^2)/R
1000=(110^2)/R
R=12.1

Power from outlet
P=(V^2)/R
P=(240^2)/12.1
P=4760.33W

Power Dissipated = 4760.33-1000
=3760.33W

The lightbulb would need to dissipate 3760.33 Watts of power.

Last edited: Aug 26, 2014
2. Aug 26, 2014

### Orodruin

Staff Emeritus
What were your answers to a and b? How did you arrive at those?

3. Aug 26, 2014

### Zondrina

Those voltage values are RMS values I believe. Usually they are controlled by a series of transformers along long power grids.

4. Aug 26, 2014

### ehild

The power dissipated on a lightbulb is the whole electric power. It transforms to heat and light. The two bulbs in series dissipate 2181 W. A single bulb dissipates half of it, 1090 W which does not exceed the nominal value too much. The bulbs will not burn out. You can use two 110 V bulbs connected in series to the 240 V system.

This is wrong. The current is not constant, it is proportional to the voltage across the lightbulb. It is the resistance that can be taken constant.

Again, P=V2/R is the dissipated power. Do not subtract the nominal value.

Plugging in a single bulb to the 240 V outlet would make the bulb burn out as the power is more than four times the nominal value.

ehild

5. Aug 27, 2014

### Staff: Mentor

6. Aug 27, 2014

### ehild

I do not think that anybody really tries to connect two bulbs in series. But the lamps are usually equipped with a transformer and they can be used with both 110 V and 240 V bulbs.

ehild

7. Aug 27, 2014

### Staff: Mentor

Which type/s of lamps and bulbs would this be?

8. Aug 27, 2014

### ehild

I only guessed... I have seen such switch for 110/230 V supply voltage on other devices.

ehild

9. Aug 27, 2014

### Staff: Mentor

Hi pstir2. I'd be interested to learn where you got this question from. Can you give further details?

10. Aug 27, 2014

### pstir2

For "A" where did you get 2181W from. Can you show all calculations like I have.

11. Aug 27, 2014

### ehild

I copied your incorrect one. I meant 2380 W.

12. Aug 27, 2014

### pstir2

That's all the information that I have. This isn't a practical question for my house, car, etc. Its a question to go on a yr 11 Physics exam that I'm writing.

13. Aug 27, 2014

### Staff: Mentor

I see. It's just a made-up scenario.

Anyway, the highlighted part is not correct. Perhaps you might think of some way to rework it so as to not mislead.
As for a plausible response to (a), I have been stumped for an answer and waiting eagerly to see the expected answer here. The only answer I can come up with is that the American manufacturer produces a version with a 240V filament for the Australian market. I can't conceive of anyone routinely operating a series pair of 110V incandescents off 240V.

Perhaps you were contemplating more of a theoretical short-term emergency measure? That would be the go!

14. Aug 28, 2014

### Devils

As an Australian this is news to me! We use bayonet & edison plugs.

I woud presume if we are talking about a single inancdescent light bulb, the filament may support 110V, 240V or more. The brightness would vary with voltage.

15. Aug 28, 2014

### ehild

As I understood, incandescent lamp bulbs are banned in Australia. The other kinds of bulbs are expensive, and look ugly. So it is worth buying old-type bulbs somewhere else.

I saw an URL where it was explained how to use two US bulbs connected in series, but I could not open it, and can not find it any more. It needs some design, but it can be solved and I think, there might be people in Australia who did it, and sell such lamps, chandeliers...

ehild

16. Aug 28, 2014

### Staff: Mentor

Nope. Though I know some people believe that.

Incandescent bulbs are available in every supermarket! Even if none was available, people would import the right voltage bulbs from China, or Brazil, or wherever, certainly not mess around with 110V bulbs on the country's 240/250V supply.

I think the "emergency" proviso would endow a suitable ambience for this question on a test paper, though, as the idea does make people think.

Wait on ..... perhaps the examiner was thinking of that other Australia? The one with no kangaroos....you know, where they filmed The Sound of Music?? Maybe incandescents are banned there?

17. Aug 28, 2014

### ehild

Incandescent lamps are banned in the EU above power of 60 or 40 W. http://en.wikipedia.org/wiki/Phase-out_of_incandescent_light_bulbs It was done gradually, first the 100 W bulbs were withdrawn and then the 75 W ones. Even the 40 W and 25 W bulbs are rare and there are no frosted bulbs. You can still buy halogen lamps but they are much more expensive then the ordinary bulbs were, and they are sensitive of switching on and of. You can buy compact fluorescent light bulbs and LED-s. The first ones look ugly, their light is not good to the eye and if burnt out, you can not throw them into the litter bin, as they contain mercury. The LEDs are better, but still look ugly and they are very expensive.
I have my last frosted candle bulbs in my chandelier and if one of the eight burns off, I have to buy a new chandelier.

I do not know what you mean on the other Australia, as I know the Sound of Music was filmed in Austria. It belongs to the EU, and the old type incandescent lamps are banned there just like in my country, Hungary.

ehild

18. Aug 28, 2014

### Orodruin

Staff Emeritus
Just to enter the incandescent ban discussion, I think it is a good thing even if the LED lights look a bit worse. The main objective of a lamp should be to provide light and the incandescent light bulb is very inefficient at doing so. In fact, a German retailer tried to circumvent the ban by selling the incandescent bulbs as "miniature heating elements". With regards to price, the LED lights have a significantly longer life-time which more than makes up for the price difference in terms of better price/usage time. I admit that the first time I bought LED lights I was under the same impression, but it is essentially a one-time investment while you regularly have to replace incandescent bulbs.

For heating, there are more efficient ways of doing it than using the electricity directly. In my house, I am using a air-to-water heat pump. Living in Sweden, this has cut my utility bill in more than half. (You can imagine the electricity cost in Sweden in winter ...)

19. Aug 28, 2014

### ehild

I like the idea of "miniature heating element".
I think that cheap and well-working things should not be banned. If there are better lighting elements, people will buy them, but deprive them from the old ones is quite cruel. The new ones cost about 100 times more... Also the factories in Hungary which produced incandescent bulbs (very good ones, working quite many years, and not polluting the environment) had to be closed down.

Halogen lamps are available still . They are hot, get burnt quite soon, and all of them are clear, bad to the eyes.
I just do not understand the whole thing.

As for heating with some more efficient way, there are lot of people who can not manage it. Living in a small flat in an apartment house you do not have too many possibilities. Gas or electricity here.

ehild

20. Aug 28, 2014

### Staff: Mentor

I don't draw a distinction between the old incandescents and the double-bulb halogens, they both rely on an incandescent filament. It's the latter that are available in supermarkets, along with the lower power novelty incandescents. I, too, would have expected halogens to be intolerant of ON-OFF cycles, but car headlight halogens seem to live on and on, so I'm not so sure. The old cheap incandescents had been showing shorter and shorter lives before they were phased out, I assumed this was because high quality bulbs had been elbowed out of the market by the cheap-and-nasty Chinese import.