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Electromagnetism 

The Maxwell stress tensor (named after James Clerk Maxwell) is a symmetric secondorder tensor used in classical electromagnetism to represent the interaction between electromagnetic forces and mechanical momentum. In simple situations, such as a point charge moving freely in a homogeneous magnetic field, it is easy to calculate the forces on the charge from the Lorentz force law. When the situation becomes more complicated, this ordinary procedure can become impractically difficult, with equations spanning multiple lines. It is therefore convenient to collect many of these terms in the Maxwell stress tensor, and to use tensor arithmetic to find the answer to the problem at hand.
In the relativistic formulation of electromagnetism, the Maxwell's tensor appears as a part of the electromagnetic stress–energy tensor which is the electromagnetic component of the total stress–energy tensor. The latter describes the density and flux of energy and momentum in spacetime.
Motivation
As outlined below, the electromagnetic force is written in terms of E and B. Using vector calculus and Maxwell's equations, symmetry is sought for in the terms containing E and B, and introducing the Maxwell stress tensor simplifies the result.
Name  Differential form 

Gauss's law (in vacuum)  
Gauss's law for magnetism  
Maxwell–Faraday equation (Faraday's law of induction) 

Ampère's circuital law (in vacuum) (with Maxwell's correction) 
 Starting with the Lorentz force law
the force per unit volume is
 Next, ρ and J can be replaced by the fields E and B, using Gauss's law and Ampère's circuital law:
 The time derivative can be rewritten to something that can be interpreted physically, namely the Poynting vector. Using the product rule and Faraday's law of induction gives
and we can now rewrite f asthen collecting terms with E and B gives
 A term seems to be "missing" from the symmetry in E and B, which can be achieved by inserting (∇ ⋅ B)B because of Gauss' law for magnetism:
Eliminating the curls (which are fairly complicated to calculate), using the vector calculus identityleads to:
 This expression contains every aspect of electromagnetism and momentum and is relatively easy to compute. It can be written more compactly by introducing the Maxwell stress tensor,
All but the last term of f can be written as the tensor divergence of the Maxwell stress tensor, giving:As in the Poynting's theorem, the second term on the right side of the above equation can be interpreted as the time derivative of the EM field's momentum density, while the first term is the time derivative of the momentum density for the massive particles. In this way, the above equation will be the law of conservation of momentum in classical electrodynamics. where the Poynting vector has been introduced
in the above relation for conservation of momentum, is the momentum flux density and plays a role similar to in Poynting's theorem.
The above derivation assumes complete knowledge of both ρ and J (both free and bounded charges and currents). For the case of nonlinear materials (such as magnetic iron with a BHcurve), the nonlinear Maxwell stress tensor must be used.^{[1]}
Equation
In physics, the Maxwell stress tensor is the stress tensor of an electromagnetic field. As derived above in SI units, it is given by:
 ,
where ε_{0} is the electric constant and μ_{0} is the magnetic constant, E is the electric field, B is the magnetic field and δ_{ij} is Kronecker's delta. In Gaussian cgs unit, it is given by:
 ,
where H is the magnetizing field.
An alternative way of expressing this tensor is:
where ⊗ is the dyadic product, and the last tensor is the unit dyad:
The element ij of the Maxwell stress tensor has units of momentum per unit of area per unit time and gives the flux of momentum parallel to the ith axis crossing a surface normal to the jth axis (in the negative direction) per unit of time.
These units can also be seen as units of force per unit of area (negative pressure), and the ij element of the tensor can also be interpreted as the force parallel to the ith axis suffered by a surface normal to the jth axis per unit of area. Indeed, the diagonal elements give the tension (pulling) acting on a differential area element normal to the corresponding axis. Unlike forces due to the pressure of an ideal gas, an area element in the electromagnetic field also feels a force in a direction that is not normal to the element. This shear is given by the offdiagonal elements of the stress tensor.
Magnetism only
If the field is only magnetic (which is largely true in motors, for instance), some of the terms drop out, and the equation in SI units becomes:
For cylindrical objects, such as the rotor of a motor, this is further simplified to:
where r is the shear in the radial (outward from the cylinder) direction, and t is the shear in the tangential (around the cylinder) direction. It is the tangential force which spins the motor. B_{r} is the flux density in the radial direction, and B_{t} is the flux density in the tangential direction.
In electrostatics
In electrostatics the effects of magnetism are not present. In this case the magnetic field vanishes, , and we obtain the electrostatic Maxwell stress tensor. It is given in component form by
and in symbolic form by
where is the appropriate identity tensor (usually ).
Eigenvalue
The eigenvalues of the Maxwell stress tensor are given by:
These eigenvalues are obtained by iteratively applying the Matrix Determinant Lemma, in conjunction with the Sherman–Morrison formula.
Noting that the characteristic equation matrix, , can be written as
where
we set
Applying the Matrix Determinant Lemma once, this gives us
Applying it again yields,
From the last multiplicand on the RHS, we immediately see that is one of the eigenvalues.
To find the inverse of , we use the ShermanMorrison formula:
Factoring out a term in the determinant, we are left with finding the zeros of the rational function:
Thus, once we solve
we obtain the other two eigenvalues.
See also
 Ricci calculus
 Energy density of electric and magnetic fields
 Poynting vector
 Electromagnetic stress–energy tensor
 Magnetic pressure
 Magnetic tension force
References
 ^ Brauer, John R. (20140113). Magnetic Actuators and Sensors. John Wiley & Sons. ISBN 9781118754979.
 David J. Griffiths, "Introduction to Electrodynamics" pp. 351–352, Benjamin Cummings Inc., 2008
 John David Jackson, "Classical Electrodynamics, 3rd Ed.", John Wiley & Sons, Inc., 1999.
 Richard Becker, "Electromagnetic Fields and Interactions", Dover Publications Inc., 1964.