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www.GOALias.blogspot.com4.8.1 The solenoidWe shall discuss a long solenoid. By long solenoid we mean that thesolenoid’s length is large compared to its radius. It consists of a longwire wound in the form of a helix where the neighbouring turns are closelyspaced. So each turn can be regarded as a circular loop. The net magneticfield is the vector sum of the fields due to all the turns. Enamelled wiresare used for winding so that turns are insulated from each other.Moving Charges andMagnetismFIGURE 4.17 (a) The magnetic field due to a section of the solenoid which has beenstretched out for clarity. Only the exterior semi-circular part is shown. Noticehow the circular loops between neighbouring turns tend to cancel.(b) The magnetic field of a finite solenoid.Figure 4.17 displays the magnetic field lines for a finite solenoid. Weshow a section of this solenoid in an enlarged manner in Fig. 4.17(a).Figure 4.17(b) shows the entire finite solenoid with its magnetic field. InFig. 4.17(a), it is clear from the circular loops that the field between twoneighbouring turns vanishes. In Fig. 4.17(b), we see that the field at theinterior mid-point P is uniform, strong and along the axis of the solenoid.The field at the exterior mid-point Q is weak and moreover is along theaxis of the solenoid with no perpendicular or normal component. As thesolenoid is made longer it appears like a long cylindrical metal sheet.Figure 4.18 represents this idealised picture. The field outside the solenoidapproaches zero. We shall assume that the field outside is zero. The fieldinside becomes everywhere parallel to the axis.FIGURE 4.18 The magnetic field of a very long solenoid. We consider arectangular Amperian loop abcd to determine the field.151