Wednesday, 10 December 2014

Turkey Power: What does it take to cook a Christmas Turkey?

Even for the most organized of us, the holiday season is a hectic one. Your time is split between getting your shopping done, visiting all your friends and family, and upping the festive factor of your house, to the point where Clark Griswold himself would be giving you a solid high five. But even within this chaotic holiday season, there’s one thing that many people look forward to – a perfectly cooked Christmas Turkey.

So, what’s the secret to the perfect Christmas Turkey? Some would say freshness, others may argue that it's seasoning. But at Sketchy Science,  we believe the key is in perfecting the Rube Goldberg-esque process by which energy is transferred between system boundaries countless times, until it reaches your cooking device, manifests itself as kinetic energy at the molecular level, that eventually enables material phase changes and the high speed molecular dance, which breaks and re-forms chemical bonds. How else would you ensure your turkey is lip-smackingly juicy and delicious?

Of course, I’m talking about cooking the turkey, and when you think about it, the molecular dance is a result of turning on your oven to 350 degrees Fahrenheit and pumping your bird full of heat energy for three to four hours (or until golden brown). But have you ever thought about how much energy it takes, where it comes from, or what else you could be doing with that turkey-cooking energy?

Probably not, and that’s why we’ve done the thinking for you.

Let’s begin with the basics. Loosely speaking, energy is the ability to do work, and we measure it in units called Joules (J). It comes in many shapes and sizes, and can be converted from one form to another. For example, the sun emits solar energy, which plants convert into chemical energy through the process of photosynthesis. When you eat the plant, your body metabolizes the sugar to power your muscles. If you then use your muscles to lift some weights over your head, you’re converting the chemical energy into gravitational potential energy. If you happen to give up on that last rep and drop the barbell, the falling weight has kinetic energy. Even the agonizing scream you may let out when the weight falls on your foot is a form of sound energy. But what kind of energy does it take to cook a turkey? Let’s take a closer look at our friend, heat energy.

There’s a simple formula to figure out how much energy it takes to heat a material to a certain temperature:

Q = mc(T2-T1),

where Q is the amount of energy in joules, T1 is the starting temperature of the material, T2 is the final temperature of the material, m is the mass of the material, and c is something called the specific heat capacity. Specific heat capacity is a property of the material in question; it tells us the energy required to raise the temperature of one gram of material by one degree Celsius. With this handy formula, we can figure out how much energy it takes to cook a turkey from room temperature (about 23 degrees Celsius) to the safe consumption temperature of 74 degrees Celsius. In case you’re wondering, the specific heat capacity of turkey is 2.81 J/goC (and yes, someone put in the effort to figure that out empirically.

Assuming an average bird mass of 12 lbs., after crunching all the numbers, we get about 780,000 joules, or approximately 800 kilojoules (kJ). Note that we don’t take the inefficiencies of the oven into account; in reality the energy usage would be a bit more. How much is 800 kJ of energy, you ask? Let’s take a look at what we can do with it.

Since energy is such a fundamental concept, we can compare its quantities across many domains. For example, Calories are a measure of food energy, and one Calorie has energy equivalent to 4.184 kJ. So, the 800 kJ in question is about the same as 191 Calories. In other words, a handful of M&M’s has enough food energy to cook a Christmas Turkey.

Electrical engineers measure the capacity of a battery in units called watt-hours. But since all a battery really does is store energy, we can always convert it back to our old friend, the joule. Knowing this, 800 kJ is enough energy to charge the 5.45 watt-hour battery of your iPhone 540 times. On the flip side, when we apply a similar analysis to the battery of a Tesla Model S, we see that the 800 kJ is only enough energy to drive the Tesla 1.3 km. This gives you an appreciation of how much energy is consumed to move a car around.

What if we were to scale the problem up a bit? In Canada, we eat approximately 3.9 million turkeys at Christmas time each year. Multiply that by the 800 kJ of energy used to cook each turkey, and we arrive at 3.12 terajoules (TJ). To give you a sense of scale, 1TJ is 1,000,000,000 kJ. This is about the same amount of energy that is released in a 1 kiloton (kt) explosion. One kiloton, as you would expect, is the amount of explosive energy released when you detonate 1000 tonne of TNT. If we gave all of that explosive energy to Santa, he would have the equivalent of 100 Davey Crocket missiles - perfect for “tactical defence” of the North Pole.

Let’s scale it up one more time.  During Christmas every year, Canada, the USA, and the UK consume a combined 36 million turkeys. At 800 kJ per turkey, that is a whopping 28.8 TJ of energy. To put this into perspective, let’s say we wanted to launch a projectile from Earth. We can calculate how much energy we can supply to it using the expression for kinetic energy:

Ek = (½)mv2,

where Ek is the amount of energy in joules, m is the mass of the object in kg, and v is the velocity of the object in m/s. Now let’s say, hypothetically of course, that our projectile is the International Space Station (m = 419,000 kg), and we give it enough energy to achieve the escape velocity of the Earth (v = 11,200 m/s).  Plugging in all the numbers, we arrive at approximately 27 TJ, which is about the same amount of energy used to cook turkeys in just three countries every Christmas.

Though to be fair, we didn’t take into account the air resistance, which would also require additional energy to overcome.  But, why try to go through the air when you can just get rid of it all together?  If we were able to build a perfectly sealed column 100 m by 100 m wide, and tall enough to reach the edge of our atmosphere (about 100 km), we could evacuate all of the air and never have to worry about air resistance ever again!  Using a few standard 10 HP air pumps that can move 60 cubic feet of air per minute, we could get rid of the air with an energy cost of about 26 TJ, which is slightly less than the global turkey energy consumption per year.  

What does this all mean? The amount of energy we use during just two Christmases to cook festive turkeys could hypothetically remove the air from a sufficiently sized tube, and shoot the ISS from a cannon with enough speed to escape the gravitational pull of the planet.

Of course, there are other types of energy we didn’t discuss. The sun uses a process known as thermonuclear fusion to keep itself shining and shower us with energy goodness. In fact, the Earth receives about 122,000 TJ of energy from the sun every second. That’s enough energy to cook the world’s Christmas turkey dinners for 4300 years!

So, the next time you are stuffing your face with Christmas turkey, take a step back and think about all the chain of energy processes that took your bird from raw to golden brown and delicious.  Perhaps do a few of your own back-of-the-envelope calculations and ponder upon the other things on which you could be spending your energy.   

You may just be surprised.


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