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Electric current, resistivity, resistance

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PH 122
Brian Jones

12 February Electric Current A battery is a source of potential difference. Chemical processes in the battery separate charges and raise them to a higher potential. The battery does work to raise the potential of the charges. The potential difference of the battery is its emf. The current is defined to be the motion of positive charges. Conservation of current dictates that the current is the same at all points in the circuit. The battery creates an electric field in the circuit that causes charges to move. Positive charges move in the direction of the electric field, in the direction of decreasing potential. When charges reach a resistor, their motion is impeded by collisions with atoms. We use the ideal wire model in which we assume that there is no resistance in the wires. Chemical energy in the battery is transofmred to potential energy of the charges, then to kinetic energy of the moving charges, and then to thermal energy as the charges collide with atoms in the resistor. The resistivity ρ is a property of a material, a measure of how good a conductor the material is. Good conductors have low resistivity. Poor conductors have high resistivity. The resistance is a property of a particular wire or conductor. The resistance of a wire depends on the resistivity and the dimensions. ρL R = A Ohm’s Law describes the relationship among potential difference and current in a resistor. ∆V I = R The power produced by a battery in a circuit is given by P = Iε The power dissipated in a resistor is given by (∆V) 2 P = IΔV = I R = R Standard electric outlets in the United States run at 120 V and can carry up to 15 A. In England, the outlets are 230 V and run at up to 13 A. An electric kettle has a coiled wire inside that dissipates power when it carries a current, warming the water in the kettle. A kettle designed for use in England runs at the maximum current of 13 A; a kettle designed for use in the United States will typically run at a maximum of 13 A as well. If a kettle that is designed to work in England is redesigned to work in the United States, how must the resistance of the coil of wire that heats the water be changed? ΔV ↓, therefore R must ↓ Will the US version of the kettle
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