ZERO-IN

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The non-italicized font represents additional information included to support the teacher’s understanding of the content being introduced within the CELL.

ANALYZE IT

• Instruct students to complete the Analysis Questions in their SDRs then discuss them as a class. Use the suggested responses below to guide students’ answers.

  1. Calculate the amount of resistance supplied by (a) the single resistor, (b) the two resistors in series, and (c) the two resistors in parallel. Record the resistances in Table B. Use Ohm’s law for the calculations.

2. Question: Study Table  B.    How does the resistance of the two resistors in series compare to the resistance provided by the single 1000 Ω resistor?

The resistance provided by the two resistors in series is approximately twice the resistance provided by the single 1000 Ω resistor.

3. Question:   How does the resistance of the two resistors in parallel compare to the resistance provided by the single 1000 Ω resistor?

The resistance provided by the two resistors in parallel is approximately one-half the resistance provided by the single 1000 Ω resistor.

4. Question: Look at how resistance and length change when resistors are in series. How do changes in the length of a resistor change resistance?

Changing the length of a resistor changes resistance. Increasing the length increases resistance. Decreasing length decreases resistance.

5. Question: Look at how resistance and cross-sectional area change when resistors are in parallel. How do changes in the cross-sectional area of the resistor change resistance?

Changing the cross-sectional area of the resistor changes resistance. Increasing the cross-sectional area decreases the resistance. Decreasing the cross-sectional area increases the resistance.

GET FOCUSED

Instruct students to complete the Focus Question in their SDRs then discuss it as a class. Use the suggested response below to guide students’ answers.

• What is the relationship between voltage, current, and resistance?

Resistance is the ratio between the voltage and the current in a circuit. As voltage changes, current changes in proportion to the voltage. However, resistance remains constant.

When voltage is held constant, current changes only if the amount of resistance in the circuit changes. Increasing resistance by changing resistors resulted in a decrease in current proportional to the change in resistance, but voltage remained constant at 6.15 V. Placing two 1000 Ω resistors in series caused current to decrease by approximately one-half, from 5.47 to 2.68 mA. At the same time, resistance doubled from 1124 to 2295 Ω.    Placing two 1000 Ω resistors in parallel caused current to double from 5.47 to 10.71 mA while resistance decreased by one-half, from 1124 to 574 Ω. In each case, the change in current and the change in resistance were inversely proportional, and voltage remained constant. These results demonstrate that Ohm’s law applies regardless of whether there is one resistor or many resistors. Ohm’s law also applies regardless of whether multiple resistors are placed in series or in parallel.

• How do the dimensions of a resistor affect current? Use your data to explain your answer.

Increasing the length of a resistor by placing two equal resistors in series doubled resistance from 1124 Ω to 2295 Ω. This caused the current to decrease by half, from 5.47 mA to 2.68 mA. Increasing the cross-sectional area by placing two equal resistors in parallel decreased the resistance by half, from 1124 Ω to 574 Ω. This change caused the current to double from 5.47 mA to 10.71 mA. Therefore, increasing the length of a resistor increases the amount of resistance and decreases current, while increasing the cross-sectional area decreases the amount of resistance and increases current.