Magnetic Effects of Electric Current – Short Answer Type Questions
CBSE • NCERT • Class 10
Physics — Chapter 12: Magnetic Effects of Electric Current — 50 Short Answer Questions
Topic-wise short-answer Q&A for thorough revision and concept clarity, aligned with NCERT and CBSE Class 10 exam pattern.
Use:
Class tests, revision, worksheets and quick revision before boards.
Class tests, revision, worksheets and quick revision before boards.
- Fundamentals & field lines (Q1–Q8)
- Magnetic field due to current & rules (Q9–20)
- Solenoid & electromagnet (Q21–29)
- Force on conductor & motors (Q30–40)
- Electromagnetic induction & applications (Q41–47)
- Mixed short answers & exam tips (Q48–50)
Fundamentals & Magnetic Field Lines (Q1–Q8)
Q1. Define magnetic field.
Answer
Magnetic field is the region around a magnet or current-carrying conductor where magnetic forces are experienced; represented by field lines.
Q2. What do magnetic field lines show?
Answer
They show direction and relative strength of the magnetic field; direction is from north to south outside a magnet, and density indicates strength.
Q3. State two properties of magnetic field lines.
Answer
(1) They form closed loops. (2) They never cross each other.
Q4. How can you visualize field lines around a bar magnet?
Answer
Sprinkle iron filings on paper over the magnet; tap gently — filings align along field lines, revealing the pattern.
Q5. Why are field lines closer near the poles?
Answer
Because the magnetic field is stronger near the poles; closer lines indicate higher field strength.
Q6. What is meant by the north pole of a magnet?
Answer
The north pole is the end of a magnet that points toward the geographic north when freely suspended; field lines emerge from it.
Q7. Do field lines exist inside a magnet? If so, direction?
Answer
Yes — inside the magnet field lines run from the south pole to the north pole, completing closed loops.
Q8. How does a compass show the direction of magnetic field?
Answer
The compass needle aligns with the magnetic field; its north-seeking end points in the field direction (from local north pole).
Magnetic Field due to Current & Direction Rules (Q9–20)
Q9. Who discovered the magnetic effect of electric current?
Answer
Hans Christian Oersted discovered that a current-carrying wire deflects a compass needle, showing current produces a magnetic field.
Q10. Describe the magnetic field around a straight current-carrying conductor.
Answer
Field lines are concentric circles centered on the wire, lying in planes perpendicular to the wire; direction given by right-hand rule.
Q11. State the right-hand thumb rule.
Answer
If the right-hand thumb points along current, the curl of fingers shows direction of magnetic field around the wire.
Q12. How does magnetic field vary with distance from a long straight wire?
Answer
The magnetic field strength decreases as distance from the wire increases; for a long straight wire B ∝ 1/r (inverse relation).
Q13. How does the magnetic field depend on current?
Answer
Magnetic field is directly proportional to current — increasing current increases field strength around the conductor.
Q14. What is the field pattern at the center of a circular current loop?
Answer
At the centre, the magnetic field is nearly straight and perpendicular to the plane of the loop; its direction given by the right-hand rule for loops.
Q15. What is the right-hand grip rule (for a coil)?
Answer
If you curl the fingers of the right hand in the direction of current in the coil, the thumb points in the direction of the magnetic axis (north pole) of the coil.
Q16. How would you use a compass to map field around a wire?
Answer
Place compass at different points around the wire and mark needle directions to trace circular field lines; nearer positions show larger deflection.
Q17. What happens to a compass needle when current is switched on in nearby wire?
Answer
It deflects from its original direction, indicating presence and direction of the magnetic field produced by the current.
Q18. Name two factors on which magnetic field around conductor depends.
Answer
(1) Magnitude of current. (2) Distance from the conductor. (Also medium/permeability and geometry.)
Q19. What is magnetic field inside a very long straight hollow tube carrying current on its surface (qualitative)?
Answer
If current flows on the surface symmetrically, the internal magnetic field can be zero (for ideal symmetric distributions), similar to hollow conductor arguments; depends on distribution.
Q20. In which direction do field lines due to a current loop emerge outside the loop?
Answer
Outside the loop, field lines emerge from the loop's north pole and return to its south pole, similar to a bar magnet's external field.
Solenoid & Electromagnet (Q21–29)
Q21. Define a solenoid.
Answer
A solenoid is a long helical coil of wire; when current passes through it, it produces a magnetic field similar to that of a bar magnet with a strong uniform field inside.
Q22. What is the expression for magnetic field inside an ideal solenoid?
Answer
B = μ₀ n I where n = number of turns per unit length (N/L), I is current, and μ₀ is permeability of free space.
Q23. What is an electromagnet?
Answer
An electromagnet is a coil wound on a soft iron core that becomes magnetised when current flows and loses magnetism when current stops.
Q24. Why is soft iron preferred as core material?
Answer
Because soft iron is easily magnetised and demagnetised; it enhances field strength when current flows and does not retain strong magnetism after current stops.
Q25. How can the strength of an electromagnet be increased?
Answer
Increase current, increase the number of turns, use a soft iron core, or make the coil more tightly wound (increase n or I).
Q26. Give one application of electromagnets.
Answer
Lifting heavy scrap-iron in scrapyards, electric bells, relays, or loudspeakers are common applications.
Q27. How does field outside a long solenoid compare to inside?
Answer
Inside the field is strong and nearly uniform; outside the field is weak and spreads out, similar to a bar magnet's external field.
Q28. What is turns per unit length (n) and its unit?
Answer
n = N/L (number of turns divided by length); unit is turns per metre (m⁻¹).
Q29. State one difference between a permanent magnet and an electromagnet.
Answer
Permanent magnet retains magnetism without power; electromagnet requires current and its magnetism can be switched on/off.
Force on Conductor & Electric Motor (Q30–40)
Q30. State Fleming's left-hand rule in brief.
Answer
Thumb = motion (force), First finger = field (N→S), Second finger = current (conventional). Used to find direction of force in motors.
Q31. Give formula for force on a straight conductor of length L carrying current I in magnetic field B (when perpendicular).
Answer
F = B I L (direction given by Fleming's left-hand rule).
Q32. A conductor of length 0.4 m carries 2 A in 0.5 T field (perpendicular). Calculate force.
Answer
F = BIL = 0.5 × 2 × 0.4 = 0.4 N.
Q33. What produces torque on a current loop in a magnetic field?
Answer
Forces on opposite sides of the loop act in opposite directions producing a couple (torque) that tends to rotate the loop.
Q34. What is the role of commutator in a DC motor?
Answer
It reverses the direction of current in the coil every half rotation so that torque acts in the same direction, ensuring continuous rotation.
Q35. How are brushes arranged in a simple motor?
Answer
Brushes are stationary contacts that press against the rotating commutator segments to supply current to the rotating coil.
Q36. Give one practical example of motor use in daily life.
Answer
Electric fans, washing machines, mixers, and ceiling fans use electric motors.
Q37. If current direction is reversed, what happens to motor rotation?
Answer
Rotation direction reverses because torque direction depends on current direction.
Q38. How can you increase torque in a motor without changing supply voltage?
Answer
Increase current (if possible), increase number of turns in coil, or use a stronger magnetic field (stronger magnets or core).
Q39. Why is the armature wound with many turns?
Answer
Multiple turns increase the effective length of conductor in magnetic field, increasing torque and output power.
Q40. Mention one safety precaution while handling motors and coils.
Answer
Avoid overcurrent which causes heating; insulate connections, keep hands away from rotating parts, and use proper fuses or circuit breakers.
Electromagnetic Induction & Applications (Q41–47)
Q41. What is electromagnetic induction (brief)?
Answer
It is the production of emf (and hence current) in a conductor due to change in magnetic flux linked with it.
Q42. State Lenz's law in one line.
Answer
The direction of induced current is such that it opposes the change in magnetic flux that produces it.
Q43. What effect does increasing the speed of relative motion have on induced emf?
Answer
It increases the induced emf because the rate of change of magnetic flux is higher.
Q44. Give one example of electromagnetic induction in everyday life.
Answer
Electric generators produce electricity by rotating coils in magnetic fields — a key example of induction.
Q45. What is Fleming's right-hand rule used for?
Answer
To find the direction of induced current when a conductor moves in a magnetic field (used for generators).
Q46. How does number of turns in coil affect induced emf?
Answer
Induced emf is proportional to number of turns: more turns → greater emf for the same change in flux.
Q47. Why are transformers important in power distribution (basic idea)?
Answer
Transformers use electromagnetic induction to step up or step down AC voltages; they enable efficient long-distance power transmission by raising voltage and lowering current.
Mixed Short Answers & Exam Tips (Q48–50)
Q48. Suggest one way to score marks easily in diagram-based questions in this chapter.
Answer
Draw neat, labelled diagrams showing current direction, field lines with arrowheads, poles, and label forces or motion where applicable.
Q49. List two quick revision points for this chapter.
Answer
(1) Remember right-hand and left-hand rules and when to use each. (2) Revise formulae: B ∝ I/r (wire) and B ∝ nI (solenoid), and F = BIL.
Q50. How should you present answers in exams to gain method marks in numericals from this chapter?
Answer
Show all steps, write the formula, substitute values with units, do unit conversions clearly, compute and state the final answer with units.