Poynting's theorem

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In electrodynamics, Poynting's theorem is a statement of conservation of energy for the electromagnetic field,^{[clarification needed]}, in the form of a partial differential equation developed by British physicist John Henry Poynting.^[1] Poynting's theorem is analogous to the work-energy theorem in classical mechanics, and mathematically similar to the continuity equation, because it relates the energy stored in the electromagnetic field to the work done on a charge distribution (i.e. an electrically charged object), through energy flux.

Statement[]

General[]

In words, the theorem is an energy balance:

The rate of energy transfer (per unit volume) from a region of space equals the rate of work done on a charge distribution plus the energy flux leaving that region.

A second statement can also explain the theorem - "The decrease in the electromagnetic energy per unit time in a certain volume is equal to the sum of work done by the field forces and the net outward flux per unit time".

Mathematically,

where ∇•S is the divergence of the Poynting vector (energy flow) and J•E is the rate at which the fields do work on a charged object (J is the current density corresponding to the motion of charge, E is the electric field, and • is the dot product). The energy density u, assuming no electric or magnetic polarizability, is given by:^[2]

${\displaystyle u={\frac {1}{2}}\left(\epsilon _{0}\mathbf {E} \cdot \mathbf {E} +{\frac {1}{\mu _{0}}}\mathbf {B} \cdot \mathbf {B} \right)}$

in which B is the magnetic flux density. Using the divergence theorem, Poynting's theorem can be rewritten in integral form:

where ${\displaystyle \partial V\!}$ is the boundary of a volume V. The shape of the volume is arbitrary but fixed for the calculation.

Electrical engineering[]

In electrical engineering context the theorem is usually written with the energy density term u expanded in the following ways, which resembles the continuity equation:

${\displaystyle \nabla \cdot \mathbf {S} +\epsilon _{0}\mathbf {E} \cdot {\frac {\partial \mathbf {E} }{\partial t}}+{\frac {\mathbf {B} }{\mu _{0}}}\cdot {\frac {\partial \mathbf {B} }{\partial t}}+\mathbf {J} \cdot \mathbf {E} =0,}$

where

• ε₀ is the electric constant and μ₀ is the magnetic constant.
• ${\displaystyle \epsilon _{0}\mathbf {E} \cdot {\frac {\partial \mathbf {E} }{\partial t}}}$ is the density of reactive power driving the build-up of electric field,
• ${\displaystyle {\frac {\mathbf {B} }{\mu _{0}}}\cdot {\frac {\partial \mathbf {B} }{\partial t}}}$ is the density of reactive power driving the build-up of magnetic field, and
• ${\displaystyle \mathbf {J} \cdot \mathbf {E} }$ is the density of electric power dissipated by the Lorentz force acting on charge carriers.

Derivation[]

While conservation of energy and the Lorentz force law can give the general form of the theorem, Maxwell's equations are additionally required to derive the expression for the Poynting vector and hence complete the statement.

Poynting's theorem[]

Considering the statement in words above - there are three elements to the theorem, which involve writing energy transfer (per unit time) as volume integrals:^[3]

So by conservation of energy, the balance equation for the energy flow per unit time is the integral form of the theorem:

${\displaystyle -\int _{V}{\frac {\partial u}{\partial t}}dV=\int _{V}\nabla \cdot \mathbf {S} dV+\int _{V}\mathbf {J} \cdot \mathbf {E} dV,}$

and since the volume V is arbitrary, this is true for all volumes, implying

${\displaystyle -{\frac {\partial u}{\partial t}}=\nabla \cdot \mathbf {S} +\mathbf {J} \cdot \mathbf {E} ,}$

which is Poynting's theorem in differential form.

Poynting vector[]

From the theorem, the actual form of the Poynting vector S can be found. The time derivative of the energy density (using the product rule for vector dot products) is

${\displaystyle {\frac {\partial u}{\partial t}}={\frac {1}{2}}\left(\mathbf {E} \cdot {\frac {\partial \mathbf {D} }{\partial t}}+\mathbf {D} \cdot {\frac {\partial \mathbf {E} }{\partial t}}+\mathbf {H} \cdot {\frac {\partial \mathbf {B} }{\partial t}}+\mathbf {B} \cdot {\frac {\partial \mathbf {H} }{\partial t}}\right)=\mathbf {E} \cdot {\frac {\partial \mathbf {D} }{\partial t}}+\mathbf {H} \cdot {\frac {\partial \mathbf {B} }{\partial t}},}$

using the constitutive relations^{[clarification needed]}

${\displaystyle \mathbf {D} =\epsilon _{0}\mathbf {E} ,\quad \mathbf {H} ={\frac {\mathbf {B} }{\mu _{0}}}}$

The partial time derivatives suggest using two of Maxwell's Equations. Taking the dot product of the Maxwell–Faraday equation with H:

${\displaystyle {\frac {\partial \mathbf {B} }{\partial t}}=-\nabla \times \mathbf {E} \ \rightarrow \ \mathbf {H} \cdot {\frac {\partial \mathbf {B} }{\partial t}}=-\mathbf {H} \cdot \nabla \times \mathbf {E} ,}$

next taking the dot product of the Maxwell–Ampère equation with E:

${\displaystyle {\frac {\partial \mathbf {D} }{\partial t}}+\mathbf {J} =\nabla \times \mathbf {H} \ \rightarrow \ \mathbf {E} \cdot {\frac {\partial \mathbf {D} }{\partial t}}+\mathbf {E} \cdot \mathbf {J} =\mathbf {E} \cdot \nabla \times \mathbf {H} .}$

Collecting the results so far gives:

${\displaystyle {\begin{aligned}-\nabla \cdot \mathbf {S} &={\frac {\partial u}{\partial t}}+\mathbf {J} \cdot \mathbf {E} \\&=\left(\mathbf {H} \cdot {\frac {\partial \mathbf {B} }{\partial t}}+\mathbf {E} \cdot {\frac {\partial \mathbf {D} }{\partial t}}\right)+\mathbf {J} \cdot \mathbf {E} \\&=\mathbf {E} \cdot \nabla \times \mathbf {H} -\mathbf {H} \cdot \nabla \times \mathbf {E} ,\\\end{aligned}}}$

then, using the vector calculus identity:

${\displaystyle \nabla \cdot (\mathbf {E} \times \mathbf {H} )=\mathbf {H} \cdot (\nabla \times \mathbf {E} )-\mathbf {E} \cdot (\nabla \times \mathbf {H} ),}$

gives an expression for the Poynting vector:

${\displaystyle \mathbf {S} =\mathbf {E} \times \mathbf {H} ,}$

which physically means the energy transfer due to time-varying electric and magnetic fields is perpendicular to the fields

Poynting vector in macroscopic media[]

In a macroscopic medium, electromagnetic effects are described by spatially averaged (macroscopic) fields. The Poynting vector in a macroscopic medium can be defined self-consistently with microscopic theory, in such a way that the spatially averaged microscopic Poynting vector is exactly predicted by a macroscopic formalism. This result is strictly valid in the limit of low-loss and allows for the unambiguous identification of the Poynting vector form in macroscopic electrodynamics.^[4][5]

Alternative forms[]

It is possible to derive alternative versions of Poynting's theorem.^[6] Instead of the flux vector E × B as above, it is possible to follow the same style of derivation, but instead choose the Abraham form E × H, the Minkowski form D × B, or perhaps D × H. Each choice represents the response of the propagation medium in its own way: the E × B form above has the property that the response happens only due to electric currents, while the D × H form uses only (fictitious) magnetic monopole currents. The other two forms (Abraham and Minkowski) use complementary combinations of electric and magnetic currents to represent the polarization and magnetization responses of the medium.

Generalization[]

The mechanical energy counterpart of the above theorem for the electromagnetic energy continuity equation is

${\displaystyle {\frac {\partial }{\partial t}}u_{m}(\mathbf {r} ,t)+\nabla \cdot \mathbf {S} _{m}(\mathbf {r} ,t)=\mathbf {J} (\mathbf {r} ,t)\cdot \mathbf {E} (\mathbf {r} ,t),}$

where u_m is the (mechanical) kinetic energy density in the system. It can be described as the sum of kinetic energies of particles α (e.g., electrons in a wire), whose trajectory is given by r_α(t):

${\displaystyle u_{m}(\mathbf {r} ,t)=\sum _{\alpha }{\frac {m_{\alpha }}{2}}{\dot {r}}_{\alpha }^{2}\delta (\mathbf {r} -\mathbf {r} _{\alpha }(t)),}$

where S_m is the flux of their energies, or a "mechanical Poynting vector":

${\displaystyle \mathbf {S} _{m}(\mathbf {r} ,t)=\sum _{\alpha }{\frac {m_{\alpha }}{2}}{\dot {r}}_{\alpha }^{2}{\dot {\mathbf {r} }}_{\alpha }\delta (\mathbf {r} -\mathbf {r} _{\alpha }(t)).}$

Both can be combined via the Lorentz force, which the electromagnetic fields exert on the moving charged particles (see above), to the following energy continuity equation or energy conservation law:^[7]

${\displaystyle {\frac {\partial }{\partial t}}\left(u_{e}+u_{m}\right)+\nabla \cdot \left(\mathbf {S} _{e}+\mathbf {S} _{m}\right)=0,}$

covering both types of energy and the conversion of one into the other.

References[]

External links[]

Wikiversity has a lesson on Poynting's theorem

• Eric W. Weisstein "Poynting Theorem" From ScienceWorld – A Wolfram Web Resource.