Energy Part 2

By Michael on September 18, 2007 at 1:30 am | In Blog Posts, Physics |

A while back I wrote a post about energy.

Here’s another example of energy that I think is interesting. When you take an automobile which is initially at rest and get it going 55 miles per hour, it gains kinetic energy. KE = (1/2)mv^2. So a car that weighs 2000 lbs. (900 kg) going 55 mpg (25 m/s) would have KE=(1/2)(900)(625)=280 kJ (kilo-joules) of energy. That energy comes at the expense of gasoline — chemical energy — exactly 281 kJ’s worth. Excluding all the messiness of friction and air resistance and such, we used exactly the same amount of chemical energy as we gained in kinetic energy. So it doesn’t matter if you use gasoline, hydrogren, electricity or biodiesel, it takes 281 kJ of energy to get your car going 55 mph.

Now you see a stop sign and you hit the brakes. You take all of that kinetic energy and put it into heat, given off by your brakes. The energy went from the gas into your car’s kinetic energy and then the friction from your brakes heated up the air. What some hybrids do, which is really cool, is instead of putting that energy into heating up the air, they put it back into the car. In some cases they literally spin up a big, heavy gyroscope. So they put the kinetic energy of the car into kinetic energy of a big spinning thing. Then when you hit the accelerator, it puts that energy back into the drive shaft and you accelerate back up to 55 mph without starting from zero energy. Cars that do this get “free” energy compared to cars that don’t. When we accelerate and brake, back and forth, all day long in our cars, we are throwing away ridiculous amounts of energy.

If you were flying in space instead, once you got going 55 mph you could turn off that gas and you’d keep going 55 mph forever (or until some force started messing with you). The reason you can’t do this on earth is because all sort of things steal the energy. The heat from your engine, friction in the moving parts, air resistance — they all steal energy that you have to keep putting back with your engine.

So one of the challenges facing us with our on-going “energy crisis” is figuring out how not to turn energy into a form that is not usable to us. The classical example is heat, like the brakes on our car. Anything that gives off heat is wasting energy. Your laptop wastes tons of energy in heat. If they could make CPU’s that didn’t heat up, your battery would last for days.

There is no shortage of energy on earth. The Sun rains down something like 1 kilowatt per square meter. There is tons of kinetic energy in the wind and ocean. We like to burn things like coal and oil –they are ridiculously finite compared to the Sun, the wind and the ocean. Renewable energy isn’t just a good idea, it’s the only long term option.