Electric
Circuit: path of an electric current. The term is usually taken to
mean a continuous path composed of conductors and conducting devices and
including a source of electromotive force that drives the current around
the circuit. A circuit of this type is termed a closed circuit, and a
circuit in which the current path is not continuous is called an open
circuit. A short circuit is a closed circuit in which a direct connection
is made, with no appreciable resistance, inductance, or capacitance,
between the terminals of the source of electromotive force.
Current
flows in an electric circuit in accordance with several definite laws. The
basic law of current flow is Ohm's law, named for its discoverer, the
German physicist Georg Ohm. Ohm's law states that the amount of current
flowing in a circuit made up of pure resistances is directly proportional
to the electromotive force impressed on the circuit and inversely
proportional to the total resistance of the circuit. The law is usually
expressed by the formula I = E/R, where I is the current in amperes, E is
the electromotive force in volts, and R is the resistance in ohms . Ohm's law applies to all electric circuits for both direct
current (DC) and alternating current (AC), but additional principles must
be invoked for the analysis of complex circuits and for AC circuits also
involving inductances and capacitances.
A
series circuit is one in which the devices or elements of the circuit are
arranged in such a way that the entire current (I) passes through each
element without division or branching into parallel circuits.
When
two or more resistances are in series in a circuit, the total resistance
may be calculated by adding the values of such resistances. If the
resistances are in parallel, the total value of the resistance in the
circuit is given by the formula
In
a parallel circuit, electrical devices, such as incandescent lamps or the
cells of a battery, are arranged to allow all positive (+) poles,
electrodes, and terminals to be joined to one conductor, and all negative
(-) ones to another conductor, so that each unit is, in effect, on a
parallel branch. The value of two equal resistances in parallel is equal
to half the value of the component resistances, and in every case the
value of resistances in parallel is less than the value of the smallest of
the individual resistances involved. In AC circuits, or circuits with
varying currents, circuit components other than resistance must be
considered.
If
a circuit has a number of interconnected branches, two other laws are
applied in order to find the current flowing in the various branches.
These laws, discovered by the German physicist Gustav Robert Kirchhoff,
are known as Kirchhoff's laws of networks. The first of Kirchhoff's laws
states that at any junction in a circuit through which a steady current is
flowing, the sum of the currents flowing to the point is equal to the sum
of the currents flowing away from that point. The second law states that,
starting at any point in a network and following any closed path back to
the starting point, the net sum of the electromotive forces encountered
will be equal to the net sum of the products of the resistances
encountered and the currents flowing through them. This second law is
simply an extension of Ohm's law.
The
application of Ohm's law to circuits in which there is an alternating
current is complicated by the fact that capacity and inductance are always
present. Inductance makes the peak value of an alternating current lag
behind the peak value of voltage; capacitance makes the peak value of
voltage lag behind the peak value of the current. Capacitance and
inductance inhibit the flow of alternating current and must be taken into
account in calculating current flow. The current in AC circuits can be
determined graphically by means of vectors or by means of the algebraic
equation
in which L is inductance, C is capacitance, and f is the frequency
of the current. The quantity in the denominator of the fraction is called
the impedance of the circuit to alternating current and is sometimes
represented by the letter Z; then Ohm's law for AC circuits is expressed
by the simple equation I = E/Z.
