PHYSICS 2016-17 April 20, 2017

Today’s Agenda (Day 145)

1. Housekeeping:

2. Homework Check: Ch 26 Notes Ch 26 Vocabulary

3. Class Activity:Ch 26 Vocabulary QuizCh 26 PPT Review – cont’d

HOMEWORK: Read Ch 26 – Electromagnetism Study for Ch 26 Test

http://highered.mheducation.com/sites/0078458137/ student_view0/self-check_quizzes.html

REMINDERS: Lab Activities Report – April 11 – POSTPONED UNTIL FURTHER NOTICE Ch 26 Test April 21

DATE ASSIGNMENTS - PHYSICS

April 17Vocabulary Quiz – Ch 25 & 26 POSTPONED UNTIL APRIL 20

April 18 Test – Ch 26 POSTPONED UNTIL APRIL 21May 16 Test – Ch 27

PHYSICS 2016-17 LAB ACTIVITY

PARALLEL RESISTANCEProcedure

1. Read the procedure and safety information, and complete the lab form.

2. Hook up a power supply, a resistor, and an ammeter in a series circuit.

3. Predict what will happen to the current in the circuit when a second, identical resistor is added in parallel to the first.

4. Test your prediction.

5. Predict the new currents when the circuit contains three and four identical resistors in parallel.

6. Test your prediction.

Analysis

7. Make a data table to show your results.

8. Explain your results. (Hint: Include the idea of resistance.)

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PARALLEL RESISTORSA parallel circuit has two or more devices connected so that all the devices are independently connected to a voltage source. If all of the devices are resistors, then you have a parallel resistance circuit. When resistors are connected in parallel, each resistor provides a path for current and each element has the same applied potential difference.

In Figure B, three resistors are connected in parallel across the voltage source. The current can pass from junction a to junction b along three paths. More current will flow between these junctions than would flow if only one or two resistors connected them. For this circuit, the total current, I, is represented by the following equation.

MATERIALS

Handle sharp ends of

connecting wires carefully.

DC power supply ordry cells

three resistors, 0.5-W,150–330- range, such as180 , 220 , and 330

one resistor, resistanceunknown

connecting wires

knife switch

0–5-V voltmeter

0–50-mA or 0–100-mA milliammeter

I I1 I2 I3

Each time another resistance is connected in parallel with other resistors, the equivalent resistance changes. The equivalent resistance of resistors in parallel can be determined by the following equation.

In this investigation you will measure current and voltage with resistors in parallel and apply the relationship R V/I to verify your results. Follow closely the circuit diagrams in Figures A and B. If you have only one ammeter and one voltmeter, you must move the meters from position to position to get your readings. For example, take the total current (A) and total voltage (V) readings, then move the meters to positions A1, and V1, and so on. For your calculations, convert milliampere readings to amperes (1 mA 0.001 A).

Figure A Figure BThe resistors in these circuits are connected in parallel. Note the positions of the voltmeter and ammeter in relation to the resistors.

MATERIALS

Handle sharp ends of

connecting wires carefully.DC power supply ordry cells

three resistors, 0.5-W,150–330- range, such as180 , 220 , and 330

one resistor, resistanceunknown

connecting wires

knife switch

0–5-V voltmeter

0–50-mA or 0–100-mA milliammeter

Objectives Measure the current in each resistor in a parallel circuit.

Compare the total current in a parallel circuit to the current in each resistor. Calculate the equivalent resistance of a parallel resistance circuit.

Infer the difference in the equivalent resistance of a circuit after adding resistors.

ProcedureA. Two Resistors

1. Set up the circuit, as shown in Figure A. Close the switch. Adjust the power supply to a set voltage on the voltmeter, such as 3.0 V. Read the current value on the ammeter. Open the switch. Record your readings in Table 1.

2. Move the meters to get the other readings. Record the readings in Table 1.

B. Three Resistors1. Set up the circuit, as shown in Figure B. Close the switch, adjust the power supply to the same

voltage as in part A, and read the meters. Open the switch. Record the readings in Table 2.2. Move the meters to get the other readings. Record the readings in Table 2.

C. Four Resistors1. Set up the circuit with the unknown resistor wired in parallel with the three resistors from

step B. Place the meters to obtain the total current in the circuit and the voltage across the circuit. Close the switch, adjust the power supply to the same voltage as in step A, and read the meters. Open the switch. Record the readings in Table 3.

2. Move the meters to obtain the voltage across and the current in the unknown resistor. Close the switch, adjust the power supply to the same voltage as in step A, and read the meters. Open the switch. Record the readings in Table 3.

Data and Observations

Table 1

R1 () R2 ()Ammeter Reading (mA) Voltmeter Reading (V)

I I1 I2 V V1 V2

Table 2

R1 () R2 () R3 ()Ammeter Reading (mA) Voltmeter Reading (V)

I I1 I2 I3 V V1 V2 V3

Table 3Ammeter Reading (mA) Voltmeter Reading (V)I IU V VU

PhysicsLAB 23-1 continued

Analysis and Conclusions

3. Use data from Table 1 to calculate the following values:

a. the measured equivalent resistance, R, of the circuit where .

b. the current, I1 I2.

c. the equivalent resistance, R, where .

4. a. Compare the current sum, I1 I2, to the measured current, I.

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b. Compare the calculated equivalent resistance to the measured equivalent resistance. Are they equal? If not, what factors might be responsible for any difference in the values?

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5. Use data from Table 2 to calculate the following:

a. the measured equivalent resistance, R, of the circuit where

b. the equivalent resistance, R, where .

6. a. Compare the value of I to the measured current sum, I1 I2 I3 .

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b. Compare the calculated equivalent resistance to the measured equivalent resistance. Are they equal? If not, what factors might be responsible for any difference in the values?

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7. As more resistors are added in parallel to an existing circuit, what happens to the total circuit current? To the equivalent resistance?

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8. Compare your data from Table 3 to your predictions in question 5. How did the total circuit current change? How did the equivalent resistance change?

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9. Use the data from Table 3 to calculate the value of the unknown resistor.

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Extension and Application

10. Tom has a sensitive ammeter that needs only 1.000 mA to provide a full-scale reading. The resistance of the ammeter coil is 500.0 . He wants to use the meter for a physics experiment that needs an ammeter that can read 1.000 A. He has calculated that an equivalent resistance of 0.5000 will produce the necessary voltage drop of 0.5000 V (V IR 1.000103 A 500.0 ), so that only 1.000 mA of current passes through the meter. What value of shunt resistor, a resistor placed in parallel with the meter, should he use?

11. A voltmeter has resistance and provides a path for current in the circuit being measured. It is often important to know the resistance of the voltmeter, especially when measuring the voltage across a resistor with very little current or a resistor with high resistance. Assume that the current in a circuit is constant and that you want to measure the voltage across a 1000- resistor. Would a voltmeter with a resistance of 10,000 be a good choice? What about a voltmeter with a resistance of 1,000,000 ? Give reasons for your answer.

ForensicsLAB 6CIRCUIT CONFIGURATIONSBackground: Jasmine called the police because she had seen a man standing outside near her kitchen window. She had no idea how long he had been there, but had felt he was watching her. She was very scared. Upon arrival, the police looked for evidence around the outside of her house. It was dark, but they used flashlights. No window screens had been cut, and the ground was frozen hard with no snow, so there were no footprints to be found. Officer Martin, the policeman who usually patrols Jasmine’s neighborhood, suggested that, since it was Christmas time, she might increase her personal security by plugging in her Christmas lights each night. Surprised that she hadn’t noticed before, Jasmine said she had plugged them in. So the police inspected to see if the cord had been cut, but it was not. Officer Martin decided they needed to have the crime lab take a look at each individual bulb, because he believed they might get fingerprints from a bulb that was unscrewed. The crime lab did find a loose bulb, lifted prints, and

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MATERIALS

• The ends of connecting

wires may be sharp and could puncture skin. Double-check setup with the teacher. Check condition of wires and apparatus.

• Always keep electrical materials away from water sources to avoid possible electrical shock.

12-V variable DC power supply

three lightbulbs in fixtures

six connecting wires

12 alligator clips (if needed)

upon investigation of the individual, found he was a wanted criminal. The key to this investigation was Officer Martin’s knowledge of the behavior of circuits.

When resistors are placed in a circuit, there is a potential difference across that resistor as charges flow through it. The total amount of potential difference per resistor will depend on how the circuit is put together along with the placement of a particular resistor. There is also interdependence between the current through a resistor or branch of the circuit, the potential difference across the resistor, and the size of the resistance. This interdependence is determined, again, by the configuration. The relationship is shown by Ohm’s law, which is

V IRwhere V is potential difference in volts, V, I is current in amps, A, and R is resistance in ohms, .

When two resistors are placed in series, the current through them is equal, while the potential difference varies with the size of each resistor. The individual potential differences within the circuit add to account for the potential difference across the circuit. Removing one resistor will break the circuit and stop the current.

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If two resistors are placed in parallel, the potential difference across the resistors is equal, and is the full amount of the potential difference across the circuit. However, the current splits across the branches, according to the size of the resistors, as indicated by Ohm’s law. Removal of one of the resistors does not stop the current through the other, since each resistor has its own complete circuit with the power supply.

Lightbulbs are resistors in a circuit. The brightness of a bulb gives a visual indicator of how current is being distributed in different circuit configurations. Keep in mind, however, that brightness is proportional to the power dissipated by the bulb, but with the resistance of the lightbulbs being equal, brightness provides a good qualitative indication of current.

ProblemHow do different circuit configurations affect the potential differences across and currents through the resistors (lightbulbs) in the circuit?

Objectives Develop the ability to read a circuit diagram.

Create real circuits based on circuit diagrams. Understand the difference between series and parallel circuit configurations.

Predict the behavior of lightbulbs in different parts of a circuit.

Procedure12. With the power supply off, connect a circuit through a single lightbulb.13. Turn on the power supply and increase potential difference to 10 V.

14. Briefly observe the brightness of the bulb.15. Turn off the power supply.

16. Connect two lightbulbs in series to the power supply.17. In the space provided, write your prediction as to the brightness each bulb will display.

18. Turn on the power supply (10 V).

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19. Record your observations about the brightness.

20. Connect two lightbulbs in parallel and repeat steps 6–8.21. Inspect the diagrams of the four different configurations possible for three lightbulbs in a circuit.

22. For each configuration, predict the brightness of each bulb by labeling the diagram with numbers, where 1 represents the brightest a bulb will be, and 2 will be the next brightest, and so on. Record your predictions in the space provided.

23. Connect each of the circuits, one at a time, and have the teacher check each configuration.

24. Turn on the power supply. If any of the bulbs appear not to be lit, you may need to increase the potential difference by small amounts.

25. Record your observations using the same numbering system.

Cleanup and Disposal

Unplug the power supply. Disconnect all wires.

Data and Observations

26. Prediction for two bulbs in series

27. Observation for two bulbs in series

28. Prediction for two bulbs in parallel

29. Observation for two bulbs in parallel

30. Three bulb circuits diagrams (with prediction labels)

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6. Three bulb circuit observations

Conclude and Apply

31. From your observations, what can be said about the equivalent resistance of the two resistors in parallel in circuit 3 for three bulbs, compared to the resistance of a single lightbulb (resistor)? Explain your answer.

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32. From your observations, what can be said about the equivalent resistance of the two resistors in series in circuit 4 for three bulbs compared to the resistance of a single lightbulb (resistor)? Explain your answer.

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Analyze and Conclude

1. Draw a Conclusion How did Officer Martin know to look for a loose lightbulb?

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2. Error Analysis Other than the fact that lightbulbs do not follow Ohm’s law, what sources of resistance may be present but unaccounted for?_____________________________________________________________________________

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