Electric current and the drift of electrons, mobility, Ohm's law at the level of the material, resistivity and conductivity, how resistance changes with temperature, and the colour code of resistors.
3 lessons, about 1 h 30 min3 simulations11 problems6 practice questionsPython in every lesson
Created by StemPy LabA current is the charge that crosses a section of a wire each second, . In a metal the free electrons move at random at great speed, and an electric field gives them a slow average drift, . The current is , the current density is , and the mobility is .
Putting into gives , with the conductivity . The resistivity is , and a wire of length and cross section has . Metals have a small resistivity, semiconductors a medium one, and insulators an enormous one.
In a metal the resistivity rises almost in proportion to the temperature, , because the ions vibrate more and the electrons hit them more often. In a semiconductor it falls as it warms, and a superconductor loses all resistance at a low temperature. Not every device obeys Ohm's law. Carbon resistors show their value in coloured bands.
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