The magnetic fields generated by currents and calculated from Ampere's Law or the Biot-Savart Law are characterized by the magnetic field B measured in Tesla. But when the generated fields pass through magnetic materials which themselves contribute internal magnetic fields, ambiguities can arise about what part of the field comes from the external currents and what comes from the material itself. It has been common practice to define another magnetic field quantity, usually called the "magnetic field strength" designated by H. It can be defined by the relationship
Magnetic fields can be produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property , their spin .   In special relativity , electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor ; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics , the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons .
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