The human heart consists largely of elongated muscle cells, some 100 ?m long and

Question

The human heart consists largely of elongated muscle cells, some 100 ?m long and 15 ?m in diameter. In its resting state, a cell contains two concentric layers of charge, which confine the electric field to the cell membrane, as in the first figure. When the heart contracts, a wave of depolarization sweeps through, depleting charge and giving each cell a dipole moment, as in the second figure. As a result, the entire organ acts like an electric dipole, producing an external field, which is indirectly detected by electrocardiography. Although the direction of the heart's dipole moment varies, the third figure is typical. In answering the questions that follow, consider the heart in isolation-don't concern yourself with the effect of surrounding tissues on its electric field. (Figure 1)

At a distance , far from the heart, the heart's electric field

a) falls off as 1/r

b) falls off as 1/r2

c) falls off as 1/r3

d) falls off as 1/r4

Explanation / Answer

c) as electric field due to a dipole always proportional to 1/r3 .

For a small dipole field can be given at axial point = 2kp/r3.. where p is magnitude of dipole moment.

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