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Transcript
Resistance How much “push” (electric potential) is needed to achieve a certain flow (electric current)? R= V V = Ω Ohm = i A A circuit element for resistor is indicated with the symbol: Resistivity and Conductivity Resistance is a property of a specific object, and so depends on the geometry of that object. Resistivity is a property of a particular material: ρ= E J [Ωm] σ= 1 Ω −1m -1 ρ G G J =σE G G E = ρJ For simple geometry: R=ρ L A 1 Temperature variations. ρ − ρ 0 = ρ0α (T − T0 ) Ohm’s Law Ohm’s Law: when the current through a device is proportional to the voltage applied across the device. When the resistance of the device is independent of the voltage across it. A material obeys Ohm’s law when the resistivity of the material is independent of the applied magnetic field. 2 Power in Circuits Remember that : Power is the amount of energy transferred per time: The amount of energy transferred to a charge q traversing a potential difference: SO: P= dE dqV = = iV dt dt dE dt U = qV P= P = i2 R For resistive circuits (resistors): V2 P= R Electromotive Force: Emf What really happens in a battery? Charge is “pumped” from a low potential to a higher potential, which requires work. We define the EMF as the amount of work per amount of charge: E = But note: E = ∆W dW = ∆q dq dW dqV = =V dq dq The correct way to interpret this last equation is: The EMF device causes a potential difference of V=E across it’s terminals. 3 Single Current Loop For the most simple circuits: I E + R E V I= = R R 4