Problem 2 Using index notation, demonstrate mathematically the following identities: 1- \underline{u} \times \underline{v} = -\underline{v} \times \underline{u} 2- \nabla \cdot (\nabla \underline{a}) = \nabla^2 \underline{a} 3- \nabla^2 (\underline{a}\underline{b}) = \underline{a}\nabla^2 \underline{b} + 2\nabla \underline{a} \cdot \nabla \underline{b} + \underline{b}\nabla^2 \underline{a} 4- \nabla \cdot (\underline{a}\underline{b}) = \nabla \underline{a} \cdot \underline{b} + \underline{a}\nabla \cdot \underline{b} 5- \nabla \times \phi = 0 6- \nabla \cdot (\nabla \times \underline{v}) = 0 7- (\underline{a} \times \underline{b}) \cdot (\underline{c} \times \underline{d}) = (\underline{a} \cdot \underline{c})(\underline{b} \cdot \underline{d}) - (\underline{a} \cdot \underline{d})(\underline{b} \cdot \underline{c})
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We can write this in index notation as: $$(uxv)_i = \epsilon_{ijk}u_jv_k$$ $$(vxu)_i = \epsilon_{ijk}v_ju_k$$ where $\epsilon_{ijk}$ is the Levi-Civita symbol. Show more…
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1.02 Using index notation, show Lagrange's identity, (A x B) · (A x B) = (A · A)(B · B) - (A · B)^2 We can treat the nabla / del operator in components as: ∇ ↦ ∂^i ≡ ∂/∂x_i. Using this, the gradient, divergence, and curl can be expressed in index notation: Gradient: (∇f)^i = ∂^if Divergence: ∇ · v = δ_ij∂^iv^j Curl: (∇ x v)^i = ε^i_jk∂^jv^k 1.03 Write out the Laplacian of a scalar function ∇^2f = ∇ · ∇f in index notation and then carry out the sum. 1.04 Prove that the curl of the gradient is zero: ∇ x (∇f) = 0. 1.05 Prove that the curl of the curl is given by ∇ x (∇ x A) = ∇(∇ · A) - ∇^2A.
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