# Direct Current Circuits

## SPECIFIC OBJECTIVES

In this experiment you will explore two ways of connecting circuit elements. One method, called a series connection, is characterized by the same current running through each element. The other method, called a parallel connection, is characterized by the same voltage across each element. (It is possible for a collection of circuit elements to be in neither the series nor the parallel configuration.)

Imagine attaching a single light bulb to a single battery. This will be our standard for comparison. If several light bulbs are connected in series, then each individual bulb will glow dimmer than the standard and if one of the bulbs in series is removed all the other bulbs go out too.

If several light bulbs are connected in parallel, then each individual bulb will glow at the same brightness as the standard and if one of the bulbs in parallel is removed the other bulbs remain glowing at the same brightness as before removal.

Other circuit elements, such as batteries, can also be connected in series or parallel.

## EQUIPMENT

Circuit board, D-cells (2), wires, resistors, light bulbs, multimeter, and probe leads.

## SYMBOLS FOR CIRCUIT ELEMENTS

In this lab you will be using many electrical components, all of which will be symbolized in schematic diagrams. You will need to recognize these components in order to perform the lab effectively.

## PROCEDURE

The underlined passages below require an answer or a sketch in your notebook.

### Part 1 - Light Bulbs

1. When you are not taking data, please disconnect the battery. This will increase the lifespan of the battery and the bulbs.

2. Use two pieces of wire to connect one light bulb to one battery in such a way as to make the light bulb glow. If unsuccessful, try in the following order a different wiring setup, a different bulb, and a different battery. Sketch, using the symbols for electrical components shown above, a schematic diagram (not a picture) of the circuit which you successfully used to light the bulb. Draw the wires running vertical or horizontal relative to the page. This will make the diagram clearer. You do not need to include springs in schematic diagrams; they act as part of the wire.

3. Reverse the two wires at the light. (i.e., take the wire at each spring connected to the light and connect it to the other spring.) Does this have any effect on the brightness of the light bulb?

4. Reverse the two wires at the battery. Does this have any effect on the brightness of the light bulb?

5. Wire a circuit in such a way as to light two of the light bulbs at equal brightness. Record the brightness level of these lights relative to the single bulb you lit in step 2. (Exact figures are not necessary, just record whether the bulbs of this circuit are brighter, dimmer, or the same brightness as the bulb in step 2.) Then sketch, in the schematic method described above, the circuit you used to light both bulbs. Remember, when not actually testing the circuit, disconnect the battery.

6. Unscrew one of the light bulbs from its socket. (It is not necessary to remove the bulb from the socket, just unscrew it until the bulb turns off and remains off.) Record the effect this action has on the other light bulb in the circuit. (i.e., does it remain on, go out, become brighter or dimmer, etc. Here again no exact numbers are needed.) Return the bulb to its socket and repeat for the other bulb.

Light Bulb Removed Effect of Removal
First Bulb
Second Bulb

7. Are the bulbs in series or parallel?

8. Using the same two lightbulbs and one battery, wire another circuit in such a way as to light two of the light bulbs at equal brightness, but a different brightness level than previously. Record the brightness level of these lights relative to the single bulb you lit in step 2. (Exact figures are not necessary, just record whether the bulbs of this circuit are brighter, dimmer, or the same brightness as the bulb in step 2.) Then sketch, in the schematic method described above, the circuit you used to light both bulbs. Remember, when not actually testing the circuit, disconnect the battery.

9. Unscrew one of the light bulbs from its socket. Record the effect this action has on the other light bulb in the circuit. (i.e., does it remain on, go out, become brighter or dimmer, etc. Here again no exact numbers are needed.) Return the bulb to its socket and repeat for the other bulb.

Light Bulb Removed Effect of Removal
First Bulb
Second Bulb

10. Are the bulbs in series or parallel?

11. Build a circuit containing three bulbs in series. Record the brightness level of these lights relative to the single bulb you lit in step 2. Diagram the circuit.

12. Build a circuit containing three bulbs in parallel. Record the brightness level of these lights relative to the single bulb you lit in step 2. Diagram the circuit.

13. Build a circuit which lights two bulbs at the same intensity and one at a different intensity. Draw the schematic diagram of the successful circuit, labeling the lights A, B, and C, as on the board.

14. Remove each of the three bulbs in turn. Describe the effect of the removal on the other two bulbs.

Light Bulb Removed Effect of Removal
A
B
C

15. Connect a single light bulb to the batteries in each of the three ways shown in the figures below. Record how bright the light bulb is for each battery configuration.

Battery Configuration Brightness
1
2
3

16. Examine how the batteries were connected in each of the figures above. (i.e. were they in series, parallel, neither...) Record for each configuration used above what type of connection it was.

Battery Configuration Type of Connection
1
2
3

17. Build the circuit shown in the figure below. Rotate the knob on the variable resistor or potentiometer (the potentiometer is the only new component in this circuit.) If you find it difficult to rotate with your fingers, use a dime or similar object. Is the light bulb in series or in parallel with the potentiometer?

18. The potentiometer is set to high resistance when the knob is fully counterclockwise, and is set to low resistance when the knob is fully clockwise. When is the light bulb brightest -- when the potentiometer is set to low or high resistance?
19. You should now have a feeling for the operation of series and parallel circuits. Record any generalizations you can make about lights wired in series and also in parallel.

20. Record any generalizations about batteries in series and in parallel.

### Part 2 - Resistors

#### An example of reading a coded resistance

Suppose the first band (starting from the left) is Yellow, the second band is Orange, the third band is Red, and the fourth is Gold. Yellow = 4, Orange = 3, and Red = 2 zeroes, so the value is 4300 ohms. The tolerance or uncertainty in the manufacturing of the resistor, as per the gold band, is 5%.

## Try the tutorial found on the web by Lori walker.

Now we will begin the measurement part of the lab. You will use your multimeter extensively in the next few sessions. Do not leave the multimeter on for long periods of time when not in use.
1. Review the precautions for using a multimeter as an ohmmeter.

2. Choose three resistors with identical coded resistance. The bands should be the same; the color and shape of the resistor body may vary.

3. Record the band colors, the coded resistance, and the tolerance in table below.

4. Connect the three resistors in the series circuit shown below, using the springs on the lower part of the board to hold the resistors. Leave the battery out of the circuit for the resistance measurements.

5. Do not connect the battery when you measure resistance! Measure the actual resistance of each of the resistors using a multimeter and record those values with errors in the table below. To discover the correct setting for the meter, start the meter off at the largest setting, then work down to smaller settings, and stop at the setting which can handle both the size of the data to be measured and has the most number of significant digits in the reading.

6. Under the column labeled "Agreement?", determine whether your value and the manufacturer's value agree. That is, do their errors overlap? See Taylor page 5 if you are confused.

Resistor

Color 1

Color 2

Color 3

Color4

Coded Resistance

Tolerance

Measured resistance

Agreement?

1

2

3

7. Copy your MEASURED resistance values for resistors 1, 2, and 3 into a table like the one below. Remember to include units and reading errors.

8. Measure R12, R23, and R123 using the multimeter and record the values in the table.

9. Add a battery to the three resistors in series, as shown below.

10. Review the precautions for using a multimeter as a voltmeter.

11. We will now measure voltages in the circuit. Be sure that your meter is on the direct voltage measurement setting and NOT the alternating setting. (Alternating is the one with a   ~   symbol by it.)

To discover the correct setting for the meter, start the meter off at the largest setting, then work down to smaller settings, and stop at the setting which can handle both the size of the data to be measured and has the most number of significant digits in the reading. Record your results in the table below.

Resistor combination

Resistance

Voltage

R12

R23

R123

R1

(copy from above)

R2

(copy from above)

R3

(copy from above)

12. Now, select three resistors that are all different from each other in resistance value. Perform the same steps for these resistors as you did for the three identical ones above and record your results in the chart below. NOTE: once you select this resistor combination, you will use it for the rest of the lab, so be sure to keep track of your resistors.

Resistor

Color 1

Color 2

Color 3

Color4

Coded Resistance

Tolerance

Measured resistance

Agreement?

1a

2a

3a

13. Measure resistances and voltages for the non-identical resistors. record your results in the table below.

Resistor combination

Resistance

Voltage

R1a2a

R2a3a

R1a2a3a

R1a

(copy from above)

R2a

(copy from above)

R3a

(copy from above)

14. Now remove the series circuit (keep track of which resistor is which) and build the parallel circuit shown below using the three identical resistors. Again record Resistance and Voltage as in the circuit above. Important: When measuring Resistance across R12 and R23, you must remove the resistor not being measured from the circuit. (Removing one of its leads from a spring is sufficient - just so it is no longer part of the circuit.) If you do not do this, you will not obtain the correct results. The dashed lines below illustrate how the connection must be broken for the R12 measurement. THIS IS ONLY FOR RESISTANCE, NOT FOR VOLTAGE. During voltage measurements, keep all of your resistors in the circuit. Connect the battery to the two vacant wires and take voltage readings.

Resistor combination

Resistance

Voltage

R12

R23

R123

R1

(copy from above)

R2

(copy from above)

R3

(copy from above)

15. Next replace the three identical resistors with the three non-identical, being careful to keep track of which one is R1a, etc. Do the measurements performed on the previous circuit again, with the same caution about removing the R3 resistor from the R1R2 measurement and the R1 resistor from the R2R3 measurement.

Resistor combination

Resistance

Voltage

R1a2a

R2a3a

R1a2a3a

R1a

(copy from above)

R2a

(copy from above)

R3a

(copy from above)

Questions:

1. Were the resistors within the manufacturer's tolerance?

2. What general rule did you observe for combining resistances in a series circuit? Did both identical and non-identical resistor patterns follow this rule?

3. What general rule did you observe for combining resistances in a parallel circuit? Did both identical and non-identical resistor patterns follow this rule?

4. How does voltage behave in a series circuit? What differences, if any, did you observe between the identical resistors and the non-identical?

5. How does voltage behave in a parallel circuit? What differences, if any, did you observe between the identical resistors and the non-identical?

Don't forget your two random and two systematic error sources.

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