where we have written [1]
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(48)
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The physical meaning of equation (47) becomes evident if in place of this equation we write, using a new notation,
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(47a)
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or, on eliminating the imaginary,
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(47b)
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When expressed in the latter form, we see that the first three equations state the principle of momentum; are the Maxwell stresses in the electromagnetic field, and is the vector momentum per unit volume of the field. The last of equations (47b) expresses the energy principle; \mathbf{s} is the vector flow of energy, and the energy per unit volume of the field. In fact, we get from (48) by introducing the well-known expressions for the components of the field intensity from electrodynamics,
- ↑ To be summed for the indices and .