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## Current

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**A charge in an electric field is subject to a force.**Acceleration Positive charges move with the field. Negative charges move against the field. - + Moving Charge E**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. Water Flow v r A**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. + Charge Flow v ne A**In a conductors electrons are free to move.**Collide with other charges Random motion A potential along a wire causes a net velocity for electrons. Negative charge moves opposite to current Current Direction**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/s • The ampere is a fundamental unit of the SI. • Coulomb is derived from it • 1 C = 1 A s**The current comes from the definition.**I = Dq/Dt (3.20 x 10-6 C)/(200 x 10-3 s) I = -16.0 mA (negative beam) The charge of one electron is used to get the number rate. (-1.60 x 10-5)/(-1.60 x 10-19) De/Dt = 1.00 x 1014 electrons/s In an electron microscope a constant electron beam transports 3.20 mC of electrons in 200 ms across a vacuum. Find the current and number of electrons per second. Electron Beam next