Ohm's Law and Resistance

Active learning works best for Ohm's Law because students often struggle with abstract proportional relationships without concrete evidence. When they measure voltage, current, and resistance themselves, the mathematical model becomes meaningful. Labs and group work transform resistance from a formula into a physical property they can observe and manipulate.

• Class 12
• Physics
• 4 activities
• 30 min–45 min

Learning Objectives

1. Calculate the current, voltage, or resistance in a simple circuit using Ohm's Law.
2. Compare and contrast the V-I characteristics of ohmic and non-ohmic conductors.
3. Evaluate the effect of temperature on the resistance of metallic conductors and semiconductors.
4. Explain the relationship between resistivity, conductivity, and the physical dimensions of a conductor.
5. Predict the change in current when voltage or resistance is altered in a circuit, assuming other factors remain constant.

Problem-Based Learning

⏱ 40 min·Pairs

Pairs Lab: Verifying Ohm's Law

Pairs connect a fixed resistor, variable power supply, ammeter, and voltmeter in series. They vary voltage from 2V to 10V in steps, record current, plot V-I graph, and calculate resistance from slope. Discuss if the graph is linear.

Prepare & details

Predict how the current in a circuit changes if the voltage is doubled while resistance remains constant.

Facilitation Tip: During the Pairs Lab, remind students to record at least five data points for each resistor to ensure accurate V-I graphs.

Setup: Standard classroom with movable furniture arranged for groups of 5 to 6; if furniture is fixed, groups work within rows using a designated recorder. A blackboard or whiteboard for capturing the whole-class 'need-to-know' list is essential.

Materials: Printed problem scenario cards (one per group), Structured analysis templates: 'What we know / What we need to find out / Our hypothesis', Role cards (recorder, researcher, presenter, timekeeper), Access to NCERT textbooks and any supplementary reference materials, Individual reflection sheets or exit slips with a board-exam-style application question

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 45 min·Small Groups

Small Groups: Factors Affecting Resistance

Groups measure resistance of wires varying length, thickness, and material using a metre bridge or multimeter. They tabulate data, graph resistance versus length or area, and derive formulas. Compare results across groups.

Prepare & details

Compare ohmic and non-ohmic conductors, providing examples of each.

Facilitation Tip: For the Small Groups activity on resistance factors, provide rulers and micrometers so students measure length and diameter precisely.

Setup: Standard classroom with movable furniture arranged for groups of 5 to 6; if furniture is fixed, groups work within rows using a designated recorder. A blackboard or whiteboard for capturing the whole-class 'need-to-know' list is essential.

Materials: Printed problem scenario cards (one per group), Structured analysis templates: 'What we know / What we need to find out / Our hypothesis', Role cards (recorder, researcher, presenter, timekeeper), Access to NCERT textbooks and any supplementary reference materials, Individual reflection sheets or exit slips with a board-exam-style application question

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 30 min·Whole Class

Whole Class Demo: Ohmic vs Non-Ohmic

Demonstrate linear graph for resistor and curved for filament lamp using data projector. Class notes differences, predicts behaviour for new voltages. Follow with pair predictions on given graphs.

Prepare & details

Evaluate the impact of temperature on the resistance of different materials.

Facilitation Tip: In the Whole Class Demo, use identical bulbs and diodes side-by-side so students compare slopes directly on the same axes.

Setup: Standard classroom with movable furniture arranged for groups of 5 to 6; if furniture is fixed, groups work within rows using a designated recorder. A blackboard or whiteboard for capturing the whole-class 'need-to-know' list is essential.

Materials: Printed problem scenario cards (one per group), Structured analysis templates: 'What we know / What we need to find out / Our hypothesis', Role cards (recorder, researcher, presenter, timekeeper), Access to NCERT textbooks and any supplementary reference materials, Individual reflection sheets or exit slips with a board-exam-style application question

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 35 min·Small Groups

Small Groups: Temperature Effect

Groups immerse wires or thermistors in hot water, measure resistance before and after using multimeter. Plot resistance versus temperature, classify materials. Discuss real applications like fuses.

Prepare & details

Predict how the current in a circuit changes if the voltage is doubled while resistance remains constant.

Facilitation Tip: During the Small Groups activity on temperature effects, instruct students to heat wires slowly and measure resistance at 10°C intervals to see the trend clearly.

Setup: Standard classroom with movable furniture arranged for groups of 5 to 6; if furniture is fixed, groups work within rows using a designated recorder. A blackboard or whiteboard for capturing the whole-class 'need-to-know' list is essential.

Materials: Printed problem scenario cards (one per group), Structured analysis templates: 'What we know / What we need to find out / Our hypothesis', Role cards (recorder, researcher, presenter, timekeeper), Access to NCERT textbooks and any supplementary reference materials, Individual reflection sheets or exit slips with a board-exam-style application question

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Teaching This Topic

Teachers should begin with simple circuits where students build confidence before introducing complex variables like temperature. Avoid rushing to the formula; let students derive Ohm's Law from their own data first. Use everyday materials like nichrome wires and LEDs to make concepts relatable, and always connect mathematical results back to physical changes in the circuit.

What to Expect

By the end of these activities, students will confidently define resistance, calculate unknowns using V = IR, and distinguish ohmic from non-ohmic conductors through experimental data. They will also explain why temperature and dimensions change resistance, linking microscopic behavior to macroscopic measurements.

Watch Out for These Misconceptions

Common Misconception

During the Pairs Lab on verifying Ohm's Law, watch for students assuming all conductors follow Ohm's Law equally.

What to Teach Instead

Have students graph data for both a metal wire and a filament bulb, then ask them to compare the linearity of both graphs. Prompt them to explain why the bulb’s curve bends, linking this to temperature changes in non-ohmic conductors.

Common Misconception

During the Small Groups activity on factors affecting resistance, watch for students predicting resistance decreases with longer wires.

What to Teach Instead

Provide a multimeter and wires of different lengths cut to identical diameters. Ask students to measure resistance for each length and plot the results, then discuss why the graph shows a rising line instead of a falling one.

Common Misconception

During any circuit-building activity, watch for students believing current exists without voltage.

What to Teach Instead

In the Pairs Lab, have students build a circuit with zero voltage and measure the current. Ask them to explain why the current is zero, reinforcing that voltage is the driving force behind current flow as per Ohm's Law.

Assessment Ideas

Quick Check

After the Whole Class Demo on ohmic vs non-ohmic conductors, show students a circuit with a known voltage and resistance. Ask them to calculate the current, then predict and explain what happens to current when voltage doubles, using their observations from the demo to justify their answers.

Exit Ticket

During the Pairs Lab, collect the V-I graphs from each pair. Provide two unlabeled graphs at the exit: one linear and one curved. Ask students to identify which represents an ohmic conductor and which a non-ohmic conductor, and write a short justification based on their lab data.

Discussion Prompt

After the Small Groups activity on temperature effects, facilitate a class discussion where students explain how heating a metal wire increases resistance, contrasting this with how heating a semiconductor like silicon may decrease resistance. Use their recorded data and microscopic models to guide explanations.

Extensions & Scaffolding

• Ask early finishers to design a circuit where doubling the voltage does NOT double the current, then explain why using their knowledge of non-ohmic conductors.
• For students struggling with proportional reasoning, provide a partially filled table for V = IR so they can complete patterns before calculating.
• Give extra time to groups who want to test how resistance changes in liquids by dipping wires in saltwater or sugar solutions, connecting to real-world applications like conductivity in solutions.

Key Vocabulary

Ohm's Law	A fundamental law stating that the current through a conductor is directly proportional to the voltage across it, provided all physical conditions and temperature remain unchanged. Mathematically, V = IR.
Resistance	The opposition to the flow of electric current in a conductor, measured in ohms (Ω). It is determined by the material's resistivity and its physical dimensions.
Resistivity	An intrinsic property of a material that quantifies how strongly it resists electric current. It is independent of the conductor's shape or size.
Conductivity	The reciprocal of resistivity, measuring a material's ability to conduct electric current. Higher conductivity means easier current flow.
Ohmic Conductor	A conductor that obeys Ohm's Law, meaning its resistance remains constant regardless of the applied voltage. Its V-I graph is a straight line passing through the origin.
Non-Ohmic Conductor	A conductor that does not obey Ohm's Law, where its resistance changes with the applied voltage or temperature. Examples include diodes and filament lamps.