Copyright © 2009 Pearson Education, Inc. Admin: Register for MasteringPhysics Course ID: MPHOLDER67874 121/132 Registered First

Copyright © 2009 Pearson Education, Inc.

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)


Today• Meters• More on sources


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


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.




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.


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.




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.


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



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.



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?


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


Practical Voltage Sources

Modelled with an ideal source and a series resistor

Ideal voltage source: rS=0

So for a practical voltage source, the output voltage depends upon RLIf rS<< RL , vL = vS , independent of RL

voltage dividerrule


Practical Current SourcesModelled with an ideal source and a parallel resistor

Ideal current source: rS=

The output current now depends upon RLIf rS>> RL , iL = iS , independent of RL

SL

EQL i

R

Ri

SLS

SL i

Rr

ri

current divider rule

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Copyright © 2009 Pearson Education, Inc. Admin: Register for MasteringPhysics Course ID: MPHOLDER67874 121/132 Registered First