Transcript Slide 1
Admin: • Register for MasteringPhysics • www.masteringphysics.com • Course ID: MPHOLDER67874 • 121/132 Registered • First graded assignment is posted. • Due Monday 23rd • Assignments make up 20% of your grade • Late assignments receive a 25% penalty (unless you get an approved extension from me) Copyright © 2009 Pearson Education, Inc. Today • Meters • More on sources Copyright © 2009 Pearson Education, Inc. Ammeters and Voltmeters An ammeter measures current; a voltmeter measures voltage. Both are based on galvanometers, unless they are digital. Luigi Galvani • current is passed through a coil in a magnetic field • coil experiences a torque proportional to the current. • If the coil's movement is opposed by a spring, then the amount of deflection of a needle attached to the coil is proportional to the current passing through the coil. Copyright © 2009 Pearson Education, Inc. Ammeters and Voltmeters An ammeter measures current; a voltmeter measures voltage. Both are based on galvanometers, unless they are digital. Ammeters are connected in series. The current in a circuit passes through the ammeter; the ammeter should have low resistance so as not to affect the current. Copyright © 2009 Pearson Education, Inc. Ammeters and Voltmeters Example: Ammeter design. Design an ammeter to read 1.0 A at full scale using a galvanometer with a full-scale sensitivity of 50 μA and a resistance r = 30 Ω. Copyright © 2009 Pearson Education, Inc. Ammeters and Voltmeters Voltmeters are connected in parallel. A voltmeter should not affect the voltage across the circuit element it is measuring; therefore its resistance should be very large. Copyright © 2009 Pearson Education, Inc. Ammeters and Voltmeters Example : Voltmeter design. Using a galvanometer with internal resistance 30 Ω and full-scale current sensitivity of 50 μA, design a voltmeter that reads from 0 to 15 V. Copyright © 2009 Pearson Education, Inc. Ammeters and Voltmeters An ohmmeter measures resistance; it requires a battery to provide a current. Copyright © 2009 Pearson Education, Inc. Ammeters and Voltmeters Summary: An ammeter (very low resistance) must be in series with the current it is to measure; A voltmeter (very high resistance) must be in parallel with the voltage it is to measure. Copyright © 2009 Pearson Education, Inc. Example: Voltage reading vs. true voltage. Suppose you are testing an electronic circuit which has two resistors, R1 and R2, each 15 kΩ, connected in series as shown in part (a) of the figure. The battery maintains 8.0 V across them and has negligible internal resistance. A voltmeter whose sensitivity is 10,000 Ω/V is put on the 5.0-V scale. What voltage does the meter read when connected across R1, and what error is caused by the finite resistance of the meter? 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. 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.