Electric Fields

In the last post, I glossed over the bit about how lightning has a hard time passing through air, so I thought I’d clarify (and hopefully this’ll be clear enough that I don’t need to keep up with this string of addenda and clarifications and can write about something new).

From the last post:

The net difference in electrical potential builds up, until the neutral air and water vapour in between the positive and negative regions can no longer sustain the difference, and a lightning bolt discharges the electrical energy. Air is a very good electrical insulator (ie, it is difficult for an electrical current to pass through the air), so a very large electric field can be sustained in the cloud before a lightning bolt discharges the stored energy, and returns at least part of the cloud to a neutral electrical state.

So what exactly is an electric field? It’s a region where, if a charged particle is placed, it will experience an electric force. It’s just like a magentic field: when a magnet (for example, a compass) is placed in a magnetic field (like the Earth’s planetary magnetic field), it experiences a force that aligns it (ie, the compass needle) in a particular way. Similarly, a charged dropped into an electric field will experience a force that pushes it in the field. Electric fields are created by a distribution of charges, either discrete or continuous:

A point charge and a lump of continuous charge, both with electric field lines.

The green lines represent the electrical field.

Of course, the force experienced by a charge dropped into a field depends on the sign (positive or negative) of the charge. A negative charge will experience the opposite force that a positive force experiences, ie, the arrow heads all point in the other direction.

With lightning, it’s not a point test charge dropped into the cloud that creates the bolt, but rather that the charge distribution itself cannot be sustained any longer, and a bolt transfers charge from one region of the cloud to another and neutralizes the field.

Heavily charged cloud with two lightning bolts.

The bolt travels through air, and air is not a vacuum, so the physical properties of the air (or any material that charge is attempting to move through) will affect how easily the charge can move through the material. Materials (and by materials I mean any state of matter, so it can include say glass, water, and air) can generally be classified as either insulators or conductors, depending on a property called conductivity. Electrical energy has a hard time travelling through insulators (like glass) which have a low conductivity, while it passes easily through conductors (like metals), which have high conductivity.

The electrical conductance (or resistance, depending on how you look at it) depends on the atomic or molecular structure of the material. Metals are excellent conductors of electricity, due to their atomic structure. Metals, by definition, have a lattice structure, with loose electrons that can propagate throughout the metal and are not confined to a single atom.

Schematic of metal lattice of positive ions, with an electron that is free to propagate through the lattice.

A very small piece of a metal lattice. The positive ions (nuclei) form the lattice, and the negative ions (valence electrons) can propagate throughout the lattice.

These loose electrons, which are negatively charged, carry electrical charge throughout the material. When a charge is applied at one end of a piece of metal, that charge quickly and efficiently propagates throughout the metal, as the loose electrons move throughout the metal in response to the introduced charge.

But not all materials are metals, and many do not have such loose electrons to acts as charge carriers. Glass, for example, is an excellent insulator, because it has an amorphous, non-crystalline molecular structure. There are no loose electrons, and so few loose charge carriers, and an applied electrical charge has difficulty propagating through the material.

A schematic of glass's crystal structure.

Glass does not have a regular lattice structure, so electrons do not propagate easily through it.

Air is like glass, in that even though there is plenty of room for particles to move freely, there are very few charge carriers to transfer electrical energy from one location to another. This means that a very large electric field can be sustained across a cloud, as small amounts of charge have a hard time flowing freely between the two regions. There is no such thing as a perfect insulator, and eventually the air cannot sustain the electric field, but since it is such a good insulator, the bolt that discharges the energy is very energetic, which we see as a bright flash and hear as a clap of thunder.


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