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Find the velocity field due to two sources of strength m: one located at (0, 0, l) andthe other – at (0, 0, −l). Where is the velocity zero? Find the velocity at an arbitrary point (x, y, 0) inthe xy-plane. Where in the xy-plane is the speed greatest?Hint: The scalar potential for the two-source system isφ(x, y, z) = φ(r) = − m|r − lk| − m|r + lk|

Question

Find the velocity field due to two sources of strength m: one located at (0, 0, l) andthe other – at (0, 0, −l). Where is the velocity zero? Find the velocity at an arbitrary point (x, y, 0) inthe xy-plane. Where in the xy-plane is the speed greatest?Hint: The scalar potential for the two-source system isφ(x, y, z) = φ(r) = − m|r − lk| − m|r + lk|

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Solution

The velocity field due to a source of strength m at a point (x, y, z) is given by the gradient of the scalar potential φ at that point. The scalar potential for a source at (0, 0, l) is φ1 = -m/|r - l| and for a source at (0, 0, -l) is φ2 = -m/|r + l|. The total scalar potential φ for the two-source system is the sum of these two potentials, i.e., φ = φ1 + φ2 = -m/|r - l| - m/|r + l|.

The velocity field v is the negative gradient of the scalar potential, i.e., v = -∇φ. Therefore, we need to compute the gradient of φ. The gradient in cylindrical coordinates (r, θ, z) is given by ∇φ = (∂φ/∂r, 1/r ∂φ/∂θ, ∂φ/∂z).

The velocity is zero where the gradient of the scalar potential is zero. This occurs when ∂φ/∂r = ∂φ/∂θ = ∂φ/∂z = 0.

To find the velocity at an arbitrary point (x, y, 0) in the xy-plane, we substitute z = 0 into the expression for the velocity field.

The speed is greatest in the xy-plane where the magnitude of the velocity is greatest. This occurs where the derivative of the magnitude of the velocity with respect to r is zero and the second derivative is negative (indicating a maximum), and similarly for θ and z.

Note: The above steps provide a general approach to solving the problem. However, the actual computations can be quite involved, depending on the specific form of the scalar potential φ.

This problem has been solved

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