By Robert M. Hazen, Ph.D., George Mason University
Static electricity is a subtle, yet pervasive force in our lives. Everyone might get a static cling or frizzy hair on a dry winter day. The little sparks one feels when one shuffles across the room on a cold day and touches a doorknob, that’s static electricity. The electrostatic and magnetism forces differ from gravity—they can both attract and repulse.
Magnetic Field Flipping
The Earth’s magnetic poles don’t correspond exactly to the rotational North and South poles. The Earth rotates on an axis, and that defines the North Pole and the South Pole. The magnetic pole, however, is several degrees off and it wanders slightly, it moves around from year to year.
Furthermore, over long periods of geological time, perhaps once every half million years or so, the Earth’s magnetic field has this remarkable property of actually flipping. The Earth’s North Pole ends up near the South, and the South Pole ends up near the North. This flip occurs, perhaps on average once every half a million years.
The idea of the Earth’s magnetic field flipping and moving around is very important in the field of geology. Some volcanic rocks freeze in the magnetic orientation of the Earth when they cool because volcanic rocks have little magnetic minerals that act like compass needles.
When the magma, the volcanic rock, is liquid, those compass needles orient to the Earth’s magnetic field, and then those orientations are frozen in, so the cold volcanic rock has its own magnetic field and tells what the magnetic field of the Earth used to be many millions of years ago. And this information is used by geologists to understand the variation of the magnetic field over millennia.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Discovery of Magnetic Field by Michael Faraday
In the early 19th century, a gifted British experimentalist, Michael Faraday, who lived from 1791-1867 performed a simple demonstration of sprinkling iron filings on a magnet. That was a 19th-century discovery by Faraday who liked to do public lectures in science. The complex curving field lines stood out starkly in this sort of experiment, and they were not simple lines radiating out from a point but rather the field lines curved around from pole to pole.
Along that axis, the force varied over distance squared, just like electricity and magnetism, but they were curving field lines. Along with other directions, the force exerted can be extremely complex. The force is a function of the size and shape of the magnet as well as the position of the object, so the exact description of the magnetic force with a magnet, especially a curving magnet like a horseshoe magnet, can be quite complex, indeed, much more difficult to describe and model mathematically than a force field coming out of a gravitational object like the Earth.
Learn more about electricity.
William Gilbert’s Findings of Static Electricity
William Gilbert did so much to improve our understanding of magnets. He found that some substances become electrically charged when they’re rubbed with fur or silk. Amber, glass, some minerals, for example, also have this process. More rubbing produced a greater charge.
It turned out that some materials, called insulators, hold this electric charge, whatever that might be. Other materials, especially metals, don’t hold their own charge and the charge drains away from these conductors. Those substances are like metals, wires, and so forth.
In this particular arrangement, there are insulating materials with suspended pith balls. There is a string that hangs down, but a little piece of conducting material at the end, insulated from its surroundings. It turned out that charges could be passed from one object to another just by touching.
Learn more about atoms.
Electric Friction Machine
One of the most efficient and effective ways of charging electricity is something called an electric friction machine. In these machines, there is a belt of rubber or cloth or fur that’s spun rapidly against another substance, and as that spinning occurs, it strips off a great deal of charge and develops very high electrostatic charges.
Such machines were used well into the 20th century. Things called Vandegraaff generators were used to develop extremely high voltages for high-energy physics experiments, so these weren’t mere curiosities in the 20th century; they were the best way to develop very high voltages.
Electrically charged objects exert forces on each other. Everyone can study these forces in a systematic manner. The way that people did this to really study this systematically was to suspend various objects that could develop a charge from strings and so forth.
Learn more about magnetism and static electricity.
Difference Between Electrostatic Force and Gravity
The law for electrostatic force is different from gravity in two important ways. First, the electrostatic force can be either attractive or repulsive, and that’s different from gravity. Remember, gravity is always attractive. The second difference is that the electrostatic force is vastly greater than gravity.
A comb with just a few billion extra electrons can pick up a piece of paper against the entire gravitational pull of the Earth. In these equations for gravitational energy for electrostatic forces, scientists use the concepts of mass and charge. We can measure these quantities, but their physical origins still remain obscure.
One might think that static electricity isn’t a central topic of physical research these days, and that’s probably true to a large extent, but people deal with the consequences of static electricity all the time. Lightning, for example, occurs whenever there is agitated rain, up in the high levels of the atmosphere.
Lightning is merely a way of compensating for that charge that’s being transferred from the upper levels of the atmosphere down to the ground. There are cloud-to-cloud, cloud-to-ground, and even in rare situations, ground-to-cloud lightning.
Common Questions about the Three Hidden Forces
William Gilbert found that a substance like glass can generate static electricity if it is rubbed with silk, fur, or rubber. He also found that the more the rubbing, the more electric charge will be generated.
The electric friction machine is an effective way of producing static electricity. This machine contains a fur or rubber belt that produces a very high electric charge when it spins around another material at high speed.
Static electricity differs from gravity for two reasons. One is that gravity is only attractive, while static electricity is both attractive and repulsive. Another difference is that static electricity is much stronger than gravity.
