CBSE Class 10 Science 2019 · Paper 31/1/2/C · Question 17

(a) Activity: Pass a thick straight copper wire vertically through the centre of a horizontal rectangular cardboard. Connect it in series with a battery, a plug key, a rheostat and an ammeter so that the current flows downward through the wire (upper end towards the positive terminal). Sprinkle iron filings uniformly on the cardboard, close the key and tap the cardboard gently. The iron filings arrange themselves in concentric circles around the wire: these are the magnetic field lines of the straight conductor. A compass placed on a circle points along it and shows that, for the downward current, the field lines run clockwise when viewed from above. (b) Right-hand thumb rule: Imagine holding the current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current; then the fingers wrapped around the conductor give the direction of the magnetic field lines. In this situation, the thumb points vertically downward along the current, and the fingers curl clockwise as seen from above. So the magnetic field lines are concentric circles around the wire in the clockwise direction when viewed from above.