Guest Post by Aaron Johnson from the Ask an Engineer series, published by MIT’s School of Engineering
You’ve probably never seen a bird straddle two wires at once, and there’s a good reason for that…
It’s not uncommon for a character in the movies to end up with a blackened face and a headful of frizzy hair after touching a live electrical wire. What makes for a good gag in the entertainment biz, however, is likely to kill you in real life—unless you’re a bird. Birds have no problem sitting, unruffled, on the high-voltage power lines you often see lining the road. This ability has nothing to do with them being birds, explains Ranbel Sun ’10, MNG ’13, a recent grad in electrical engineering who currently teaches at Phillips Academy in Andover, Massachusetts. It’s all about the connections they’re making—or, more importantly, not making.
“Electrical current is the movement of electrons,” explains Sun. The movement of electrons through a device like your TV is what gives it the energy to display images and produce sound. Sun describes the long process these moving electrons take to get to your house. “The electrons are essentially being pulled from the ground by the power station,” she says. “They move through the power lines, through your TV, and eventually they make their way back into the ground from where they came.” This creates a closed loop, which is required for electricity to flow.
The other thing electrons need in order to move is motivation—or, more specifically, a difference in what’s called electrical potential. “Imagine lugging a bunch of bowling balls up a mountain,” Sun explains. “If you give them a path, the balls will naturally roll down the mountain to a lower position.” At the top of the mountain, the bowling balls (which represent the electric current) have a high potential, and they will travel down any path that becomes available. When a bird is perched on a single wire, its two feet are at the same electrical potential, so the electrons in the wires have no motivation to travel through the bird’s body. No moving electrons means no electric current. Our bird is safe, for the moment anyway. If that bird stretches out a wing or a leg and touches a second wire, especially one with a different electrical potential, it will open a path for the electrons—right through the bird’s body.
There are other perils for our feathered friends, Sun points out. “The wood pole supporting the wires is buried deep in the ground,” she says, “so it would also be dangerous for a bird to sit on the pole and touch a wire.” Read more.
Authored by Aaron Johnson. Thanks to Naveen Surisetty from Visakhapatnam, India, for this question. Visit the MIT School of Engineering’s Ask an Engineer site for answers to more of your questions.