Investigating resistance

Grade 9 Science

Content

• Voltage, current and resistance
• Circuits

Curricular competencies

Questioning and predicting

• Demonstrate a sustained intellectual curiosity about a scientific topic or problem of personal interest
• Make observations aimed at identifying their own questions, including increasingly complex ones, about the natural world

Planning and conducting

• Collaboratively and individually plan, select, and use appropriate investigation methods, including field work and lab experiments, to collect reliable data (qualitative and quantitative)
• Select and use appropriate equipment, including digital technologies, to systematically and accurately collect and record data
• Ensure that safety and ethical guidelines are followed in their investigations

Processing and analyzing

• Seek and analyze patterns, trends, and connections in data, including describing relationships between variables (dependent and independent) and identifying inconsistencies
• Analyze cause-and-effect relationships

Evaluating

• Evaluate their methods and experimental conditions, including identifying sources of error or uncertainty, confounding variables, and possible alternative explanations and conclusions
• Describe specific ways to improve their investigation methods and the quality of the data

Applying and innovating

• Transfer and apply learning to new situations
• Generate and introduce new or refined ideas when problem solving

Communicating

• Formulate physical or mental theoretical models to describe a phenomenon

Any electrical component with electrical resistance restricts the flow of electrons in a circuit and transforms electrical energy into other forms of energy.

A resistor is an electrical component with a specific resistance value. Resistors can be used to control current or to provide a specific voltage and current to other components in a circuit.

Students should use different strengths of resistors in their circuits. If the resistors have the same resistance value in a series circuit, the voltage drop in each resistor will be the same (each will be half of the applied voltage). For parallel circuits, the current will be the same through each resistor, although each will be half of the total current. Using resistors with different values emphasizes how the voltage and current change in series and parallel circuits.

Students should be able to recognize the following relationships emerging from the data collected during their investigations:

• The total resistance of a circuit depends on the number of individual resistances
• Increasing the number of resistors in a series circuit increases the total resistance and reduces the current
• Increasing the number of resistors in a parallel circuit decreases the total resistance and increases the current flow by creating additional pathways

• Observe students during the group work and use the "Lab investigations and safety" evaluation rubric to assess them.

Worksheet Answer Key:

Observations

Table A - Series Circuit Measurements

*Resistor values will depend on resistors used.

**Sum of the voltages across the three resistors should equal approximately 3.0 V.

Table B - Parallel Circuit Measurements

* Resistor values will depend on resistors used.

Analysis

1. Calculate the total resistance of your series circuit using Ohm’s Law (R = V / I). Be sure to use the voltage across the dry cells. Show your calculations.

• Answers will vary.   R = ________________

2. How does the resistance calculated above compare to that of the individual resistors used in the circuit?

• Total resistance calculated above should be equal (or close) to the sum of the three resistor values.

3. How does the voltage compare across each resistor in a series circuit?

• The voltage is different across each resistor in a series circuit. Higher resistance yields higher voltage. The increase in voltage is proportional to the increase in resistance.

4. Add the voltages from across each of the resistors. How does this total compare to the voltage of the dry cells?

• The sum of the voltages across the resistors is equal (or close) to the total voltage of the dry cells.

5. Using Ohm’s Law, calculate the total resistance of your parallel circuit. Be sure to use the voltage across the dry cells as V in your calculation.

• Answers will vary. R = ____________

6. How does the resistance calculated above compare to that of the individual resistors used in the parallel circuit?

• The total resistance in the parallel circuit is less than any of the resistors used.

7. How does the voltage compare across each resistor in a parallel circuit?

• The voltage stays the same across each resistor in a parallel circuit.