Physics · Class 12

Active learning ideas

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.

• CBSE Learning Outcomes
• CBSE: Moving Charges and Magnetism - Class 12

• 30–45 min
• Pairs → Whole Class
• 4 activities

Activity 01

• 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.

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.

What to look for

Present students with a diagram showing a current loop in a magnetic field at different angles (e.g., 0, 30, 60, 90 degrees). Ask them to calculate the torque in each case using τ = NIAB sinθ, assuming N=1, I=1A, A=0.1m², B=0.5T. Then, ask which orientation yields maximum torque.

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

Activity 02

• 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.

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

What to look for

Pose the question: 'A galvanometer shows full-scale deflection for a current of 1 mA. How would you modify it to measure a current of 1 A? What about measuring a voltage of 10 V?' Facilitate a class discussion on the roles of shunt and series resistances.

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

Activity 03

• 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.

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

What to look for

On a small slip of paper, ask students to write down the formula for the torque on a current loop and briefly explain why a radial magnetic field is important for the working of a moving coil galvanometer.

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

Activity 04

• 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.

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

What to look for

Present students with a diagram showing a current loop in a magnetic field at different angles (e.g., 0, 30, 60, 90 degrees). Ask them to calculate the torque in each case using τ = NIAB sinθ, assuming N=1, I=1A, A=0.1m², B=0.5T. Then, ask which orientation yields maximum torque.

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

A few notes on teaching this unit

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.

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.

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Watch Out for These Misconceptions

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

  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.

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

  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.

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

  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.

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Methods used in this brief

• Case Study Analysis