Free How to Destroy the Universe by Paul Parsons Page A

volition but moves to or from concentrations of electric charge. This happens because of the way charges attract or repel one another. So a negatively charged electron will tend to drift away from a concentration of negative electric charge (because like charges repel) and move toward an areaof positive charge (because opposite charges attract). This is referred to as an “electromotive force,” or emf. Sometimes also known as a “potential difference,” it is measured in volts—after the Italian physicist Alessandro Volta. Batteries are a source of emf.
    How much current flows through a particular conductor, say a piece of wire, when it is connected up to a battery is given by another property known as electrical resistance. This is the opposition that a current experiences as it tries to flow through the conductor, as the electrons jostle and squeeze between the lattice of atoms from which it is made—rather like commuters at a busy railway station. Resistance is particular to different materials and is measured in ohms, after the 19th-century German physicist Georg Simon Ohm. He also came up with a mathematical relationship, now known as Ohm’s law, revealing that resistance is simply given by dividing the voltage applied to a conductor by the current that this voltage produces.
    Resistance is also the reason lightbulbs work. The resistance of the filament inside the bulb causes it to get hot—as the electrons all trying to squeeze through it rub against one another and against the atoms in the material. The rate at which heat and light are generated is measured in watts (after Scottish engineer James Watt) and is just given by the current times the resistance of the filament squared. Light-bulb filamentsgenerally have a high resistance to maximize the amount of energy they give off.
What is lightning?
    Lightning happens when the undersides of clouds acquire a large quantity of negative charge, building an emf of hundreds of millions, and in some cases even billions of volts between the underside of the cloud and the ground. The reason this happens is thought to be all down to ice crystals. These tend to gather positive charge, and are then carried to the top of the cloud by swirling currents within it. At the same time, heavier pieces of ice and water sink to the bottom of the cloud, carrying with them negative charges. This process of separation induces a massive negative electrical charge on the underside of a storm cloud. As this charge grows, it attracts positive charges, causing an equal but opposite charge to gather on the ground below. The high electrical resistance of the intervening air stops the charges from coming together and canceling out—until, that is, the accumulated charge gets so great that it overcomes the resistance in one almighty discharge.
    The air’s resistance breaks down because of a phenomenon called ionization, where the huge electrical forces literally rip electrons from their parent atoms, gradually turning the air into a conductor. The process begins gradually. Tendrils of negatively ionized air called“leaders” begin to snake their way down from the bottom of the thundercloud toward the ground. At the same time, on the ground, the storm bashes electrons from atoms to make positively charged ions, which also begin wending their way upward from high points such as trees, telegraph poles—and people. When a leader from the cloud and one from the ground finally meet, current can flow and lightning strikes.
Where to shelter
    The first thing you might know about a storm on the way is the distant rumble of thunder. Sometimes you can see flashes of lightning, too. Light travels much faster than sound in air (300,000,000 m/s compared with 343 m/s), and counting the number of seconds between seeing the flash and hearing the rumble is a good measure of the distance between you and the storm—about a kilometer for every three seconds. Normally