Magnetic field and Circular loop
In case of a circular current carrying conductor, the magnetic field is produced in the same manner as it is in case of a straight current carrying conductor.
In case of a circular current carrying conductor, the magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor. Since, magnetic field lines tend to remain closer when near the conductor, so the magnetic field would be stronger near the periphery of the loop. On the other hand, the magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. Finally, at the centre, the arcs of big circles would appear as a straight lines.
This figure shows a conductor in the shape of a circle or a loop. You can imagine that a straight conductor has been twisted to make a loop. The way magnetic field lines make circles around a straight conductor, they make similar circles around the loop-like conductor. You can see that circles are smaller near the conducting wire. As we move away from the conductor, i.e. towards the centre of the loop, the magnetic field lines take the form of straight lines. In fact, these lines are part of a bigger circle, and appear as straight lines because of very big diameter of the circle.
The direction of magnetic field can be identified using Right Hand Thumb's Rule. Let us assume that the current is moving in anti-clockwise direction in the loop. In that case, the magnetic field would be in clockwise direction, at the top of the loop. Moreover, it would be in anticlockwise direction at the bottom of the loop.
Clock Face Rule
A current carrying loop works like a disc magnet. The polarity of this magnet can be easily understood with the help of clock face rule. If the current is flowing in anti-clockwise direction, then the face of the loop shows north pole. On the other hand, if the current is flowing in clockwise direction, then the face of the loop shows south pole. The figure on the left hand side shows anti-clockwise current and North Pole. The figure on the right hand side shows clockwise current and South Pole.
Magnetic field and number of turns of coil
Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are n turns of coil, magnitude of magnetic field will be n times of magnetic field in case of a single turn of coil.
Magnetic Field due to a current in a Solenoid:
Solenoid is the coil with many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. In other words, a solenoid is a device which converts electrical energy into magnetic force. The term solenoid was coined by Andre Marie Ampere in 1823.
A current carrying solenoid produces similar pattern of magnetic field as a bar magnet. One end of solenoid behaves as the north pole and another end behaves as the south pole. Magnetic field lines are parallel inside the solenoid, similar to a bar magnet. It shows that magnetic field is same at all points inside the solenoid.
By producing a strong magnetic field inside the solenoid, magnetic materials can be magnetized. Magnet formed by producing magnetic field inside a solenoid is called electromagnet.
Uses of Solenoid
Solenoid or electromagnet is used in many devices. It is used in call bell. It is used in electric motor. In fact, a solenoid is used wherever a controlled magnetic field is required.