Going to Ground

A short circuit on a large scale

Here's a vivid example of an electrical short circuit in a beautiful natural setting. In brief, a short circuit occurs when the normal path of current is bypassed via an alternate route with very low resistance. Since current likes to take the path of least resistance, most of it will flow through the short circuit. Also, according to Ohm's Law (V = IR), reducing the resistance of the circuit will drive up the current. Large currents result in excessive resistive heating in circuits, and we usually want to avoid them.

In the video an unfortunate tree has fallen onto a high-voltage power line. This provides a direct path for current to go into the ground, rather than travelling the normal route through the electrical grid. The resulting surge of moving charge results in sufficient resistive heating to ignite the tree and potentially start a forest fire.

But why does the charge want to get to the ground in the first place, and for that matter, what do we really mean when we talk about "voltage"? It's a common misconception that voltage refers to an amount of something. But it doesn't make sense to say "30,000 volts travelled through the victim's body."

Current carries electric charge. Electric charge is something and it can travel through bodies, wires, etc. -- but voltage can't.

Voltage is known affectionately in physics circles as "electric potential difference." Electric potential difference is defined as the change of electrical potential energy per charge, as charges move through an electric field. Let's make an analogy with gravitational potential energy (PEgrav) which depends on the position of an object in a gravitational field. The higher a ball is off of the ground the more PEgrav it has relative to the ground. It's the same idea with PEelectric. The more PEelectric you have, the higher up you are on the "electrical hill".

A high-voltage power line is at the top of that electrical hill, while a low-voltage cable, or the ground, would be at the bottom. However, the charge in the wire doesn't know the ground is there until you connect a conducting path between them. As we are all aware, birds can sit on high-voltage lines all day without any ill effects. This is because they aren't connected to the bottom of the electrical hill. However, if you provide a relatively low-resistance path, the charge is going to charge down that hill in the form of a large electric current. As we can see, in some cases your conducting path might be a tree.

Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.