Copyright © 2009 Pearson Education, Inc. Admin: Register for MasteringPhysics Course ID: MPHOLDER /132 Registered First graded assignment is posted. Due Monday 23 rd 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. An ammeter measures current; a voltmeter measures voltage. Both are based on galvanometers, unless they are digital. Ammeters and Voltmeters 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. Luigi Galvani

Copyright © 2009 Pearson Education, Inc. 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. Ammeters and Voltmeters

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. 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. Ammeters and Voltmeters

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. An ohmmeter measures resistance; it requires a battery to provide a current. Ammeters and Voltmeters

Copyright © 2009 Pearson Education, Inc. 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. Ammeters and Voltmeters

Copyright © 2009 Pearson Education, Inc. Example: Voltage reading vs. true voltage. Suppose you are testing an electronic circuit which has two resistors, R 1 and R 2, 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 R 1, 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 battery

Copyright © 2009 Pearson Education, Inc. Practical Voltage Sources Modelled with an ideal source and a series resistor Ideal voltage source: r S =0 So for a practical voltage source, the output voltage depends upon R L If r S << R L, v L = v S, independent of R L voltage divider rule

Copyright © 2009 Pearson Education, Inc. Practical Current Sources Modelled with an ideal source and a parallel resistor Ideal current source: r S =  The output current now depends upon R L If r S >> R L, i L = i S, independent of R L current divider rule