Torque on a Current Loop and Moving Coil Galvanometer

Students often find torque and magnetic effects abstract, so hands-on experiments make the concept tangible. Active learning lets them connect the formula τ = N I A B sinθ with real rotations, helping them visualise why orientation matters.

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

Learning Objectives

1. Calculate the torque on a current loop in a uniform magnetic field for various orientations.
2. Explain the principle of operation of a moving coil galvanometer based on torque balance.
3. Compare the magnetic field patterns produced by a current loop and a bar magnet.
4. Design a circuit modification to convert a galvanometer into an ammeter with a specific range.
5. Analyze the effect of changing the angle between the magnetic moment and the magnetic field on the torque experienced by a current loop.

Case Study Analysis

⏱ 35 min·Small Groups

Demo Setup: Current Loop Torque

Provide soft straws, insulated wire, battery, and bar magnets. Students wind 10-20 turns on straw, connect circuit, place in field, and observe rotation by varying current or angle. Measure equilibrium angle with protractor and compare to sinθ predictions.

Prepare & details

Predict how the torque on a current loop changes with the orientation of the loop in a magnetic field.

Facilitation Tip: During Angle Variation Experiment, use a protractor fixed to the base and ask students to note angles in degrees before recording torque values.

Setup: Standard classroom with movable furniture preferred; works in fixed-desk classrooms with pair-and-share adaptations for large classes of 35 to 50 students.

Materials: Printed case study packet with scenario narrative and guided analysis questions, Role assignment cards for structured group work, Blank analysis worksheet for individual problem definition, Rubric aligned to board examination application question criteria

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

Case Study Analysis

⏱ 45 min·Pairs

Model Build: Simple Galvanometer

Use thin aluminium strip as pointer, wind coil on frame, suspend with thread over magnet. Pass low DC current, note deflection. Adjust suspension tension and record θ vs I, plotting graph to verify linearity.

Prepare & details

Explain the principle behind the operation of a moving coil galvanometer.

Setup: Standard classroom with movable furniture preferred; works in fixed-desk classrooms with pair-and-share adaptations for large classes of 35 to 50 students.

Materials: Printed case study packet with scenario narrative and guided analysis questions, Role assignment cards for structured group work, Blank analysis worksheet for individual problem definition, Rubric aligned to board examination application question criteria

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

Case Study Analysis

⏱ 40 min·Pairs

Design Challenge: Instrument Conversion

Give galvanometer specs (G=30Ω, Ig=1mA). Pairs calculate shunt Rs for 5A ammeter and series R for 10V voltmeter. Sketch circuits, simulate with multimeter or breadboard if available, test predictions.

Prepare & details

Design a method to convert a galvanometer into an ammeter or a voltmeter.

Setup: Standard classroom with movable furniture preferred; works in fixed-desk classrooms with pair-and-share adaptations for large classes of 35 to 50 students.

Materials: Printed case study packet with scenario narrative and guided analysis questions, Role assignment cards for structured group work, Blank analysis worksheet for individual problem definition, Rubric aligned to board examination application question criteria

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

Case Study Analysis

⏱ 30 min·Small Groups

Angle Variation Experiment

Fix loop area and current, rotate in field at 0°, 45°, 90°. Measure torque via deflection scale or spring balance. Groups tabulate sinθ vs torque, discuss equilibrium positions.

Prepare & details

Predict how the torque on a current loop changes with the orientation of the loop in a magnetic field.

Setup: Standard classroom with movable furniture preferred; works in fixed-desk classrooms with pair-and-share adaptations for large classes of 35 to 50 students.

Materials: Printed case study packet with scenario narrative and guided analysis questions, Role assignment cards for structured group work, Blank analysis worksheet for individual problem definition, Rubric aligned to board examination application question criteria

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

Teaching This Topic

Start with a real-world hook, like a small DC motor, to show torque in action. Avoid heavy derivations; instead, focus on pattern recognition through measurements. Research shows students grasp sinusoidal relationships better when they plot data themselves rather than memorise graphs.

What to Expect

Students will confidently relate loop orientation to torque magnitude and explain how a galvanometer converts current to deflection. They should use the formula correctly and justify design choices for instrument modification.

Watch Out for These Misconceptions

Common Misconception

During Demo Setup: Students may think torque is the same at all angles.

What to Teach Instead

Ask students to rotate the loop slowly and observe the spring balance reading. When the reading peaks at 90 degrees, remind them to link this to the sinθ factor in the formula.

Common Misconception

During Model Build: Students may confuse galvanometer function with voltage measurement.

What to Teach Instead

While assembling the model, provide a multimeter and let students measure current through the coil and voltage across it. Ask them to explain why the needle deflects based on current, not voltage.

Common Misconception

During Angle Variation Experiment: Students may think a net force causes rotation.

What to Teach Instead

Ask students to draw free-body diagrams of the loop at different angles. Highlight that opposite sides experience equal and opposite forces, creating a couple but no net force.

Assessment Ideas

Quick Check

After Angle Variation Experiment, give students a worksheet with loop orientations at 0°, 45°, and 90°. Ask them to calculate torque values and predict which orientation will produce the loudest 'click' sound when the loop snaps into place.

Discussion Prompt

After Model Build, ask students to discuss in pairs: 'Why does the galvanometer needle return to zero when current stops? How would the design change if we used a stronger magnet?' Facilitate a whole-class sharing of ideas.

Exit Ticket

During Demo Setup, collect students' sketches showing the direction of forces on the loop at θ=30° and θ=90°. Use these to assess if they can identify the couple producing torque.

Extensions & Scaffolding

• Challenge: Ask students to design a torque sensor using a current loop and calibrate it for different magnetic field strengths.
• Scaffolding: Provide a partially completed data table for Angle Variation Experiment with missing values for θ=30° and θ=60°.
• Deeper exploration: Have students research how torque sensors are used in robotics and present their findings to the class.

Key Vocabulary

Magnetic dipole moment	A measure of an object's tendency to align with a magnetic field, for a current loop it is given by NIA, where N is the number of turns, I is the current, and A is the area of the loop.
Torque	A twisting force that tends to cause rotation. In this context, it's the force that rotates the current loop in a magnetic field.
Radial Magnetic Field	A magnetic field that is directed radially outward from or inward toward a central axis. In a galvanometer, this ensures the torque is proportional to the current, regardless of coil orientation.
Shunt Resistance	A low resistance connected in parallel with a galvanometer to divert most of the current, allowing it to function as an ammeter.
Series Resistance	A high resistance connected in series with a galvanometer to limit the current flowing through it, enabling it to function as a voltmeter.