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Current

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Moving Charge A charge in an electric field is subject to a force. Acceleration Positive charges move with the field. Negative charges move against the field. E

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Water Flow Moving water in a pipe forms a current. Steady flow at constant velocity The mass flow is the density times velocity times area. Pressure provides the force. v A

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Charge Flow Moving charge forms a current. Charge density ne instead of mass density The charge flow is the density times velocity times area. Defined for positive charge Potential provides the force. v A ne

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Current Direction In a conductors electrons are free to move. Collide with other chargesCollide with other charges Random motionRandom motion A potential along a wire causes a net velocity for electrons. Negative charge moves opposite to currentNegative charge moves opposite to current

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Ampere Current is the average amount of charge flowing through an area in a unit of time. The unit of electric current is the ampere (A). 1 A = 1 C/s1 A = 1 C/s The ampere is a fundamental unit of the SI. Coulomb is derived from itCoulomb is derived from it 1 C = 1 A s1 C = 1 A s

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Electron Beam The current comes from the definition. I = q/ t (3.20 x 10 -6 C)/(200 x 10 -3 s) I = -16.0 A (negative beam) The charge of one electron is used to get the number rate. (-1.60 x 10 -5 )/(-1.60 x 10 -19 ) e/ t = 1.00 x 10 14 electrons/s In an electron microscope a constant electron beam transports 3.20 C of electrons in 200 ms across a vacuum. Find the current and number of electrons per second. next

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