Case Study:

In a typical household experiment, a student placed a small magnetic compass on a table near a mobile phone charger connected to a live socket. Surprisingly, the compass needle showed a noticeable deflection even though no permanent magnet was present nearby. When the charger wire was moved farther away, the deflection reduced significantly and eventually disappeared.

The student repeated the observation by varying the distance between the charging cable and the compass. It was noted that the deflection was maximum when the wire was very close (within 2 - 3 cm) and decreased as distance increased. Interestingly, when the charging current was switched off, the compass needle immediately returned to its normal north - south alignment.

This observation raised a question: How can an ordinary current-carrying wire influence a compass needle? According to NCERT principles in Moving Charges and Magnetism, a current flowing through a conductor produces a magnetic field around it. This magnetic field can be described using the Biot–Savart Law and direction determined by the Right-Hand Thumb Rule. Even a small current in a charging cable generates a detectable magnetic effect at short distances, causing the compass needle to deflect.

Thus, the phenomenon highlights the direct relationship between electric current and magnetism, forming the basis of electromagnetism.

This magnetic field can be described using the Biot - Savart Law and direction determined by the Right-Hand Rule explained with easy diagrams.

Thus, the phenomenon highlights the direct relationship between electric current and magnetism in daily life, forming the basis of electromagnetism.

CASE-BASED QUESTIONS

MCQ

A1. Why does the compass needle deflect near a charging cable?
A. Due to electric charge on the wire
B. Due to gravitational force
C. Due to magnetic field produced by current
D. Due to heat from the wire

A2. The direction of magnetic field around a straight current-carrying wire is determined by:
A. Fleming’s Left Hand Rule
B. Right-Hand Thumb Rule
C. Newton’s Third Law
D. Ohm’s Law

Assertion - Reason

A3. Assertion (A): A current-carrying wire produces a magnetic field around it.
Reason (R): Moving charges always generate magnetic effects in the surrounding space.

Options:
A. Both A and R are true, and R explains A
B. Both A and R are true, but R does not explain A
C. A is true, R is false
D. A is false, R is true

Application-Based Question

A4. Why does the deflection of the compass decrease when the charging cable is moved away?

Application-Based Question

A5. What will happen to the compass needle if the charging current is switched off? Explain briefly.

Data/Logic-Based Question

A6. If the compass deflection is maximum at 2 cm, reduced at 5 cm, and negligible at 10 cm, what conclusion can be drawn about the magnetic field strength with distance?

Exam Prep Tip: After solving this case study, test your knowledge with these top 20 important questions from magnetic force on charges.

3. ANSWER KEY WITH EXPLANATION

A1. C - A current in the wire produces a magnetic field, which interacts with the compass needle.

A2. B - The Right-Hand Thumb Rule gives the direction of magnetic field lines around a straight conductor.

A3. A - Both statements are correct; moving charges (current) produce magnetic fields, explaining the assertion.

A4. Magnetic field strength decreases with distance from a current-carrying wire, so the torque on the compass needle reduces.

A5. The needle returns to north–south direction because the magnetic field due to current disappears when current is off.

A6. Magnetic field strength decreases as distance increases from the current-carrying conductor.

5. HOTS EXTENSION QUESTIONS

A. If the charging cable carries alternating current (AC), how would the compass behavior differ compared to DC current? Explain logically.

B. Suggest a method to minimize magnetic interference from household wiring while conducting sensitive compass experiments.

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