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QUESTION: I am not clear on what different electrical measures are. I was wondering. For a person who weighs 200lbs (1lbs = 453.592grams)how long could they keep a 100 volt light bulb burning? Assuming the 200lbs is measured at sea level making weight and mass the same thing. I assume volts are actual energy? When I hear terms like watt hour I get confused. Also lightyear is upseting too. lol. What measure simply means energy that is released?

ANSWER: A volt is a measure of the difference in potential energy at two different locations per unit of charge. For example, a 6 volt battery has electrons on the negative terminal that have 6 more Joules of potential energy (per Coulomb of charge - 6.25 x 10^18 electrons) than the electrons on the positive terminal. Since they have more potential energy they can do work (in fact 6 Joules of work per Coulomb of charge) as they are allowed to return to the positive terminal. where they then again gain energy from the battery only to repeat the process.
A kilowatt hour (kWhr) is also a unit energy where 1 kilowatt hour is equivalent to:
1 kWhr = 1000W *1hr = 1000J/s*3600s = 3,600,000 Joules of energy. Most middle class homes use about 1000 kWhr per month, on average.
As for a light year cyr, a light year is the distance that light moves in a year. Since the speed of light is 3.0x10^8m/s the corresponding distance will be:
1 cyr = 3.0 x 10^8m/s * 1yr = 3.0 x 10^8m/s * (365*24*3600s) = 9.5 x 10^15m
As for how long a 200 kg person could keep a 100 Watt bulb lit is a question that would need to be answered by a physiologist, not a physicist.

---------- FOLLOW-UP ----------

QUESTION: Why would I need a physiologist? I am talking about if a person of 200lbs was converted into pure energy. I'm not talking about their bodies' energy efficiency. Their body would be no more. They'd become energy from every atom. Shouldn't a physicist be able to calculate this just from the mass in grams?

Sorry, I misunderstood. It did not occur to me that you wanted to completely convert the mass of the person into pure energy.
In any case, since the weight of the person is 200 pounds and since 1 pound is 0.454kg the mass of this person is:
m = 200lb x 0.454kg/lb = 90.8kg
According to Einstein's equation this mass is equivalent to:
E = m*c^2 = 90.8kg*(3.0 x 10^8)^2 = 8.17 x 10^18Joules
Assuming that the light bulb is a 100Watt bulb where:
100Watts = 100Joules/sec
This total mass would run the light bulb for:
t = 8.17 x 10^18J/100J/s = 8.17 x 10^16 sec = 2.27 x 10^15 hours = 9.46 x 10^13 days = 2.59 x 10^11 years = 18.9 ages of the universe


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James J. Kovalcin


I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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