I'm sitting here wishing my phone would charge faster. Somehow I forgot and now I face a couple of hours of recharging time. The frustrating thing is how much energy is being transferred to the nearly depleted battery.
My iPhone 4 has a 5.25 watt-hour capacity battery - the power rate for charging is only on the order of a couple of watts. By comparison a night light is usually a watt or two.
Too bad I can't crank the power to a kilowatt or two and charge the phone very quickly - at a kilowatt you could charge the iPhone in less than twenty seconds.1 The power outlets in your home can provide up to about 1,800 watts, so this is easy in theory, but the little battery in the phone can't absorb this much power and would catch on fire or possibly explode.
Many people don't have a good feeling for watts and watt hours, but energy is just energy and power is just power, so you can look at them in other units. Most of us are comfortable with nutrition Calories - we generally have to eat about 2,000 a day to stay alive. A candy bar is about 250 Calories, a piece of bread about 150 and so on ... Getting down to the scale of the iPhone's battery a useful unit is the M&M at about 3.5 Calories or approximately 4 watt-hours.2
Wouldn't it be sweet if Apple could make an iPhone that ran on chocolate or some other food? Even if this food fueled battery (a fuel cell of sorts) was as inefficient as human body, we'd only use 7 of the candies.
Melts in your mouth, powers your phone...
But what about the power rate a snack involves? A bag of M&M's is about 250 Calories or about 290 watt-hours. Say it takes you a minute to dispatch a bag, the rate of energy transfer is 290 watt-hours * 60 minutes/hour or 17,400 watts - 17.4 kilowatts.
Now we're transferring power! This is about ten times the power a 120 volt 15 amp line will provide before the circuit breaker trips to protect it - also what the average electric range burner uses. And that's just you with a small bag of M&Ms.
Many of our foods are basically hydrocarbons, which are wonderfully dense energy stores. We can get away with sustaining our roughly 2.4 kWh daily energy need in a few minutes of meals a day should we choose.3
That's impressive - the average home in the US uses an average of about 1 kilowatt of power. It may burst to 5 or 6 kilowatts for brief periods, but the average is only ten times higher than the average power requirement of a person. A family sitting down to dinner can have a power rating that would probably blow out the local line transformer if we used electricity rather than food.
Our vehicles have much greater power requirements that we do and gasoline stations have amazing power capacities - fueling your car turns out to be 11 million watts!4 A busy gas station may be serving ten cars at a time, so it has a power rating of over 100 million watts - a good sized nuclear power plant is only ten times larger. And it gets much more impressive at an airport. A 747-400 can take on 50,000 gallons of fuel and the ground pumps are generally rated at about 2,000 gallons per minute. The power being transferred during a refueling is over 4 billion watts per aircraft.
whew - huge numbers that are only loosely tied to anything we can make sense of
Let's go in the other direction and think about how much power a parcel of land gets from the Sun and how much of that can be stored. That will give us a sense of how much land is required for agriculture and is tightly bound to historic limits on population.
Doing that properly will push me way over my one hour limit, so it will have to wait a bit..
In the mean time I'm going to have a cup of chocolate gelato for a treat. Maybe 400 Calories - say 465 watt-hours of energy. I'll savor it over perhaps five minutes - so I'm transferring power at about 5.6 kilowatts.
... and my iPhone and its charger toil away at under 2 watts.
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1 Power is just the amount of energy converted or transferred per unit time so time = 5.25 watt-hours/1000 watts.
2 1.0 watt--hours is about 0.86 Calories
3 The energy density of carbohydrates and proteins are roughly similar - about 4 Calories per gram. Fats are 9 Calories per gram, which is similar to petroleum based fuels like diesel fuel, gasoline and turbine fuel. Also remember that the average daily energy requirement of a person is about 2,400 watt-hours (a bit over 2,000 Calories), so a person averages a 100 watt of nutritional power requirement. Our personal power transfer rates are much higher than the electrical wiring in our homes can permit.
4 Filling station pumps in the US are limited by law to 10 gallons per minute, but most are throttled to half of that. Gasoline has about 36.6 kWh of energy per gallon, so 5 gallons per minute is a bit less than 11 million watts.
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Recipe Corner
A reader asked for another kid's recipe. This is a very simple cake for a single person. It uses commercial cake mixes and a microwave. It works because there is a lot of leavening in an Angel Food cake mix, so no eggs or ingredients other than water are required.
Ridiculously Simple Cakelets
Ingredients
° 1 box Duncan Hines Angel Food Cake (the kind that only requires water)
° 1 box of another commercial cake mix. A richer flavor like chocolate fudge or yellow cake works well
° a bit of butter or Pam to grease a mug
° chopped nuts, dried fruit, jam, chocolate chips, etc (optional)
Technique
° combine the two cake mixes - I pour them in a 1 gallon plastic bag and mix well. Seal it well!
° to make a caklet, mix 30g (about an ounce) of the mix and 30g of water in a greased microwave safe mug. Mix in any add-ins.
° microwave at about 1,000 watts for a minute
° let it cool in the cup for about 10 minutes and pop out.
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And finally a plug for a something of a friend. Christopher Lydon does some of the best interviews in the business. Really really good. He has been a weekly companion during my rowing sessions with his podcasts and I've learned an enormous amount from him.
His is proposing a potentially fantastic kickstarter project that deeply deserves funding.
Recommended! I pledged...
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