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# Investigation 2 – Lab

### MINDSET

This Investigation is designed to:

• reinforce for you that the amount of current in a circuit is proportional to the amount of voltage supplied by the power source,
• allow you to discover that the ratio of voltage to current changes when voltage is held constant but resistance changes,
• reinforce that the ratio of voltage to current is resistance,
• reinforce that the relationship between voltage, current, and resistance is captured by Ohm’s Law,
• guide you to conclude that increasing the length of a resistor decreases the amount of current in a circuit by increasing resistance, and
• demonstrate to you that increasing the cross-sectional area of a resistor increases the amount of current in a circuit by decreasing the resistance.

### BE PREPARED

Student Preparation for the Investigation includes gathering the following materials.

Note: The materials are listed in your SDR. They are also listed below for your reference.

• 1 multimeter
• 4 D cell batteries
• 4 D cell battery joiners
• 1 resistor A
• 1 resistor B
• 2 Resistor C
• 1 roll of masking tape
• 1 clear plastic metric ruler

Direct one student from each lab group to collect the materials listed in their SDRs.

### INVESTIGATE

• In Trial 1, you will construct a circuit to test how changing the resistance affects the current.

• Reflect on the PreLab video as you move through the procedural steps.
• During the Experiment, every procedural step is important. If one step is skipped, data can become invalid. To help you keep on track, read each step thoroughly, complete the step, then check it off (Read it – Do it – Check it).
• Complete the procedural steps in your SDR.

Note: The procedural steps are listed below for your reference.

Note: Most of the illustrations in the lab protocol can be enlarged for detail by clicking on the image.

#### Trial 1:

1. Construct a circuit with four batteries and one Resistor A.

a. Attach one end of an alligator test leads to the tab at the positive terminal of the group of battery joiners.

b. Attach one end of the second alligator test lead to the tab at the negative terminal of the group of battery joiners.

c. Create a small loop at each end of Resistor A.

2. Insert the ammeter (multimeter that will measure current) into the circuit. Refer to the Multimeter Use and Operation procedure for assistance if necessary.

a. Place the connector of a red probe into the outlet marked “10ADC” on the front of the multimeter.

b. Place the connector of the black probe into the outlet marked “COM” on the front of the multimeter.

c. Turn the selection dial on the multimeter to the 10A mark in the A section. This sets the multimeter to measure Amps.

3. Measure the current of the circuit.

a. Insert the tip of the red probe from the multimeter into the loop on the free end of Resistor A.

b. Insert the tip of the black probe from the multimeter into the alligator test lead connected to the negative terminal of the battery.

c. Observe the current flowing through the circuit by reading the display on the multimeter.

d. If the current is less than 0.2 A, adjust the multimeter to the 200 mA (milliamp) range to change the sensitivity of the multimeter.

Open the circuit. Remove the black probe from the alligator test lead and the red probe from Resistor A

• Turn the selection dial to the “Off “ position. Remove the connector of the red probe from the 10ADC outlet and into the VΩmA outlet.

• Turn the selection dial to the 200 mA mark in the DCA range.

Close the circuit. Insert the black probe into the alligator test lead connected to the negative terminal of the battery.

• Insert the red probe into the loop on the free end of Resistor B.

• Record the current in units of mAmps (mA).

e. If the current is less than 20 mA, open the circuit. Turn the selector dial to 20 mA then close the circuit and record the current in units of mAmps (mA).

4. Record the current displayed on the multimeter in Table A.

5. Determine the voltage of the circuit across Resistor A as follows:

a. Open the circuit. Remove the black probe from the alligator test lead. Remove the red probe from Resistor A.

b. Make sure the connector of the black probe is in the outlet marked “COM” on the front of the multimeter.

c. Make sure the connector of the red probe is in the outlet marked VΩmA.

d. Turn the selection dial to 20V in the voltage section.

e. Close the circuit. Place the tip of the red probe against the alligator test lead connecting the resistor to the positive terminal of the battery.

f. Place the tip of the black probe into the alligator test lead connecting the resistor to the negative terminal of the battery.

g. Record the voltage in Table A of the Student Data Record.

h. Open the circuit by removing the black probe from Resistor A. Turn the selection dial on the multimeter to “Off.”

6. Replace Resistor A with one Resistor B. Determine the current and voltage of the circuit. Record your data in Table A.

7. Replace Resistor B with one Resistor C. Determine the current and voltage of the circuit. Record your data in Table A.

8. Question: What effect did increasing resistance have on the current flowing through the circuit?

9. Question: How can you explain the results of your experiment in relation to Ohm’s law?

#### Trial 2:

1. Build a circuit with four batteries and one Resistor C.

2. Refer to the diagram below for assistance if necessary.

3. Measure the length of the resistor barrel. Record the value in Table B of your SDR.

4. Calculate: Determine the cross-sectional area of the resistor barrel:

a. Measure the diameter of the barrel.

b. Divide the diameter by 2 to get the radius.

c. Use the following formula to calculate the area and record the area in Table B of the Student Data Record.

5. Determine the voltage and current as you did in Trial 1. Record the values in Table B.

#### Trial 3:

1. Construct a circuit with two 1000 Ω resistors in series as shown in the figures below.

2. Record the voltage and current in Table B.

3. Question: How did adding a second resistor in series with the first resistor affect the current in the circuit?

4. Calculate: Determine the length and cross-sectional area of the two 1000 Ωresistors when placed end to end or in series as follows:

a. Measure the length of the barrel of each resistor. Add the two lengths to get the length.

b. Measure the diameter of one resistor barrel.

c. Calculate the cross-sectional area of the barrel using the formula:

d. Record the data in Table B.

#### Trial 4:

1. Construct a circuit with two resistors in parallel as shown in the picture and diagram below and record the current and voltage in Table B.

2. Question: How did adding a resistor in parallel affect the amount of current compared to the single 1000 Ω resistor?

3. Measure the length of the barrel of one resistor. Record this length in Table B.

4. Calculate: Determine the cross-sectional area of the two 1000 Ω resistors when placed in parallel as follows:

a. Measure the diameter of each resistor barrel.

b. Calculate the cross-sectional area of each barrel using the formula:

c. Add the two areas to determine the total cross-sectional area of the resistors in parallel.

d. Record the data in Table B.

5. Question: Look at the data for resistor length. Do you think the length of a resistor affects the current? Explain your answer.

6. Question: Look at the data for the cross-sectional area. Do you think the cross-sectional area affects current? Explain your answer.

7. Question: Do the cross-sectional area and length of the resistor affect current the same way? Explain your answer.

### CLEAN UP

Be sure that you and your group clean up the lab bench after you finish your experiments.