Magnetic Effects of Electric Current CBQs — CBSE Class 10 (2027) with step-wise solutions

Sample Magnetic Effects of Electric Current CBQs

1. Question 1MCQ1 mark
   A student winds many turns of insulated copper wire closely on a plastic tube to make a long solenoid and connects it to a cell. She records four observations about its magnetic field. Which one of her observations is INCORRECT?
   • A compass needle placed inside the solenoid sets itself along the axis of the solenoid.
   • Iron filings around the solenoid form a pattern similar to that around a bar magnet.
   • Reversing the connections of the cell interchanges the north and south poles of the solenoid.
   • The magnetic field is strongest just outside the middle of the solenoid and weakest inside it.
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   1. [1 mark] The incorrect observation is that the field is strongest just outside the middle of the solenoid: inside a long solenoid the field lines are parallel straight lines and the field is strong and uniform, while the field outside it is weak.

   Answer: The magnetic field is strongest just outside the middle of the solenoid and weakest inside it.

2. Question 2Assertion-Reason1 mark
   Assertion (A): A compass needle placed near a straight wire is deflected when current flows through the wire, and the deflection reverses when the direction of the current is reversed. Reason (R): The direction of the magnetic field produced by a current-carrying conductor depends on the direction of the current through it.
   • (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
   • (B) Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion.
   • (C) Assertion is true but Reason is false.
   • (D) Assertion is false but Reason is true.
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   1. [1 mark] (A) Both are true and R explains A: the current produces a magnetic field that deflects the needle, and since the direction of this field depends on the direction of the current, reversing the current reverses the deflection.

   Answer: (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

3. Question 3Short answer2 marks
   A straight vertical wire carries a current flowing downwards. It is placed in a uniform horizontal magnetic field directed from west to east. (a) In which direction does the wire experience a force? Name the rule you used. (b) How should the wire be placed in this field so that it experiences no force?
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   1. [1 mark] (a) By Fleming's left-hand rule — forefinger along the field (towards east), middle finger along the current (downwards) — the thumb points towards the south, so the force on the wire is towards the south.
   2. [1 mark] (b) The wire should be placed along the direction of the magnetic field (horizontal, along the west–east line); the force is zero when the current is parallel to the field.

   Answer: (a) Towards the south (Fleming's left-hand rule); (b) along the field, in the west–east direction.

4. Question 4Case study4 marks
   Tenzin, a Class 10 student in Gangtok, placed a bar magnet under a sheet of white paper and sprinkled iron filings over it. On tapping the sheet, the filings arranged themselves in a definite pattern of curves. She then moved a small compass around the magnet and drew the field lines by joining the positions of the compass needle. She noticed that the filings were crowded together near the two ends of the magnet and spread far apart midway between the ends. In her notebook, two of the lines she drew crossed each other at one point. Her teacher marked this part of the drawing wrong. (i) Where is the magnetic field of the bar magnet strongest, and how does the pattern of filings show this? (ii) Why did the teacher mark the two crossing field lines as wrong? (iii) State the direction of the magnetic field lines outside the magnet and inside the magnet. Why are magnetic field lines described as closed curves?
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   1. [1 mark] (i) The field is strongest near the poles (ends) of the magnet; the field lines (iron filings) are most crowded there, and the relative strength of the field is shown by the degree of closeness of the field lines.
   2. [1 mark] (ii) No two field lines can cross. If they crossed, a compass needle placed at the point of intersection would point in two directions at the same time, which is not possible — the field has only one direction at a point.
   3. [1 mark] (iii) Outside the magnet, the field lines emerge from the north pole and merge at the south pole (N → S).
   4. [1 mark] (iii) Inside the magnet, the field lines run from the south pole to the north pole (S → N); hence each field line is a continuous closed curve.

   Answer: (i) Near the poles, where the lines are most crowded; (ii) field lines never intersect — a compass cannot point in two directions at one point; (iii) outside N → S, inside S → N, so the lines are closed curves.

5. Question 5Long answer5 marks
   A school robotics club is making an electromagnetic lifter to pick up iron nails. They wind 200 turns of insulated copper wire closely on a cardboard tube and connect the coil to a battery through a switch. (a) Describe the pattern of the magnetic field lines inside the tube when current flows. What does this pattern tell you about the field there? (b) State two ways in which the club can make the lifter pick up more nails. (c) Should a soft iron rod or a steel rod be placed inside the tube? Justify your choice. (d) A student facing one end of the tube sees the current in the coil flowing clockwise. Is this end a north pole or a south pole? Justify using the right-hand thumb rule. (e) Why must the copper wire used for the coil be insulated?
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   1. [1 mark] (a) Inside the tube the field lines are parallel straight lines, which shows that the magnetic field is the same at all points inside, i.e. uniform.
   2. [1 mark] (b) Any two: increase the number of turns of the coil; increase the current through the coil; place a soft iron core inside the tube.
   3. [1 mark] (c) A soft iron rod: it becomes strongly magnetised while current flows and loses its magnetism when the current is switched off, so the nails can be dropped; steel would stay magnetised (become a permanent magnet).
   4. [1 mark] (d) South pole: curling the fingers of the right hand along the clockwise current, the thumb points away from the student into the tube, so the field lines enter at this end, as they do at the south pole of a bar magnet.
   5. [1 mark] (e) The wire is insulated so that adjacent turns do not touch electrically; the current then flows around every turn of the coil instead of taking a short path between turns, and all turns add to the magnetic field.

   Answer: (a) Parallel straight lines — uniform field; (b) more turns, larger current (or soft iron core); (c) soft iron — loses magnetism when current is off; (d) south pole; (e) so the current flows through every turn.

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