PPE20_Current - Current, Resistance, Current, Resistance,...

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Unformatted text preview: Current, Resistance, Current, Resistance, and Circuits and Circuits Current RHJansen Charge Carriers RHJansen The charge carriers are the electrons . Protons are trapped in the nucleus and are not easily removed. The valence electrons are far from the nucleus, and in conductors they move from atom to atom easily. Electron move from low potential (negative) to high potential (positive). They fall up, which is a bit awkward. Since this is backwards compared to mechanics it is easier to pretend the positive charges are moving. If the positive charges were able to move they would fall from high potential (positive) to low potential (negative), just as a mass falls from a high height to a low height. Since the charge on protons and electrons are exactly the same, it does not matter mathematically which charge is moving. The only difference is the direction of charge flow. Motion of charges in a conductor RHJansen Motion of charges is caused by a potential difference. Batteries and generators create a potential difference (a height difference so to speak) that positive charges can fall through. As they fall they do work and run electrical devices. If we accept that positive charges are moving, then charges move from the positive plate to the negative plate of the battery. When a battery is connected to wires an electric field surges through the wires from the positive to the negative terminals of the battery. This takes place at the speed of light as a field has no mass. The electric field creates a force on the charges causing them to move. Theoretically they should accelerate to the speed of light. However, as the charges move they start colliding with one another and with imperfections in the wire. The collisions slow the charges to a very slow speed in cm/second. Current and Drift Velocity The flow of charge Variable: I Units: A (amperes or amps) Example: the current in the circuit is 2 amps, I = 2 A . I = N Number of charge carriers per volume, charges/m 3 e charge of an electron, 1.6 10 19 C v d the average velocity of the charges as drift down the wire A the area of the wire RHJansen Current and Drift Velocity Electron are so small and they move randomly down the wire colliding with each other and the imperfection in the wire....
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This note was uploaded on 07/21/2009 for the course PHYSICS 7B taught by Professor Packard during the Spring '08 term at University of California, Berkeley.

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PPE20_Current - Current, Resistance, Current, Resistance,...

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