Magnetic Field due to Current Carrying Wire
Straight wire :
It means that when the current flows in a straight wire, the magnetic field produced has circular lines of force surrounding the wire, having their centres at the wire as shown in fig. The plane of circular lines is perpendicular to the length of the wire. Their direction is marked by arrows.
Current straight, magnetic field circular
It means that when the current flows in a circular wire (coil), the magnetic field produced has straight lines of force near the centre of the coil, as shown in figure. The parallel lines are in a plane perpendicular to the plane of the coil. Their direction is marked by the arrows.
Rule : The direction of the magnetic lines of force is related with the direction of the current by the right hand thumb rule.
The rule states :
Curl the four fingers of the right hand on the palm, keeping the thumb stretched out at right angles. The thumb is straight and the fingers are circular.
In case 1. thumb represents the direction of the current in the straight wire and curling of fingers represents the direction of the circular magnetic lines of force (fig.).
In case 2, curled fingers represents the direction of the current in circular wire and thumb represents the direction of the straight magnetic lines of force. (Fig.)
Right hand thumb rule for direction of magnetic field.
Definition : A solenoid is a straight cylindrical core on which a large number of turns of a insulated copper wire are wrapped. It is shown in fig.
Solenoid carrying a current and polarity of its face