Dear Straight Dope:
Can you settle an argument my wife and I are having? She say's that birds don't get electrocuted when they land on a live wire because they are not grounded--a pedestrian answer if you ask me. I say it is because electricity follows the path of least resistance and since the wire offers less resistance than the bird, the current ignores the bird and continues through the wire. So who is right or are we both right or both wrong or are we each partially right and partially wrong?
This reminds me of an exploding squirrel that once took out a transformer on a telephone pole in the back alley. It was just running along the wire, got to the end by the transformer, stepped off, and BOOM! All the lights go off. Especially for the squirrel.
A bird or squirrel on a wire gets an electric charge (assuming the wire is uninsulated). But it doesn’t get electrocuted because no current flows through it. It’s like those Van De Graaff generators you see in museums. They’re the big balls you touch that make your hair stand on end. Touch one, and you get filled up with electrons, including your hair. All the electrons being of like charge, they repel each other and turn your head into a dandelion puff. But the electrons don’t go anywhere. No current flows through your body, and you don’t feel any pain.
Now, after touching the Van De Graaff generator, the next step is to touch your sister. What happens? A spark jumps and burns you both. It’s the flow of the electrons that burns–or rather, whatever resistance there is to the flow. The energy lost due to the resistance of the medium through which the electrons are flowing (you, the air, your sister), is converted to heat. Get enough current, enough resistance, and enough heat, and BOOM. No more squirrel. Even without the heat, the moving electrons create magnetic fields that yank the squirrel’s electrons and ions around and do its nervous system no good.
Back to you. In the interests of marital harmony, I’ll declare you both right. You’re right because although the bird is full of electrons, the electrons see no point in going up one leg of the bird and down the other. The bird’s resistance is much higher than the resistance of the wire between its legs, so no current flows through the bird. The key here though is that the bird’s legs and the wire are equally charged–they’re at the same voltage level, or "potential," as the engineers say. Should the bird touch one of its legs, or anything else, to something that can suck up infinite numbers of electrons, e.g., the ground, this will no longer be the case. Suddenly, one leg will be charged and the other won’t. The electrons in the charged leg that repel each other now have somewhere to go, the uncharged leg. Current flows, heat is generated, and, well, you get the idea. Seeing as how current flows from source to ground, your wife is right. You’re explaining why the “downstream” foot of the bird isn’t on a good ground, which is true enough, but your wife’s got a better grasp of the general principle.
Send questions to Cecil via firstname.lastname@example.org.
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