Question

a) Outside a house run high-voltage lines (sometimes called “high tension” lines) that carry the electrical power throughout our neighborhood. These are typically many kilovolts -- much higher in voltage than the 120 V at the electrical receptacles in our walls, so they are even more dangerous. However, birds sit on these wires all the time and squirrels run across them quite safely. Explain why it’s safe for a bird to alight with its feet on a high voltage wire. Assume you can’t count on the dielectric strength of the plastic insulation covering these wires to protect you. (In fact, you can’t! The insulation is so thin that the electric field can easily exceed its dielectric strength.)

b) Would the bird still be safe if the line sagged close to the ground after a storm (without breaking) and the bird perched with one foot on a high voltage line and one foot on the ground? Explain why or why not. (Moral of the story: stay away from downed power lines!)

Answer 1

(a) Birds and squirrels stting or running on one of the lines do not get an electrical shock because the one line is at the same potential through out its length. So a bird to get an electric shock (an electric current passing through the bird's body) there must be some potential difference between its feet. But sitting on one line, the potential difference between its feet is zero, so there won't be any current flow through the body of the bird and it will not get any shock.

(b) The bird with one foot on the high voltage line and one foot on the ground will get a shock because the potential of the wire and the Earth are different and now there will be non zero potential difference between the feet of the bird.