## electrical circuitclosed
circuit; if not, an open circuit; and if resistance
between them is virtually zero, a short circuit: a electric
switch when off is a closed circuit, when on is a short circuit. Short
circuits between the terminals of a power source are dangerous (see circuit
breaker; fuse). See also Kirchhoff's
laws. ## Analysis of electric circuitsThe analysis of electric circuits, to find the voltages and currents at every point, employs two basic laws, illustrated here. The circuit shown supplies power from a battery (V) to three components (having resistance R_{1}, R_{2}, R_{3}) through power lines which each
have a resistance (R). First, Kirchoff's "current law" states that at every
junction, electric current neither appears nor vanishes: current is conserved,
thus (center left) I = I_{3} + I_{2}; and so on around the
circuit. Second Kirchoff's "voltage law" states that the total voltage around
any constituent circuit is zero: thus (center right,applying Ohm's law)
V + IR + I_{3}R_{3} + IR = 0; the same applies to the other
two circuit loops. Given the battery voltage and the resistances, the currents
can be calculated. The bottom diagram shows the circuit of an electrical
resistance thermometer, in which changes in the probe resistance, r, are
used to measure the temperature. Contact C is moved along a resistance-wire
(ab) until the detector in CD shows zero current. If the two resistances
R are equal, it is then true that R_{c} + a = b + r; r is read off
from a pre-calibrated scale along the wire ab. ## Related category• ELECTRICITY AND MAGNETISM | ||||||

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