Electricity Current & Potential Difference Current - the rate of flow of charged particles The SI unit of electric charge is the coulomb, C, and the SI unit of current is the ampere, A. (1A=1C/s) Potential Difference (Voltage, V) - a mesure of energy transfer between two points in an electric circuit W is work done (ie. energy transfered) The SI unit of potential difference is the volt, V (1V=1J/C) Emf - (Electromotive force) a mesure applied to a source of electrical power - such as a battery Ohm's Law & Current - Voltage graphs Ohm's law is: The SI unit of Resistance is the Ohm, A graph that produces a linear relationship shows us that it obeys ohm's law The resistance of metallic conductors increase with increasing temperature Metals - the resistance of most metals increase with temperature Because...

there are many free electrons in the metals

electrons move through the metal - collide with vibrations of the lattice and are scattered

metal get hotter - atoms vibrate more

therefore more collisions between atoms and electrons

so, current decreases, resistance increases Semiconductors - contain fewer free electrons than metals semiconductors with a NTC have resistance that decreases with temperature because... as the material gets hotter, increased vibration of the atoms releas more electrons - this increases current which decreases resistance semiconductors with a PTC have resistance that increases with temperature Power & Work Power is the rate of doing work or the rate of transfer of energy. Resistance & Resistivity Resistors in Series Current - same

Voltage - adds up so, Resistors in parallel Current - adds up

Voltage - same so, Where is resistivity the units for resistivity are is a measure of how strongly a material opposes the flow of electric current Resistivity Number

density of

conduction electrons Number density - the number of charge carriers that can move per Potential Dividers A potential divider consists of two (or more) resisters connected in series. Because they are in series, the same current flows through both resisters, and the p.d is divided between them Internal Resistance The emf of a power supply is the total energy it supplies to each coulomb of charge.

Many power supplies have some internal resistance, so some of the energy supplies to each coulomb is 'lost' due to heating with in the power supply the formula for finding the internal resistance is:

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# Physics AS - Electricity

AS Level EdExcel Physics (Unit 2)