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Generators EMF from Motion A moving conductor causes electrons to move. • Magnetic force applied to electrons The electrons drift to one end. • Create an electric field • Matching EMF At equilibrium forces balance. FM qvBsin FE qE EL FE L q vBL sin Falling Bar A bar 1.0 m long is dropped in an east-west orientation. The earth’s field is 2.0 x 10-5 T. Find the induced EMF after the bar falls for 4.0 s. The problem gives the length and the field, but the velocity is needed. • • • • Use basic kinematics v = v0 + gt v = (9.81 m/s2)(4.0 s) v = 39 m/s. Now use the equation for induced EMF. • E = vBL • E = 0.78 mV Cutting Lines As the sliding bar moves the circuit increases in size. The area increases by a rate equal to DA/Dt = vL. The number of field lines cut per second is the area change times the field. • vLB in a uniform field The EMF equals the number of field lines cut per second. Power Bar The sliding bar creates an EMF • Potential difference in volts The potential can create a current in a resistor. There is power output from the moving bar. BDA DV vBL Dt V 1 BDA P IDV R R Dt 2 2 Alternating Generator A turning loop of wire in a magnetic field generates EMF. • Loop area changes with respect to field lines AC Generation The moving loop alternates creating current one way then the other. • • • • Sinusoidal pattern Loop turns at rate w=/t EMF based on loop area Extra turns increase the EMF This is called alternating current or AC. v wr vBL sin wrBL sin wt NABw sin wt DC Generator A DC generator can be made by using a commutator and brushes. • Like a motor • Voltage pulses but doesn’t reverse Better DC generators use multiple commutators. next