Physical Significance of gradient.
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WOW200. k. h. l. b..............
Physical Significance of Gradient
A scalar field may be represented by a series of level surfaces each having a stable value of scalar point function θ. The θ changes by a stable value as we move from one surface to another. These surfaces are known as Gaussian surfaces. Now let the two such surfaces are very close together, be represented by two scalar point functions and (θ + d θ). Let ‘r’ and (r + d θ) be the position vectors of points A and B, on the surfaces θ and (θ + d θ) correspondingly with respect to an origin 0 as shown in Figure. Clearly, the vector AB will be dr. Let the least detachment between the two surfaces ‘dn’ be in the direction of unit usual vector n at A.
Physical Significance of Gradient
dn = dr cos θ
= | n | dr | cos θ =n .dr
Dϕ = ∂ϕ/ dn =∂ϕ/dn n .dr…… (1)
Since the continuous scalar function defining the level surfaces (Gaussian surfaces) has a value θ at point A (x, y, z) and (θ + dθ) at point (x + dx, y + dy, z + dz), we have
dϕ = ∂ ϕ/dx dx +dϕ/∂y + ∂ϕ/∂x dz
= (I ∂ϕ/∂x +j ∂ϕ/vy +k ∂ϕ/∂z) .(idx +jdy +kdz)
= ∆ ϕ. dr … … (2)
From equations (1) and (2), equating the values of d θ,
We obtain ∆θ .dr =∆ ϕ= ∂ϕ/ ∂n n .dr
As dr is an arbitrary vector, we have
∆ϕ =∂ϕ/∂ n
Grad ϕ = ∂ ϕ/∂n n
Therefore, the gradient an of a scalar field at any point is a vector field, the scale of which is equal to the highest rate of increase of θ at that point and the direction of it is similar as that of usual to the level surface at that point.
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