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Electrical resistivity Electrical resistivity (also known as resistivity, specific electrical resistance, or volume resistivity) is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current. A low resistivity indicates a material that readily allows the flow of electric current. Resistivity is commonly represented by the Greek letter p (rho). The SI unit of electrical resistivity is the ohm-metre (Q-m) although other units like ohm-centimetre (-cm) are also in use. As an example, if a 1m x 1 m * 1 m solid cube of material has sheet contacts on two apposite faces, and the resistance between these contacts is 1 Q, then the resistivity of the material is 1 Q«m. Many resistors and conductors have a uniform cross section with a uniform flow of electric current, and are made of one material. In this case, the electrical resistivity p (Greek: rho) is defined as: a t p—rR where R is the electrical resistance of a uniform specimen of the material (measured in ohms, Q) fis the length of the piece of material (measured in metres, m) A is the cross-sectional area of the specimen (measured in square metres, m’). The reason resistivity is defined this way is that it makes resistivity an intrinsic property, unlike resistance. All copper wires, irrespective of their shape and size, have approximately the same resistivity, but a long, thin copper wire has a much larger resistance than a thick, short copper wire. Every material has its own characteristic resistivity — for example, resistivity of rubber is far larger than capper's. In a hydraulic analogy, passing current through a high-resistivity material is like pushing water through a pipe full of sand—while passing current through a low-resistivity material is like pushing water through an empty pipe. If the pipes are the same size and shape, the pipe full of sand has higher resistance to flow. Resistance, however, is not solely determined by the presence or absence of sand. It also depends on the length and width of the pipe: short or wide pipes have lower resistance than narrow or long pipes. The above equation can be transposed to get The resistance of a given material increases with length, but decreases with increasing cross-sectional area. From the above equations, resistivity has SI] units of ohm-metre. This table shows the resistivity, conductivity and temperature coefficient of various materials at 20°C (293 K) 
