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Drawing of an electrical circuit showing Ohm's law relating voltage, current,
 and resistance.

Electrical circuits are used throughout aerospace engineering, from flight control systems, to cockpit instrumentation, to engine control systems, to wind tunnel instrumentation and operation. The most basic circuit involves a single resistor and a source of electric potential or voltage. Electrons flow through the circuit producing a current of electricity. The resistance, voltage, and current are related to one another by Ohm's law, as shown in the figure. If we denote the resistance by R, the current by i, and the voltage by V, then Ohm's law states that:

V = i R

Resistance is a circuit property that offers opposition to the flow of electrons through a wire. It is analogous to friction in a mechanical system or aerodynamic drag. The resistance is measured in ohms and depends on the geometry of the resistor and the material used in the resistor. At the atomic level, free electrons in a material are in constant random motion continually colliding with one another and the surrounding atoms of the material. When an electric field is applied, the electrons preferentially move in the direction opposite to the field. The atoms form a matrix through which the electrons move. Depending on the spacing, size, and orientation of the matrix, the speed of the flow of electrons will vary. Different materials have different values of electrical conductivity. The resistivity of the material is the inverse of the conductivity and denoted by rho. If the material has a length l and a cross-sectional area A, the resistance is given by:

R = (rho * l ) / A

As the electrons move through the material colliding with each other and with the atomic matrix, the electrons generate random thermal energy or heat. Because of the random nature of the motion, the collisions are irreversible and generate an increase in entropy as described in the second law of thermodynamics. The irreversible generation of heat by the resistor dissipates power from the circuit. The power P is measured in watts and is given by:

P = i V = i^2 R

A resistor therefore carries two ratings: 1) its ohmic value, and 2) its power dissipation ability.

Because the resistance depends on the geometry of the resistor or wire, and the geometry can be changed by an applied force, we can construct an electric circuit for detecting forces using the change in resistance. Electric strain gages are one of the most common types of instruments used in wind tunnel testing. When building a practical circuit, there is normally more than one resistor. The resistors may be connected in parallel or in series with a power source. A special circuit called the Wheatstone bridge is used in wind tunnel testing to eliminate temperature bias in strain gages.

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 05 2015

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