Page 334 - NS-2 Textbook

PHYSICAL SCIENCE 329

in an electric circuit are designed to keep electrical resis- ates current. These cells may be restored to their original
tance as low as possible. A wire; s resistance depends on condition (recharged) by forcing an electric current
its length, diameter, composition, and temperature. through them in the opposite direction to that of dis-
Manufactured circuit elements that provide a defi- charge. The automobile storage battery is a common ex-
nite specified amOlmt of resistance are called resistors. Re- ample of a battety composed of secondary cells.
sistance is measured in ohllls (symbol: n, the Greek letter
Olllega). One ohm is the resistance of a circuit element (or
THE ELECTRICAL CIRCUIT
circuit) that permits a steady current of one ampere to
flow when a potential difference of one volt is applied to Whenever two unequal charges are connected by a con-
that circuit. ductOl; a pathway for electrons and thus current flow is
An instrument designed to nleasure resistance in an created. An electric circllit is a conducting pathway con-
electrical circuit is called an ohlllllleter. sisting of the conductor and the path through the voltage
source. For example, a lamp connected by wires to a dry
cell's terminals forms a simple electric circuit. The elec-
BATTERIES
tron current flows from the negative (-) terminal of the
A battety consists of a number of cells assembled in a battety through the lamp to the positive (+) battelY ter-
common container and connected to function as a source minal, and continues by going through the battety from
of electrical power. A cell is the fundamental unit of a the (+) terminal to the (-) terminal. As long as this path-
battery. A silllple cell consists of two electrodes placed in way is tmbroken, it is a closed circuit and current will
a container holding the electrolyte. flow.
The electrodes are the conductors by which the cur- When electrical experiments were first conducted in
rent leaves or returns to the electrolyte. In a simple cell, the 1800s, the electron was as yet undiscovered, so it
they are carbon and zinc strips, placed in the electrolyte. was assumed that current was a flow of positive charges
In the dry cell there is a carbon rod in the center and a zinc from the positive to the negative terminals of a source
container in which the cell is assembled. The electrolyte in a circuit. This idea became so widespread that still
may be a salt, an acid, or an alkaline solution. In the au- today for many applications electrical current is said to
tomobile storage battery the electrolyte is in liquid form. proceed from positive to negative terminals. In the scien-
In the dry cell battery, the electrolyte is a paste. tific community hOV\.Tever, the idea of an electron current
A prilllary cell is one in which the chemical action eats flowing from negative to positive is more prevalent, and
away one of the electrodes, usually the negative. Eventu-
ally the electrode must be replaced or the cell discarded.
In the case of the common dry cell, as in a flashlight bat-
tery, it is usually cheaper to buy a new cell.
,
A secol1dary cell is one in which the electrodes and the ,
electrolyte are altered by the chemical action that gener- I I
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ClOSED CIlCUIT OPEN CIRCUIT
Dry cell battery, cross-section. (AJ Simple electric circuit (closed); (B) Simple electric circuit (open).