Since the direction of current in the straight conductor is from Q to P, then according to Maxwell's right hand thumb rule, magnetic field at point A is inside the paper and at point B is outside the paper. Since rl < 12 the strength of the magnetic field at A is more than at B because greater the distance of a point from the current carrying wire, weaker will be the magnetic field produced at that point.
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Answer:
Current is generally defined as the rate of flow of charge. We already know that stationary charges produce an electric field which is proportional to the magnitude of the charge. The same principle can be applied here, moving charges produce magnetic fields which are proportional to the current and hence a current carrying conductor produces magnetic effect around it. This magnetic field is generally attributed to the sub-atomic particles in the conductor, for e.g. the moving electrons in the atomic orbitals.
A magnetic field has both magnitude and direction. Hence, it is a vector quantity and is denoted by B (in the diagram given below). Magnetic field due to a current-carrying conductor depends on the current in the conductor and distance of the point from the conductor. The direction of the magnetic field is perpendicular to the wire. If you wrap your right hand’s fingers around the wire with your thumb pointing in the direction of the current, then the direction in which the fingers would curl will give the direction of the magnetic field. This will be clearer with the diagram shown below where the red lines represent the magnetic field lines.
Explanation: