Figure (a) and (b): The diagrams show the magnetic field pattern around a straight conductor carrying current.
The Magnetic field lines around a straight conductor carrying current are concentric circles whose centres lie on the wire. The direction of magnetic field lines can be determined using the Right-Hand Thumb Rule.
Factors affecting the strength of the magnetic field are:
1. Magnetic field strength is directly proportional to the magnitude of the current flowing in the conductor. i.e. B∝I, greater the current in the conductor, stronger will be the magnetic field produced.
2. Magnetic field strength is inversely proportional to the distance from the wire i.e. B∝1r, greater the distance from the current-carrying conductor, weaker will be the magnetic field.
Consider an infinitely long conductor AB through which current I flow. Let P be any point at a distance from the centre of the conductor. Consider dl to be the small current-carrying element at point c at a distance r from point p. α be the angle between r and dl. l be the distance between the centre of the coil and elementary length dl.
From biot-savant law, magnetic field due to current carrying element dl at point P is