Board question: The given figure shows the current passing through the straight conductor XY. (i) Copy the diagram and draw the magnetic field lines when current flows from conductor X to Y. (ii) Name and state the rule used in determining the direction of the magnetic field lines in the situation given above.
Paper: CBSE 2025 Main 31/2/1 Q36(a)(i) and (ii), 3 of the question’s 5 marks (the same question is 31/2/2 Q35(a) and 31/2/3 Q34(a); its part (iii), Fleming’s left-hand rule, is left out here). In the paper’s figure, X is at the top of the wire and Y at the bottom.
What the question asks: draw the magnetic field lines round the wire XY, with the right shape and the right direction for a current flowing from X to Y, then write the name of the rule that gives that direction and state it.
Key words: the conductor is a straight wire passing through a flat card. Field lines show the magnetic field: a compass needle placed on a line points along it. The current flows from X at the top down to Y.
Step 1, the shape: the field lines are circles on the card, all centred on the wire. Circles with one common centre are called concentric circles. Why circles: iron filings sprinkled on the card settle into these rings round the wire.
Step 2, the direction: point your right thumb along the current, down from X to Y. Your fingers curl clockwise, seen from above, so every arrow on the circles goes clockwise.
Step 3, the spacing: draw the circles close together near the wire and wider apart farther out. Why: the magnetic field is strongest near the wire and gets weaker with distance from it. A larger current makes the field stronger.
Step 4, the rule: the right-hand thumb rule. Imagine holding the current-carrying straight wire in your right hand with the thumb pointing along the current; then your fingers wrap round the wire in the direction of the magnetic field lines.
Answer, marking scheme value points (CBSE 2025 Main 31/2/1 Q36(a)): (i) diagram: concentric circles round XY with their direction (1½). (ii) Right-hand thumb rule (½); its statement (1).
The trap: drawing the arrows anticlockwise. Anticlockwise (seen from above) is the field of a current flowing up. Here the current flows down, from X to Y, so the arrows go clockwise.
Homework: question 4 (i) and (ii) of the 22 Oct set is this same figure and question.
Quick recall (Magnetism): Field inside a long solenoid? → Uniform (parallel straight lines, the same strength everywhere inside)
Comment challenge (based on CBSE 2023 Main 31/2/2 Q25): a circular loop of wire lies flat on a table and the current goes round it anticlockwise, seen from above. Inside the loop, does the magnetic field point up or down? Answer in the pinned comment.
References: NCERT Class 10 Science, Ch 12 Magnetic Effects of Electric Current, sec 12.2.1 (Fig 12.6: the field lines round a straight current-carrying wire are concentric circles; the field gets stronger as the current increases and weaker farther from the wire) and sec 12.2.2 (Fig 12.7, right-hand thumb rule: “Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field”); CBSE Class 10 board questions and marking schemes.
Narration uses a licensed AI voice (Azure AI Speech, Indian English). Script and physics research by ScoreSpark.
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Board question (CBSE 2025 Main 31/2/1 Q36(a)(i) and (ii)): The given figure shows the current passing through the straight conductor XY. (i) Copy the diagram and draw the magnetic field lines when current flows from conductor X to Y. (ii) Name and state the rule used in determining the direction of the magnetic field lines in the situation given above.
Board answer (CBSE marking scheme 2025 Main 31/2/1 Q36(a), NCERT Fig 12.6 and Fig 12.7): (i) concentric circles on the card round XY, closer together near the wire, with arrows clockwise seen from above, because the current flows down from X to Y (1½). (ii) Right-hand thumb rule (½). Imagine holding a current carrying straight conductor in the right hand such that the thumb points towards the direction of current; then the fingers wrap around the conductor in the direction of the field lines of the magnetic field (1).
Comment challenge (based on CBSE 2023 Main 31/2/2 Q25): a circular loop of wire lies flat on a table and the current goes round it anticlockwise, seen from above. Inside the loop, does the magnetic field point up or down?
Answer: up, out of the table. Use the right-hand thumb rule on each part of the loop: with the current anticlockwise seen from above, the fingers point upward everywhere inside the loop, so all the field lines inside point up (and outside the loop they come back down). CBSE 2023 Main 31/2/2 Q25 marking scheme: direction of current ½, direction of the field lines ½, pattern of the field lines 1.
Quick recall (Magnetism): Field inside a long solenoid? → Uniform (parallel straight lines, the same strength everywhere inside)