Bend a straight wire into a loop, and its concentric circular fields overlap and straighten out into one uniform field at the centre.
Every point on the loop adds its own circular field: near the wire the loops are small, but at the centre they all line up into one straight, uniform field.
A current-carrying straight wire's field weakens with distance: the field circles get bigger and bigger farther from the wire. Now bend that same wire into a circular loop: every point along the loop still produces its own set of concentric circles, but because the whole loop is curved, these fields add up in a new way.
Near the wire itself, the field still looks like small circles wrapping around it. But moving toward the centre of the loop, contributions from every point on the loop line up in the same direction, so the circles effectively become straight, parallel lines, all pointing the same way. That means the field at the centre of a current loop is strong and uniform.
If the loop has n turns instead of just one, the field at the centre is n times as strong, since each turn's current contributes in the same direction and the effects simply add up.
The right-hand rule still applies to figure out the field's direction: just apply it to the current at any point along the curved loop, and you'll find every part of the loop contributes to the field in the same direction inside the loop.
Key exam points
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Class 10 Physics Chapter 13 | Magnetic Field Due To A Current Through A Circular Loop · Magnet Brains