Copyright © 2009 Pearson Education, Inc. An ammeter measures current; a voltmeter measures voltage. Both are based on galvanometers, unless they are digital. 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. 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. Check if the scale is linear.

Copyright © 2009 Pearson Education, Inc. 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. 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. Is the scale linear?

Copyright © 2009 Pearson Education, Inc. An ohmmeter measures resistance; it requires a battery to provide a current.

Copyright © 2009 Pearson Education, Inc. Summary: An ammeter must be in series with the current it is to measure; a voltmeter 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, 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, part (b) of the figure, and what error is caused by the finite resistance of the meter?

Copyright © 2009 Pearson Education, Inc. A source of emf transforms energy from some other form to electrical energy. A battery is a source of emf in parallel with an internal resistance. Resistors in series: Summary of Chapter

Copyright © 2009 Pearson Education, Inc. Resistors in parallel: Kirchhoff’s rules: 1.Sum of currents entering a junction equals sum of currents leaving it. 2.Total potential difference around closed loop is zero.

Copyright © 2009 Pearson Education, Inc. RC circuit has a characteristic time constant: To avoid shocks, don’t allow your body to become part of a complete circuit. Ammeter: measures current. Voltmeter: measures voltage.