electrode Electric conductor, usually metal, used as one of two termi¬ nals to conduct electric current through a conducting medium. A simple voltaic cell, or battery, consists of two electrodes, usually one zinc and one copper, immersed in an electrolytic solution (see electrolyte). When a chemical reaction occurs in the solution, electrons gather on the zinc elec¬ trode, or cathode, which becomes negatively charged. At the same time, electrons are drawn from the copper electrode, the anode, giving it a posi¬ tive charge. The difference in charge sets up a potential difference, or voltage, between the two electrodes. When they are connected by a con¬ ducting wire, electrons flow from the cathode to the anode, producing a current.
electrodynamics, quantum See quantum electrodynamics
electroencephalography U-.lek-tro-in-.sef-o-Tag-ro-feV Technique for recording electrical activity in the brain, whose cells emit distinct pat¬ terns of rhythmic electrical impulses. Pairs of electrodes on the scalp transmit signals to an electroencephalograph, which records them as peaks and troughs on a tracing called an electroencephalogram (EEG). Differ¬ ent wave patterns on the EEG are associated with normal and abnormal waking and sleeping states. They help diagnose conditions such as tumours, infections, and epilepsy. The electroencephalograph was invented in the 1920s by Hans Berger (1873-1941).
electrolysis \i-,lek-'tra-l9-s3s\ Process in which electric current passed through a substance causes a chemical change, usually the gaining or losing of electrons (see oxidation-reduction). It is carried out in an electrolytic cell consisting of separated positive and negative electrodes (anode and cath¬ ode, respectively) immersed in an electrolyte solution containing ions or in a molten ionic compound. Electric current enters through the cathode; posi¬ tively charged cations travel to it and combine with electrons. Negatively charged anions give up electrons at the anode. Both thus become neutral molecules. Electrolysis is used extensively in metallurgy to extract or purify metals from ores or compounds and to deposit them from solution (electro¬ plating). Electrolysis of molten sodium chloride yields metallic sodium and chlorine gas; that of a strong solution of sodium chloride in water (brine) yields hydrogen gas, chlorine gas, and sodium hydroxide (in solution); and that of water (with a low concentration of dissolved sodium chloride or other electrolyte) yields hydrogen and oxygen.
electrolyte Substance that conducts electric current as a result of disso¬ ciation of its molecules into positively and negatively charged particles called ions. The most familiar electrolytes are acids, bases, and salts, which ionize when dissolved in polar solvents such as water. Many salts, includ¬ ing sodium chloride, behave as electrolytes when melted in the absence of solvent, since they have ionic bonds. The most commonly used electro¬ lytes are dissolved metal salts (for electroplating metals) and acids (in electric batteries). See also electrolysis.
electromagnet Device consisting of a core of magnetic material such as iron, surrounded by a coil through which an electric current is passed to magnetize the core. When the current is stopped, the core is no longer mag¬ netized. Electromagnets are particularly useful wherever controllable mag¬ nets are required, as in devices in which the magnetic field is to be varied, reversed, or switched on and off. Suitably designed magnets can lift many times their own weight and are used in steelworks and scrap yards to lift loads of metal. Other devices that utilize electromagnets include particle accelerators, telephone receivers, loudspeakers, and televisions.
electromagnetic field Property of space caused by the motion of an electric charge. A stationary charge produces an electric field in the sur¬ rounding space. If the charge is moving, a magnetic field is also produced. A changing magnetic field also produces an electric field. The interaction of electric and magnetic fields produces an electromagnetic field, which
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has its own existence in space apart from the charges involved. An elec¬ tromagnetic field can sometimes be described as a wave that transports ELECTROMAGNETIC RADIATION.
electromagnetic force One of the four known basic forces in the universe. Electromagnetism is responsible for interactions between charged particles that occur because of their charge, and for the emission and absorption of photons (electromagnetic radiation). The phenomena of electricity and magnetism are consequences of this force, and the relation¬ ships between them were first described by James Clerk Maxwell in the 1860s. The physical description of electromagnetism has since been com¬ bined with quantum mechanics into the theory of quantum electrodynamics. The electromagnetic force is about 10 36 times as strong as the gravita¬ tional force (see gravitation), but significantly weaker than both the weak force and the strong force.
electromagnetic induction Induction of an electromotive force in a circuit by varying the magnetic flux linked with the circuit. The phenom¬ enon was first investigated in 1830-31 by Joseph Henry and Michael Fara¬ day, who discovered that when the magnetic field around an electromagnet was increased or decreased, an electric current could be detected in a sepa¬ rate nearby conductor. A current can also be induced by constantly mov¬ ing a permanent magnet in and out of a coil of wire, or by constantly moving a conductor near a stationary permanent magnet. The induced electromotive force is proportional to the rate of change of the magnetic flux cutting across the circuit.
electromagnetic radiation Energy propagated through free space or through a material medium in the form of electromagnetic waves. Examples include radio waves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays. Electromagnetic radiation exhibits wavelike properties such as reflection, refraction, diffraction, and interfer¬ ence, but also exhibits particlelike properties in that its energy occurs in discrete packets, or quanta. Though all types of electromagnetic radiation travel at the same speed, they vary in frequency and wavelength, and inter¬ act with matter differently. A vacuum is the only perfectly transparent medium; all others absorb some frequencies of electromagnetic radiation.
electromagnetic spectrum Total range of frequencies or wavelengths of electromagnetic radiation. The spectrum ranges from waves of long wavelength (low frequency) to those of short wavelength (high fre¬ quency); it comprises, in order of increasing frequency (or decreasing wavelength): very-low-frequency to ultrahigh-frequency radio waves, MICROWAVES, INFRARED RADIATION, visible UGHT, ULTRAVIOLET RADIATION, X-RAYS, and gamma rays. In a vacuum, all waves of the electromagnetic spectrum travel at the same speed: 299,792,458 m/sec (186,282 mi/sec).
red orange yellow green blue violet
microwaves ultraviolet gamma rays
radio, TV waves infrared x-rays
The spectrum of electromagnetic waves ranges from low-frequency radio waves to high-frequency gamma rays. Only a small portion of the spectrum, representing wavelengths of roughly 400-700 nanometers, is visible to the human eye.
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electromagnetism Branch of physics that deals with the relationship between electricity and magnetism. Their merger into one concept is tied to three historical events. Hans C. Orsted’s accidental discovery in 1820 that magnetic fields are produced by electric currents spurred efforts to prove that magnetic fields can induce currents. Michael Faraday showed in 1831 that a changing magnetic field can induce a current in a circuit, and James Clerk Maxwell predicted that a changing electric field has an associated