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Transcript
Admin: Lab today: Light bulb and resistors II: current - voltage (I - V) curves Don’t forget to complete the pre-lab. Proposed mid-term date: Wednesday March 18th. • Assignments: Still a few people not registered for Mastering Physics • Second graded assignment is posted. • Due Monday March 2nd • Grading setting changed from 3 to 4 attempts. • If you’re not completely sure of your answer, use the hints. • Don’t forget the other resources available to you – my office hours, my email, the discussion TAs and their office hours and emails. Copyright © 2009 Pearson Education, Inc. Today • Sources • Finishing up combination rules • Kirchoff’’s laws Copyright © 2009 Pearson Education, Inc. More on Sources • Sources and Loads – – – – – Source + load = simplest description of any electrical system Voltage sources and current sources provide prescribed voltages / currents Ideal sources: can provide any amount of energy In the ideal case, Voltage / Current values are not affected by load – Ideal voltage source: • Output voltage does not change with current • Current supplied is determined by load • Zero internal resistance – Ideal current source: • Output current does not change with voltage • Voltage generated is determined by load • Infinite internal resistance Copyright © 2009 Pearson Education, Inc. battery Practical Voltage Sources Modelled with an ideal source and a series resistor Ideal voltage source: rS=0 vL = RL vS rS + RL voltage divider rule So for a practical voltage source, the output voltage depends upon RL If rS<< RL , vL = vS , independent of RL Copyright © 2009 Pearson Education, Inc. Some voltage sources Copyright © 2009 Pearson Education, Inc. Series and Parallel EMFs EMFs in series in the same direction: total voltage is the sum of the separate voltages. Copyright © 2009 Pearson Education, Inc. Series and Parallel EMFs; Battery Charging EMFs in series, opposite direction: total voltage is the difference (but the lowervoltage battery is charged). Copyright © 2009 Pearson Education, Inc. Series and Parallel EMFs EMFs directly in parallel only make sense if the voltages are the same; this arrangement can produce more current than a single emf. Copyright © 2009 Pearson Education, Inc. Some current sources Copyright © 2009 Pearson Education, Inc. Practical Current Sources Modelled with an ideal source and a parallel resistor Ideal current source: rS= iL REQ = 1 REQ RL iS current divider rule rS RL = æ1 1 ö r +R L ç + ÷ S è rS RL ø iL rS iS rS RL The output current now depends upon RL If rS>> RL , iL = iS , independent of RL Copyright © 2009 Pearson Education, Inc. Series and Parallel EMFs Current sources in parallel provide a current equal to their sum. Current sources in series does not happen. Copyright © 2009 Pearson Education, Inc. Circuit analysis method 1: Apply element combination rules Series resistors Parallel resistors Series voltage sources Parallel current sources Copyright © 2009 Pearson Education, Inc. Another Example R=22Ω R1=10Ω R2=20Ω R3=30Ω V=10V Find the equivalent resistance and the current at I Copyright © 2009 Pearson Education, Inc. I= 10V/22Ω = 0.45V