Brain Teaser

How long could you power 3 light bulbs using only 210 watts?

[limbo]

OK not really a brain teaser, more a question for someone with a better understanding of energy and electricity than my own.

I am trying desperately to gain a hook line for a recycling campaign that revolves around the amount of energy saved by recycling 1 Alooo-minum Can. So far we have established it will power a 100 watt light bulb for 20hours, does that mean it will power 3 for 6.66 hours? (I'm guessing it's not that simple)

We know that the energy saved by recycling just one can is the equivalent saving 209 watts*, could that really power 3 100W bulbs for over 6 hours?

A mind with more power than mine is required...

*figures:
1kg of Recycled Alu = 14,000 Watts saved
1 can = 14.9g
1000g / 14.9g = 67 cans per kilo
14,000 / 67 = 209 watts per can

[trillianjedi]

100 watt light bulb for 20hours, does that mean it will power 3 for 6.66 hours? (I'm guessing it's not that simple)

If you ignore resistance then yes (which will create a minute difference), essentially yes.

Watts = Volts x Amps

Amp hours = Amps x time (hours).

Assuming US 100 volts (rounded down to make the math easier):-

100 watts = 100 volts x Amps

Amps = 1

You are producing 1 amp x (for) 20 hours = 20 amp Hours (aH).

You can divide that 20 amp hours up however you like, substitute 50 watt bulbs and see what happens:-

50 watts = 100 volts x Amps

Amps = 0.5

20 aH / 0.5 = 40 hours.

So your 50 watt bulb will run twice as long as the 100 watt bulb for the same amount of energy.

could that really power 3 100W bulbs for over 6 hours?

3 x 100 watt bulbs is a total power consumption of 300 watts.

300 watts = 100 volts * Amps

Amps = 3

20 aH / 3 = 6.66 hours

However, I think your original maths may not be correct. I suspect the 14,000 watts figure is a measure of joules per second, not "electrical" wattage which makes no reference to time.

So while you're correct in the way that you could run three 100 watt bulbs for one third of the time you could run a single bulb for the same amount of energy, your statement:-

We know that the energy saved by recycling just one can is the equivalent saving 209 watts*

..... is not referring to the same measurement as the "watts" you use for lightbulbs, electric heaters etc (which is "power"), it's referring to "energy" (joules per second).

So you need to convert your joules measurement to watt hours (or amp hours) before you can work out some real world examples.

You may have already done that, I'm just not sure if your maths there is correct.

Some good bedtime reading for you:-

[nmsea.org...]

;-)

TJ

[smellystudent]

We know that the energy saved by recycling just one can is the equivalent saving 209 watts*, could that really power 3 100W bulbs for over 6 hours?

You're a factor of 10 out.

I assume that your figures are measured in watt-hours. Watts are a measurement of power, not energy.

If recycling your can will save 209 watt-hours, then that will power a 100 watt bulb for 2.09 hours, or a 50 watt bulb for 4.18 hours, and so on.

Electricity used is normally measure in kilowatt-hours (kWh). I don't know what the US system is, but in the UK one 'unit' of electricity is actually 1 kWh, and costs roughly 10p. This should allow you to put a dollar value on your can :-)

[smellystudent]

As an answer to your mail question, 209 watt-hours will power 3 x 100watt bulbs for 41.8 minutes.

3 x 100 watt bulbs = 300 watts being used
209 watt-hours / 300 watts = 0.6967 hours
0.6967 x 60 minutes = 41.8 minutes

[limbo]

This place never ceases to amaze me :o

Cheers for both of your very quick, asute and informed conclusions. The Answer you gave TJ is a real lifesaver, I can now (armed with extracts of your working) present my concept without the fear of complete buffoonary.

Smelly Student - excellent explaination, I make to many assumptions, based on my figures I think it is safe to say we'd have had a few complaints. When you put the wrong info on bill boards people tend to take notice more than when you don't.

I know why I chose to draw pictures for a living, not work at a power station... I rather liken myself to Homer Simpson after today's lessons ;)

[trillianjedi]

You're a factor of 10 out.

I assume that your figures are measured in watt-hours.

I think it's more likely to be joules.

Kilo-Watt hours = Joules * 0.0002778

1 can (209 watts) is 0.058 kilo-watt hours

That will run a 100 watt bulb for 34.8 minutes, or 3 of them for 11.6 minutes.

We need clarification on joules or kilo-watt hours on the original figures, but it just sounds far more likely to be joules based on the numbers?

TJ

[httpwebwitch]

wow, this is related to a question I was asked this morning for an energy-related marketing project.

If I have an energy consumption amount in BTU, can I translate that into KwH?

What the heck is a BTU?

[olwen]

A BTU is a British Thermal Unit. It's the imperial equivalent of a Joule. Google for conversions. I found a useful looking one with a search on btu joule