Question
To study the pattern of magnetic field lines around a straight current carrying wire.
✓
Answer
→ As shown in figure 1 , consider a straight thick wire, passed through the centre of big size square shaped card board.
→ Now connect a $12 V$ battery, variable resistance (in the form of rheostat), an ammeter with a range of 0 to $5 A$ with above wire as shown in figure 1 (keeping wire $X Y$ vertical)
→ Now, sprinkle iron filings on above card board uniformly around the wire.
→ " Now, when key is closed, current starts passing through the vertical wire from $X$ to $Y$.
→ Now tap the card board gently few times. We notice that iron filings get aligned in the form of concentric circles.
→ These circles represent circular magnetic field lines. When a compass is placed at any point on any of these circles, its north pole points along the tangent to that circle at that point, which gives direction of magnetic field at that point.
→ "When current in the wire increases, deflection of needle in the compass increases proportionally. But when distance of compass from the wire is increased, deflection of needle in the compass decreases.
→ As shown in figure 2, when direction of current passing through the wire is reversed, north pole of needle in the compass points in opposite direction.
•Conclusion :
→ If strength of magnetic field is $B$, current through the wire is $I$, radius of circular field line is $r$ then we conclude that :
(i) $B \propto I$
(ii) $B \propto \frac{1}{r}$
→ Also, when direction of current is reversed, direction of magnetic field also gets reversed.
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