Equations for the Magnetic Field Produced by One or More Rectangular Loops of Wire in the Same Plane.

Beginning with expressions for the vector potential, the equations for calculating the magnetic flux density from up to three rectangular loops of wire in the same plane are derived. The geometry considered is the same as that found in some walk-through metal detectors and electronic article surveillance systems. Equations for more or fewer loops can be determined by inspection. A computer program for performing the magnetic field calculation is provided in an appendix.

1. Introduction

The expression for the magnetic flux density from a single rectangular loop of wire of many turns can be found in text books and various publications [1-3]. The rectangular geometry is convenient, in part, because the expressions for the three spatial components of the flux density are in closed form. Single square coils have been used for calibration of extremely low frequency magnetic field meters for applications that require uncertainties of a few percent [2]. Multiple rectangular loops with a common axis have found applications in a number of fields, including biological exposure systems for in vivo and in vitro studies [3,4]. It is also noteworthy that a square Helmholtz coil produces a greater volume of nearly uniform magnetic field than a circular Helmholtz coil of comparable dimensions [5]. This paper develops expressions for the magnetic flux density produced by three rectangular loops of wire that lie in the same plane, i.e., loops that are not co-axial. The geometry is similar to that used in some walk-through metal detectors and electronic article surveillance systems. By inspection, the expressions for more or fewer loops are easily determined. We consider static and time varying fields that are quasi-static. In the latter case, the wavelength λ of the time varying field is much greater than any dimension or distance of interest. For example, a 1 MHz alternating field (λ ≈ 300 m) is well approximated as being quasi-static a few meters or less from loops of comparable dimensions. The quasi-static condition allows us to solve the static field problem first and, with negligible error, introduce the time dependence as a multiplicative factor, e.g., the direct current in the field equations could be replaced with an alternating current. The field equations are for rectangular loops with a single turn of wire. The magnetic flux density for loops with more than one turn are found by multiplying the equations by the appropriate number of turns.

2. Field Equations

We follow the development of Weber [1] by first considering the vector potential for a rectangular loop of wire in the x-y plane, A and A, and then calculating the vector components of the magnetic flux density using the relations For a single rectangular loop of wire of negligible wire cross section, designated as loop 1, with side dimensions 2a1 by 2b1 as shown in Fig. 1, the components of the vector potential are [1] and where μ0 is the magnetic constant (also called the magnetic permeability of vacuum), and I1 is the current in the loop.

Fig. 1

Geometry for a single rectangular loop of wire in the x-y plane. The magnetic flux density is evaluated at point P(x, y, z).

The parameters r1, r2, r3, and r4 are the distances from the corners of the loop to the point P(x, y, z) where the magnetic flux density will be evaluated (see below and Fig. 1).

The z-component of the magnetic flux density at P(x, y, z) is where Equation (4) is equivalent to that given in Ref. [1], but perhaps in a more convenient form for writing a computer program to calculate the magnetic flux density.

From Eqs. (1) to (3), the expressions for the x- and y-components of the magnetic flux density can be readily derived and are and

The x-component of the vector potential for a second loop of wire of side dimensions 2a2 by 2b2 that is displaced from the origin by a distance s2 and bisected by the y-axis (see Fig. 2) is given by [1] where and I2 is the current in loop 2.

Fig. 2

Geometry for a second rectangular loop of wire in x-y plane. The point P(x, y, z) coincides with that in Fig. 1 (note that the scales of Figs. 1 and 2 are not the same).

The integrals can be solved using elementary methods and yield where , , , and are the distances from the corners of loop 2 to the point P(x, y, z) where the magnetic flux density will be evaluated (see below).

The expression for A2 can be similarly determined and is given by Taking the appropriate derivatives of Eqs. (8) and (9), the expression for the z-component of the magnetic flux density at P(x, y, z) associated with loop 2 is where From Eqs. (1), (8), and (9), the x- and y-components of the magnetic flux density due to loop 2 are and

The equations for the flux density components at P(x, y, z) from a third rectangular loop with side dimensions 2a3 by 2b3, displaced from the origin by a distance s3 and bisected by the y-axis follow by inspection. That is where and I3 is the current in loop 3.

The x- and y-components of the magnetic flux density due to loop 3 are and

The spatial components of the magnetic flux density at P(x, y, z) due to all three loops (Fig. 3) are found by summing the respective contributions from each loop, i.e.,

Fig. 3

Geometry for three rectangular loops of wire in a vertical plane. The origin of the coordinate system is at the center of loop 1.

For direct currents in the loops, the direction of the magnetic flux density will remain fixed and is described by the vector where , , and are unit vectors along the x, y, and z directions, respectively. The magnitude of the magnetic flux density vector will also be constant and equal to

For alternating currents in the loops that are in phase, for example I1sin(ωt), I2sin(ωt), and I3sin(ωt), the magnetic flux density is described by the vector where I1, I2, and I3 are current amplitudes, ω is the angular frequency, and t is the time. The flux density is said to be linearly polarized because of its oscillatory motion along a straight line. The magnitude of the vector will be time dependent and equal to

If the alternating currents in the various loops are not in phase, the magnetic flux density vector will rotate and the point of the vector will, in general, trace an ellipse [6]. The magnitude and direction of the magnetic flux density at a given point in space will change as a function of time. For this case, the flux density is said to be elliptically polarized.

As a convenience to the reader, a program for calculating the static magnetic flux density from three coils in the x-y plane as shown in Fig. 3 is provided in Appendix A.