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HOW to Understood magnetic field DUE to A Current CARRYING conductor ?

When a conductor Carries an electric current, a magnetic field is Produced all along it's length. The magnetic lines of force concetric circles around the conductor. The direction of current and the direction of magnetic field are at right angles to each other. 
The magnetude of the magnetic field associated with a current carring conductor depends upon the direction of flow of current. 
The direction of the lines of force
( magnetic field ) around a straight current carrying conductor can be determined by any of the following rules.

*Ampere's rules. If an observer imagines himself the electric current, with the current flowing in the direction From feet to his head, then on facing the compass needle the North Pole will be deflected towards his left.

*Cork Screw rules

Cork Screw rules
Cork screw rules

If the Right handed cork Screw held with its axis parallel to the conductor pointing the direction of flow of current and the head of the Screw is rotated in such a direction that the Screw moves in the direction of flow of current then the the direction in which the need of screw is rotated, will be the direction of magnetic field or lines of force.

*Right Hand Rules. If the current carrying conductor is held in right hand by the observer so that is it encycled by fingers stretching the thumb at right angle to the fingers in the direction of flow of current then finger tips will point the direction of magnetic lines of force as shown in fig.

Right hand Rules
Right hand Rules

How generat a magnetic field Due to a circular loop ?
If a single turn wire carrying Current is bent in the form of a loop (or ring), as shown in fig. 

Magnetic field due to a circular loop
Magnetic field due to a circular loop

The lines of magnetic induction around it will be consentric circles, leaving the plane of the loop (or ring) on one side and and entering on the other. The loop acts as the true magnet having north and south poles.
The direction of magnetic field May be determined by the applying eighter of the two rules namely

   I) Right hand Rules.
   II) Cork screw rules, already explained in Magnetic field Due to a current carrying conductor.

How generat magnetic field in solenoid coil?

Solenoid Coil

The current carrying wire wound spirally in the form of helix about an axis as shown in image, is known as solenoid Coil. Magnetic field Produced due to current carrying solenoid is fairly uniform over a Small region in the middle of the coil. It acts just like a baar magnet having both and south poles. 
There are several methods used to determine the polarity of the solenoid.


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