Category: flashlights

How does lightning damage electrical appliances that are properly grounded and have their power switches in the off position? Doesn’t that eliminate a path for the electricity? — RDU, Atlanta, Georgia

When lightning strikes a power line, it pours enormous amounts of electric charge onto that wire. These like charges repel one another and they quickly spread out all over the wire. If this wire enters your home, the charges traveling along it will flow into any appliance that’s plugged in, whether it’s turned on or not. But if the appliance is turned off, this charge will reach the open switch and it will come to a stop, at least temporarily.

What matters then is just how much charge enters the appliance. The open switch would normally block the passage of electricity, which is why the appliance doesn’t operate while it’s turned off. But as charge accumulates on one side of the switch, the voltage at that point rises higher and higher. When the voltage becomes high enough, as it easily does after a lightning strike, the charges can leap into the air and travel to the other side of the switch even though the two sides don’t touch one another. Another view of this disaster is that the like charges on one side of the switch repel one another so vigorously that some of them are pushed through the air to the other side of the switch. As a result of this movement of charges through the air—an electric arc—current passes through the appliance as though it were turned on. If this current exceeds what the appliance can tolerate, the appliance will be destroyed. Even grounding the appliance may not help—charges can flow uncontrollably through the appliance and, while some charges take paths to ground, others flow through sensitive components and destroy them.

What is the speed with which electric power is transmitted through the power grid? Believe it or not, the education center at an important nuclear power plant claims that “electrons travel at the speed of light,” an obvious impossibility for current in a copper wire. What is the maximum speed of an electron in a commercial electric power grid? in a superconductor? — AW, Alexandria, VA

Amazingly enough, the speed at which electric power travels through a wire is very different from the speed at which electrons move through that wire. In most wires, electric power travels at very nearly the speed of light while the electrons themselves travel only millimeters per second! This statement is true whether the electricity is traveling in a copper wire or a superconductor!

To understand how this difference in speeds is possible, think about what happens when you turn on the water to a long hose. If that hose is already filled with water, water will immediately begin pouring out of the hose’s end even though the water is flowing quite slowly through the hose. While the water itself moves slowly, the water’s effects travel through the hose at the speed of sound in water—several miles per second! Water at the end of the hose “knows” that you have opened the faucet long before new water from the faucet arrives.

Similarly, when you turn on a flashlight, electrons begin to flow out of the battery’s negative terminal at speeds of only a few millimeters per second. But these electrons don’t have to travel all the way to the light bulb for the bulb to light up. When these electrons leave the battery, they push on the electrons in front of them, which push on the electrons in front of them, and so on. They produce an electromagnetic wave that rushes through the wire at an incredible speed. As a result, electrons begin flowing through the light bulb only a few billionths of a second after the first electron left the battery. So while the electrons that carry electricity through the power grid flow rather slowly, the power they deliver moves remarkably fast.

How does a relay work? — CS, Fairfax, Virginia

A relay is an electromagnetically operated switch. It contains a coil of wire that acts as an electromagnet. Since electric currents are magnetic, this coil of wire develops north and south magnetic poles whenever current passes through it. A metal core is often placed inside the coil of wire to enhance its magnetism. Adjacent to the coil of wire is a moveable piece of iron. While iron normally appears nonmagnetic when it’s by itself, it becomes highly magnetic whenever it’s exposed to a nearby magnetic pole. The iron piece becomes magnetic as current flows through the coil and the two are attracted toward one another. As the iron piece shifts toward the coil, it moves various electric contacts that are attached to it. These contacts close some circuits while opening others. The coil remains magnetic and continues to hold the iron piece near it until current stops flowing through the coil. When the current does stop, the coil loses its magnetism and so does the iron piece. A spring in the relay then pulls the two apart and the electric contacts return to their original positions.

What is the most effective way to electronically measure the level of charge of a lead acid battery? — RS

The voltage of any battery—the amount of energy it gives to each positive charge that it transfers from its negative terminal to its positive terminal—increases slightly when the battery is fully charged. That’s because when the battery is fully charged and its chemicals are highly ordered, the laws of thermodynamics that encourage the development of disorder act to increase the battery’s disorder through effects that also increase the battery’s voltage. But as the battery discharges, these thermodynamic effects fade and the battery’s voltage diminishes slightly. So the easiest way to determine the battery’s charging status electronically is to look at the voltage rise across the battery when little or no current is flowing through it. The higher the voltage, the more fully charged the battery is.

Is it possible to charge batteries using static electricity? Can lightning or atmospheric charges be stored in a capacitor and then released into a cell for charging? — JM, Lafayette, NT

Yes, static electricity has energy associated with it and that energy can be used to charge batteries, at least in principle. Static electricity is literally stationary separated electric charges—essentially separated charges stored on capacitor-like surfaces. As you suggest, it may be easiest to transfer these separated charges into a real capacitor and then to use this charged capacitor to recharge an electrochemical cell. Whether such a procedure can be carried out efficiently and in a cost-effective manner isn’t clear to me. The charges involved in lightning have so much energy per charge—so much voltage—that they’re hard to use for anything. Even the charges that you accumulate when you rub your feet on a wool carpet on a cold, dry winter day acquire an enormous amount of energy per charge. To charge most batteries, you need lots of low energy charges, not the small numbers of high-energy charges that are typical of static electricity. Using this tiny current of high-energy charges to charge a battery is equivalent to trying to fill a swimming pool with water from a high-pressure car-washing nozzle—too little water under too much pressure. You can do it, but there are better ways.

What are some unusual conductors of electricity?

How about graphite and cadmium sulfide? Graphite, such as that in the lead of a pencil, conducts electricity even though it’s not formally a metal. If you draw a dark line on a sheet of paper, that line can act as a wire for sensitive electric circuits. Cadmium sulfide is a photoconductor—a material that is electrically insulating in the dark but that conducts electricity when exposed to light. Photoconductors of this sort are used in some light sensors, as well as in xerographic copiers and laser printers.

How does electricity work?

I’ll assume that you are asking about moving or dynamic electricity, the type that lights the bulb in a flashlight (as opposed to static or stationary electricity). In that case, you are referring to a flow of electric charges that is generally called an electric current. This movement of electrically charged particles carries with it energy, both as kinetic energy (energy of motion) in the charged particles and as potential energy in the electrostatic attractions and repulsions of these particles. The particles typically acquire this energy from a battery. The battery pulls opposite charges away from one another and pushes like charges together. These actions increase the energy of those charges. The charges then rush through electrically conducting materials, generally metals, in order to bring opposite charges closer together. This flow of charges releases the energy given them by the battery.

In a flashlight, the batteries provide the charges with power and the light bulb makes use of the power. The charges first flow through the battery (which gives them energy), then through wires to the light bulb, then through the light bulb (where they give up their energy), and finally back through wires to the battery. The charges move in a loop—a circuit—so that they don’t accumulate anywhere. They travel endlessly between battery and bulb, shuttling energy from the battery to the bulb. As is always the case in electric circuits, two wires connect the battery and bulb—one wire to carry charges to the bulb and one wire to return them to the battery to begin their trip over again.

How can you run a clock off of a potato?

The classic technique is to insert two dissimilar metal strips into the potato in order to build a simple battery. You can then run an electronic clock with the power provided by that battery. But the energy in that battery is coming from chemical reactions of the metals and not really from the potato. If you really want to use a potato as the power source for a clock, you should dry the potato out and burn it. You can use the heat of the fire to run a steam engine or to generate electricity.

In the simplest terms, how does a basic electrical circuit work? — CC, Port St. Joe, FL

An electric circuit is racetrack for electric charges. It must be a complete loop—a “circuit”—so that the charges don’t pile up somewhere along the track. The simplest circuit has a source of energy for the electric charges (e.g., a battery) and a device that takes energy away from the electric charges (e.g., a light bulb). When the charges are in motion through the circuit, they are an electric current. By convention, current points in the direction of positive charge flow, so you can imagine a stream of positive charges circling this circuit over and over again, with current pointing always in the direction that those positive charges are moving. As the current passes through the battery, entering it at the battery’s negative terminal and leaving it at its positive terminal, the charges pick up energy. The battery is converting some of its stored chemical potential energy into electric energy and giving that energy steadily to the current flowing through it. The battery is “pumping” the charges from its negative terminal to its positive terminal. The current continues around the circuit and then passes through the light bulb. In the light bulb, the charges give up most of their energies to the filament and the filament becomes white hot. The current continues out of the bulb and returns to the negative terminal of the battery to pick up more energy. This simple circuit is present in a flashlight. The same charges complete this circuit millions of times each second, shuttling energy from the battery to the bulb.

Can we make an electric fence with no physical wire? — AW, Karachi, Pakistan

No. An electric fence needs at least one real wire. When you put a large electric charge on this wire, anyone who touches it and the ground at the same time will serve as the path through which that charge will flow into the ground. They will receive a shock. But without either the charged wire or the ground, they won’t carry any electric current and they won’t receive a shock.