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Electric charges are source of electric fields. An electric field exerts force on an electric charge, whether the charge happens to be moving or at rest.
One could similarly think of a magnetic charge as being the source of a magnetic field. However, isolated magnetic charge ( or magnetic monopoles) have never been found to exist. Magnetic poles always occur in pairs ( dipoles) - a north pole and a south pole. Thus, the region around a bar magnet is a magnetic field. What characterizes a magnetic field is the qualitative nature of the force that it exerts on an electric charge. The field does not exert any force on a static charge. However, if the charge happens to be moving (excepting in a direction parallel to the direction of the field) it experiences a force in the magnetic field.
It is not necessary to invoke the presence of magnetic poles to discuss the source of magnetic field. Experiments by Oersted showed that a magnetic needle gets deflected in the region around a current carrying conductor. The direction of deflection is shown in the figure below. |
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Thus a current carrying conductor is the source of a magnetic field. In fact, a magnetic dipole can be considered as a closed current loop. |
