electric charge: see charge.

depth charge, explosive device used against submarines and other underwater targets, either rolled into the water from rails on the stern of a ship or propelled from depth charge throwers. The charge is detonated by water pressure at a predetermined depth. It does not have to come into actual contact with the target to destroy it, since the concussion can accomplish this if the charge explodes near enough. First used by the British navy in World War I, it contributed significantly to the defeat of the German U-boat campaign.

charge, property of matter that gives rise to all electrical phenomena (see electricity). The basic unit of charge, usually denoted by e, is that on the proton or the electron; that on the proton is designated as positive (+e) and that on the electron is designated as negative (-e). All other charged elementary particles have charges equal to +e, -e, or some whole number times one of these, with the exception of the quark, whose charge could be 1/3e or 2/3e. Every charged particle is surrounded by an electric field of force such that it attracts any charge of opposite sign brought near it and repels any charge of like sign, the magnitude of this force being described by Coulomb's law (see electrostatics). This force is much stronger than the gravitational force between two particles and is responsible for holding protons and electrons together in atoms and for chemical bonding. When equal numbers of protons and electrons are present, the atom is electrically neutral, and more generally, any physical system containing equal numbers of positive and negative charges is neutral. Charge is a conserved quantity; the net electric charge in a closed physical system is constant (see conservation laws). Whenever charges are created, as in the decay of a neutron into a proton, an electron, and an antineutrino, equal amounts of positive and negative charge must be created. Although charge is conserved, it can be transferred from one body to another. Electric current, on which much of modern technology is dependent, is a flow of charge through a conductor (see conduction). Although current is usually treated as a continuous quantity, it actually consists of the transfer of millions of individual charges from atom to atom, typically by the transfer of electrons. A precise description of the behavior of electric charge in crystals and in systems of atomic and molecular dimensions requires the use of the quantum theory.

Quantity of electricity that flows in electric currents or that accumulates on the surfaces of dissimilar nonmetallic substances that are rubbed together briskly. It occurs in discrete natural units, equal to the charge of an electron or proton. It cannot be created or destroyed. Charge can be positive or negative; one positive charge can combine with one negative charge, and the result is a net charge of zero. Two objects that have an excess of the same type of charge repel each other, while two objects with an excess of opposite charge attract each other. The unit of charge is the coulomb, which consists of 6.24 × 10¹⁸ natural units of electric charge.

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or depth bomb

Weapon used by ships or aircraft to attack submerged submarines. Developed by the British in World War I for use against German submarines, it consisted of a canister filled with explosives and dropped off the stern of a ship near a submerged submarine. It rarely exploded close enough to sink the submarine, but its shock waves loosened the submarine's joints and damaged its instruments, forcing it to the surface, where naval gunfire could finish it off. Modern depth charges can be fired as far as 2,000 yards (1,800 m) from a ship's deck or launched from aircraft. Atomic depth charges have a nuclear warhead and a vastly increased killing radius.

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Small card that authorizes the person named on it to charge goods or services to his or her account. It differs from a debit card, with which money is automatically deducted from the bank account of the cardholder to pay for the goods or services. Credit-card use originated in the U.S. in the 1920s; early credit cards were issued by various firms (e.g., oil companies and hotel chains) for use at their outlets only. The first universal credit card, accepted by a variety of establishments, was issued by Diners' Club in 1950. Charge cards such as American Express require cardholders to pay for all purchases at the end of the billing period (usually monthly). Bank cards such as MasterCard and Visa allow customers to pay only a portion of their bill; interest accrues on the unpaid balance. Credit-card companies get revenue from annual fees and interest paid by cardholders and from fees paid by participating merchants.

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