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Resistance and Resistivity. Explain the concept of resistivity. Use resistivity to calculate the resistance of specified configurations of material. Use the thermal coefficient of resistivity to calculate the change of resistance with temperature. 20.4. Electric Power and Energy.
The flowing charges inside wires are negatively charged electrons that move in the opposite direction of the current. Electric currents flow in conductors: solids (metals, semiconductors), liquids (electrolytes, ionized) and gases (ionized), but the flow is impeded in non-conductors or insulators.
Figure 4.1: Electric current: Charge per time passing through a cross-s ection of a wire. Electric current can be related to the number density of electrons in a conductor and the speed with which they move by: I = nqv dA (4.3) where n is the number density of charge carriers (electrons, usually), q is the value of their charge, v
The resistivity is a characteristic of the material used to fabricate a wire or other electrical component, whereas the resistance is a characteristic of the wire or component. To calculate the resistance, consider a section of conducting wire with cross-sectional area A , length L , and resistivity \(\rho\).
a longer wire (of any material) creates greater resistance, and a fatter wire (greater cross-section) reduces total resistance. We can easily test the length dependence, and simultaneously find the resistivity of an unknown wire. OBJECTIVES ๏ Use Ohm’s Law to establish the dependence of resistance on length
We learned that the resistance of a piece of material is a function of length, L, cross-section area, A, and material dependent resistivity, r. Consider a copper wire with length L, cross-section area A, with a resistance R.
Resistance is the electric property that impedes a current. A current flowing through a wire (or resistor) is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop which pushes water through the pipe.
