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Use of Ammeters and Voltmeters

Use of Ammeters and Voltmeters. Not really ideal. Some V gets used up in ammeter. Some current leaks through voltmeter. Potential Divider Circuits. Sometimes elements on circuit need less than the total voltage potential (pd) .

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Use of Ammeters and Voltmeters

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  1. Use of Ammeters and Voltmeters Not really ideal Some V gets used up in ammeter. Some current leaks through voltmeter.

  2. Potential Divider Circuits • Sometimes elements on circuit need less than the total voltage potential (pd) . • Circuit that can be adjusted to divide the p.d. (voltage) called potential divide. • Which type of circuit divides voltage? • Series circuits divide total input voltage between resistors.

  3. 12V R R 6V 6V 12V R R R 4V 4V 4V 12 V R 2R 4V 8V

  4. Potentiometers & variable resistance • Sometimes called rheostats or variable resistors.

  5. Variable R can divide V to dim bulb 9 V If r = 40W, it gets 2/3 or 6 V. Bulb gets 3V. If r = 0W, it gets 0 or 0 V. Bulb gets 9V. 0 - 40 W 20 W Can it make bulb go off?

  6. Variable Resistors in parallel

  7. Envision a long resisting wire. It’s as if wire is many resistors in series. The voltmeter can read the pd around some of the resistance by touching specific points. V 6.0 V

  8. Simplest potentiometer has slider contact. • At top (S) V = 2 V • Halfway V = 1 V • At bottom V = 0V. S 2V 2V • This is equivalent. • Why?

  9. Ex : If there are 30cm above & 70cm below the contact, what is the voltmeter reading? • Find the V reading around70 cm. • Can set up proportion. • Partial R = 70 cm • Total R = 100 cm • V = 2V x (70/100) • 1.4 V 30cm 2V 70cm

  10. Ex 1: If there are 40cm above & 60cm below the contact, what is the voltmeter reading? • The reading around 60cm. • Partial R = • 60 cm. • Total R = • 100 cm. • V = 2V x (60/100) • 1.2 V 40cm 2V 60cm

  11. I can add another resistor to the circuit and determine the reading on the voltmeter.

  12. Ex 2. The maximum resistance of this variable resistor is 100 W. It is placed in series with the 100 W R. When the sliding contact, S, is at contact A, what will the voltmeter read? • It will read 6V (the variable resistor is bypassed): the maximum reading of voltage in this circuit. • What is the reading of the voltmeter when the sliding contact is moved to B?

  13. We have, in effect, the following situation. Therefore, the voltmeter will read 3V. It is not possible to make the reading of the voltmeter vary from zero up to the full voltage of the supply using this circuit.

  14. Ex 3: When all 3 connections of the 100 W variable resistor are used as below, what will the voltmeter reading be? X is a wire, the contact is at B. • Zero. The contact bypasses the bulb like a short circuit. The wire x is in parallel with the bulb. Wire and bulb have 0V.

  15. Ex 4. The variable resistor is 100 W. Approximately what resistance does it supply on the circuit below? What will the voltmeter read? • 50 W, since resistance is directly proportional to length. • 4 V

  16. Ex 5:What is the voltage around the bulb? 9V 40 W 20 W 9 V

  17. Uses of potentiometers • To control volume. • To control lights (dimmer switch).

  18. Kerr 145 potentiometer • Demo, explanation, problem. • Nov 04 pg 15 • Kerr pg 150 #32.

  19. Use your data booklets to sketch the circuit with the potentiometer, voltmeter, and bulb, using the IB symbols on the boards.

  20. Electrical Sensors • Devices whose resistance changes with changing physical conditions.

  21. Strain GaugeWhen stretched R increases b/c it gets longer & skinnier. Find symbol put in notes.

  22. Light SensorLDR – light dependent resistor Resistance decreases when light shines on it. See symbol.

  23. Thermistor – resistance varies with temperature. Usually the R decreases w/increasing temperature (counter to our wires). Symbol.

  24. switch Using Potential Divide with Sensors LDR - When light stops shining, its resistance increases and V2 increases. If it V2 gets high enough, the switch will activate perhaps putting on lights. The p.d. divide is needed to create a p.d. to activate switch. 12V

  25. switch Use of SensorsEx: The cell has an emf of 12-V and no internal resistance. The p.d. required to activate the switch is 5-V. Find the value of R1 that will cause the switch to activate when the resistance of the LDR is 200 kW.

  26. Rearrange the potential divide equation:Vout = (Vin) R2 R1 + R2.solve for R1. R1 = (R2) Vin - Vout V out.

  27. Solve for R1. 280 kW.

  28. Which would be a good sensor to use with a fire alarm?

  29. What would be a good use for a strain gauge?

  30. Hwk. Read Hamper.115 – 123do pg 123 #21

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