Electrical resistivity

 
Electrical resistivity

What is electrical resistivity? Electrical resistivity is a measure of a material’s property to oppose the flow of electric current. This is expressed in Ohm-meters (Ω⋅m). The symbol of resistivity is usually the Greek letter ρ (rho). A high resistivity means that a material does not conduct well electric charge. Electrical resistivity is defined as the relation between the electrical field inside a material, and the electric current through it as a consequence: in which ρ is the resistivity of the material (Ωm),E is the magnitude of the electrical field in the material (V/m),J is the magnitude of the electric current density in the material (A/m2) If the electrical field (E) through a material is very large and the flow of current (J) very small, it means that the material has a high resistivity. Electrical conductivity is the inversion of resistivity, and is a measure of how well a material conducts electric current: in which σ is the conductivity of the material expressed in Siemens per meter (S/m). In electrical engineering often κ (kappa) is used instead of σ. Electrical Resistance Electrical resistance is expressed in Ohms, and is not the same as resistivity. While resistivity is a material property, resistance is the property of an object. The electrical resistance of a resistor is determined by the combination of the shape and the resistivity of the material. For example, a wirewound resistor with a long, thick wire has a higher resistance then with a shorter and thinner wire. A wirewound resistor made from a material with high resistivity has a higher resistance value then one with a low resistivity. An analogy with a hydraulic system can be made, where water is pumped through a pipe. The longer and thinner the pipe, the higher the resistance will be. A pipe full [... read more]

Wirewound resistor

 
Wirewound resistor

What is a wire-wound resistor? A wire wound resistor is an electrical passive component that limits current. The resistive element exists out of an insulated metallic wire that is winded around a core of non-conductive material. The wire material has a high resistivity, and is usually made of an alloy such as Nickel-chromium (Nichrome) or a copper-nickel-manganese alloy called Manganin. Common core materials include ceramic, plastic and glass. Wire wound resistors are the oldest type of resistors that are still manufactured today. They can be produced very accurate, and have excellent properties for low resistance values and high power ratings. Definition of a wirewound resistor A wire wound resistor is a resistor where a wire with a high resistivity is wrapped around an insulating core to provide the resistance. Construction Wire wound resistor construction varies widely. The manufacturing and choice of materials used is dependent on the way the resistor will be used in a circuit. All are made by winding a wire around a core. The resistance value is dependent on the resistivity of the wire, the cross section and the length. Since these parameters can be accurately controlled, a high precision can be achieved. For high tolerance requirements, the resistance value is measured to determine exactly the cut to length of the wire. To create a high resistance, the wire diameter needs to be very small and the length very long. Therefore wire wound resistors are mainly produced for lower resistance values. For low power ratings, very thin wire is used. The handling of the wire is for this matter critical. Any damage may sever contact. After winding the wire is well protected from access of moisture to prevent electrolytic corrosion. Next to precision, there are also wire wound resistors with high power rating for 50W or more. [... read more]