Chapter 7 Electricity (Section 3)

7.3 Electric Currents—Superconductivity An electric current is a flow of charged particles. The cord on an electrical appliance encloses two separate metal wires covered with insulation. When the appliance is plugged in and operating, electrons inside each wire move back and forth.

7.3 Electric Currents—Superconductivity Inside a television picture tube, free electrons are accelerated from the back of the tube to the screen at the front. There is a near vacuum inside the picture tube, so the electrons can travel without colliding with gas molecules.

7.3 Electric Currents—Superconductivity When salt is dissolved in water, the sodium and chlorine ions separate and can move about just like the water molecules. If an electric field is applied to the water, the positive sodium ions will flow one way (in the direction of the field), and the negative chlorine ions will flow the other way.

7.3 Electric Currents—Superconductivity Regardless of the nature of the moving charges, the quantitative definition of electric current is as follows. Current: The rate of flow of electric charge. The amount of charge that flows by per second. The SI unit of current is the ampere (A or amp), which equals 1 coulomb per second. Current is measured with a device called an ammeter.

7.3 Electric Currents—Superconductivity

7.3 Electric Currents—Superconductivity Either positive charges or negative charges can comprise a current. The effect of a positive charge moving in one direction is the same as that of an equal negative charge moving in the opposite direction. Formally, an electric current is represented as a flow of positive charge. This is because it was originally believed that positive charges moved through metals. Even after it was discovered that it is negatively charged electrons that flow in a wire to comprise the current, the convention of defining the direction of current flow as that which would be associated with positive charges was retained.

7.3 Electric Currents—Superconductivity If positive ions are flowing to the right in a liquid, then the current is to the right. If negative charges (like electrons) are flowing to the right, then the direction of the current is to the left.

7.3 Electric Currents—Superconductivity The ease with which charges move through different substances varies greatly. Any material that does not readily allow the flow of charges through it is called an electrical insulator. Substances such as plastic, wood, rubber, air, and pure water are insulators because the electrons are tightly bound in the atoms, and electric fields are usually not strong enough to rip them free so they can move. Our lives depend on insulators: the electricity powering the devices in our homes could kill us if insulators, like the covering on power cords, didn’t keep it from entering our bodies.

7.3 Electric Currents—Superconductivity An electrical conductor is any substance that readily allows charges to flow through it. Metals are very good conductors because some of the electrons are only loosely bound to atoms and so are free to “skip along” from one atom to the next when an electric field is present. In general, solids that are good conductors of heat are also good conductors of electricity.

7.3 Electric Currents—Superconductivity Liquids such as water are conductors when they contain dissolved ions. Most drinking water has some natural minerals and salts dissolved in it and so conducts electricity. Solid insulators can become conductors when wet because of ions in the moisture. The danger of being electrocuted by electrical devices increases dramatically when they are wet.

7.3 Electric Currents—Superconductivity Semiconductors are substances that fall in between the two extremes. The elements silicon and germanium, both semiconductors, are poor conductors of electricity in their pure states, but they can be modified chemically (“doped”) to have very useful electrical properties. Transistors, solar cells, and numerous other electronic components are made out of such semiconductors.

7.3 Electric Currents—Superconductivity The electronic revolution in the second half of the 20th century, including the development of inexpensive calculators, computers, sound-reproduction systems, and other devices, came about because of semiconductor technology.

7.3 Electric Currents—Superconductivity What makes a 100-watt light bulb brighter than a 60-watt bulb? The size of the current flowing through the filament determines the brightness. That, in turn, depends on the filament’s resistance. Resistance A measure of the opposition to current flow. Resistance is represented by R, and the SI unit of measure is the ohm (W).

7.3 Electric Currents—Superconductivity In general, a conductor will have low resistance and an insulator will have high resistance. The actual resistance of a particular piece of conducting material—a metal wire, for example—depends on four factors: Composition. The particular metal making up the wire affects the resistance. For example, an iron wire will have a higher resistance than an identical copper wire.

7.3 Electric Currents—Superconductivity Length. The longer the wire is, the higher its resistance. Diameter. The thinner the wire is, the higher its resistance. Temperature. The higher the temperature of the wire, the higher its resistance. The filament of a 100-watt bulb is thicker than that of a 60-watt bulb, so its resistance is lower. This means a larger current normally flows through the 100-watt bulb, so, it is brighter.

7.3 Electric Currents—Superconductivity Resistance can be compared to friction. Resistance inhibits the flow of electric charge, and friction inhibits relative motion between two substances. In metals, electrons in a current move among the atoms and in the process collide with them and give them energy. This impedes the movement of the electrons and causes the metal to gain internal energy. The consequence of resistance is the same as that of kinetic friction—heating. The larger the current through a particular device, the greater the heating.

7.3 Electric Currents—Superconductivity In 1911, Dutch physicist Heike Kamerlingh Onnes made an important discovery while measuring the resistance of mercury at extremely low temperatures. He found that the resistance decreased steadily as the temperature was lowered, until at 4.2 K (–452.1F) it suddenly dropped to zero.

7.3 Electric Currents—Superconductivity Electric current flowed through the mercury with no resistance. Onnes named this phenomenon superconductivity for good reason: mercury is a perfect conductor of electric current below what is called its critical temperature (referred to as Tc) of 4.2 K. Subsequent research showed that hundreds of elements, compounds, and metal alloys become superconductors, but only at very low temperatures. Until 1985, the highest known Tc was 23 K for a mixture of the elements niobium and germanium.

7.3 Electric Currents—Superconductivity Superconductivity seems too good to be true: electricity flowing through wires with no loss of energy to heating. Once a current is made to flow in a loop of superconducting wire, it can flow for years with no battery or other source of energy because there is no energy loss from resistance.

7.3 Electric Currents—Superconductivity A great deal of the electrical energy that is wasted as heat in wires could be saved if conventional conductors could be replaced with superconductors. But the superconducting state for a given material has limitations. Resistance returns if the temperature is raised above the superconductor’s Tc, if the current through the substance becomes too large, or if it is placed in a magnetic field that is too strong.

7.3 Electric Currents—Superconductivity Practical superconductors were developed in the 1960s and are now widely used in science and medicine. Most of them are copper oxide compounds that contain calcium, barium, yttrium, and other rare-earth elements. Superconducting electromagnets, the strongest magnets known, are used to study the effects of magnetic fields on matter and to direct high-speed charged particles.

7.3 Electric Currents—Superconductivity The Large Hadron Collider (LHC), an enormous particle accelerator located near Geneva, Switzerland, uses superconducting electromagnets to guide and focus protons as they are accelerated to nearly the speed of light. An entire experimental passenger train was built that levitated by superconducting electromagnets. Magnetic resonance imaging (MRI) uses superconducting electromagnets to form incredibly detailed images of the body’s interior.

7.3 Electric Currents—Superconductivity Widespread practical use of these superconductors is severely limited because they must be kept cold using liquefied helium. Helium is very expensive and requires sophisticated refrigeration equipment to cool and to liquefy. Once a superconducting device is cooled to the temperature of liquid helium, bulky insulation equipment is needed to limit the flow of heat into the helium and the superconductor. These factors combine to make the so-called low-Tc superconductors unwieldy or uneconomical except in certain special applications when there are no alternatives.

7.3 Electric Currents—Superconductivity But hope for wider use of superconductivity blossomed beginning in 1987 when a new family of “high-Tc” superconductors was developed with critical temperatures that now reach as high as about 140 K. This was an astounding breakthrough because these materials can be made superconducting through the use of liquid nitrogen (boiling point 77 K).

7.3 Electric Currents—Superconductivity Liquid nitrogen is widely available, is inexpensive to produce compared to liquid helium, and can be used with much less-sophisticated insulation. However, the new high-Tc superconductors are handicapped by a couple of unfortunate properties: they are brittle and consequently are not easily formed into wires, and they aren’t very tolerant of strong magnetic fields or large electric currents. If these problems can be overcome, a new revolution in superconducting technology will occur.