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1.3 Current (I) and Magnetic Field (H) 5 1.3 Current (I) and Magnetic Field (H) It is a very well known phenomenon that a current carrying wire generates a magnetic field. Figure 1.3-1 Magnetic Field for a current carrying wire As shown in Figure 1.3-1, the lines of the magnetic field are concentric circles for a long, straight, current-carrying wire and their direction is given by the right hand rule. Most of you will be familiar with the Biot-Savart law which allows the vector field B to be calculated. Its modulus is shown in equation (1.3-1) | | Where represents the distance from the wire and the magnetic permeability of space. In as much as the B field is considered appropriate to represent magnetic fields in free space, 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. In free space a very simple relationship exists between the H and B fields Equations (1.3-1) and (1.3-2) show that B and H have the same direction and orientation but the magnitude of H is obtained by dividing the magnitude of the B field by the scalar quantity . However when magnetic materials are present, this relationship no longer holds true and B and H may be different not only in magnitude but also in direction and orientation. Because of this, the H field is a more general representation of the magnetic field and hence, from here forth, by magnetic field we will intend the H field, unless otherwise specified. This is also the field which is used at radio frequency. Equation (1.3-1) shows that a scalar quantity, current, may be used to describe the flow of charge in an electric circuit. This, as in the case of voltage and electric field, greatly simplifies the analysis of the circuit but it is appropriate only under specific assumptions which are generally satisfied at low frequency. At higher frequencies however, a current alone may not be sufficient to describe the effect of a magnetic field and hence the vector H may need to be used instead. (1.3-2) (1.3-1) Conquer Radio Frequency 5 www.cadence.com/go/awr