Poyntings Theorem Power Flow and Plane Waves.
A propagating electromagnetic (EM) wave carries energy with it. Physically this makes sense to us when we listen to the radio or talk on a cell phone. These types of wireless communications are possible because EM waves carry energy.
In these examples, some of this EM energy is used to oscillate electrons in the metal parts of the receiving antenna of our radio or cell phone, which ultimately results in wireless communications.
There is a precise mathematical definition of the time rate of energy flow (i.e., power flow) for EM waves. Before getting to this, we first need to digress briefly to first discuss Poynting’s theorem.
Poynting’s Theorem
Poynting’s theorem is a hugely important mathematical statement in electromagnetics that concerns the flow of power through space. We’ll derive it now for time-domain fields. We begin with Maxwell’s curl equations
Next, we employ the vector calculus identity
Ñ(`E´ `H) = `H (Ñ´ `E)- `E (Ñ´ `H)
Substituting (1) and (2) into (3) we have:
Using the constitutive equations `B = m`H , `D =e`E , and `J =s`E in (4) gives
assuming m ¹ f (t) and e¹ f (t)
thinking of the chain rule of differentiation. Consequently, using this result and a similar one for the E ¶E/ ¶t term, (5) becomes
This is the point form of what is called Poynting’s theorem. Lastly, we integrate this point form equation (6) throughout a volume V bounded by the closed surface S:
where V, S, and ds are related as
Applying the divergence theorem to the LHS of this last equation gives
where in moving ¶ / ¶t outside of the integral we’re assuming that V is not a function of time. This result is called the integral form of Poynting’s theorem.
Discussion of Poynting’s Theorem
To understand the physical significance of the LHS of (7), we’ll begin by looking at the RHS, which has elements you’ve seen before in EE 381. In particular, the first and second terms in the RHS of (7) are the time rates-of-change of the stored energy in the magnetic and electric fields inside V. The third term is the Ohmic power dissipated in V due to the flow of conduction current. We can now interpret the RHS of (7) as the rate of decrease in the magnetic and electric power stored in V, and further reduced by the Ohmic power dissipated in V.
OK, so now here is the payoff: By the conservation of energy law, all of this represented by the RHS of (7) must equal the power leaving the volume through the bounding surface S. Consequently, the quantity`E ´`H in the LHS of (7) is a vector that must represent the power flow of the EM field leaving the volume V per unit area. We define this vector
The LHS is the power flow into S. The first term in the RHS is the increase in the stored power in the `H and `E fields in V, while the second term is the increase in the Ohmic power dissipated in V.
Power Flow for UPWs
We will now apply this Poynting vector concept to uniform plane waves. In Example N25.1, we found for a certain UPW that
`E(z,1) = -a_{y} 43.501 cos (6p´10^{8} t – 21.780z)V/m `H(z,1) = -a_{x} 0.1 cos (6p´10^{8} t – 21.780z)A/m
This UPW is propagating in the +z direction:
The instantaneous Poynting vector associated with this UPW using (8) is then
`S(z,t) = `E(z,t) ´ `H(z,t)= (-a_{y} ´ a_{x} )4.350 cos ^{2} (6p ´ 10^{8}t -21.780z)W/m^{2}
such that
`S(z,t) = a_{z} 4.350 cos ^{2} (6p ´ 10^{8} t -21.780 z)W/m^{2}
The direction of this S (z,t ) is a_{z} . This indicates that the flow of power of this UPW in the same direction as the wave propagation: in the a_{z} direction.
Time Average Power Flow
Notice in (11) that while S (z,t ) oscillates in time, it has a nonzero time average value. (As an aside, this is one of the reasons why S (z) is not a phasor quantity.) In particular, using the trigonometric identity
The second term in the RHS oscillates in time (at twice the frequency of E and H ) and has a zero time average value, while the first term is constant and does not vary with time. Consequently, the time average value of this Poynting vector in
This UPW – on time average – carries or transfers power in the direction that the wave is propagating.
Sinusoidal Steady State Time Average Power Flow
It turns out that there is another way to calculate this time average Poynting vector for sinusoidal steady state, and to calculate it directly from phasor fields. We derive this expression beginning with (8) and writing E(t ) and H (t ) in terms of their phasor forms
For evaluating (14), note that
Rather, we can employ
in (14) to give
which we can write as
We can recognize the RHS as a term plus its complex conjugate. So, once again using (15)
Integrating this expression over one time period as in (13), we find from (16) that
Using this equation, we can compute a time averaged quantity ( AV S ) directly from phasor domain quantities (`E and `H ). For the UPW of Example N25.1, the phasor form of `E and `H Are
Using (17), the time average Poynting vector is then
This is the same result we found in (13) using the time domain forms of `E and `H . Here in (17) we find a time averaged quantity directly from the phasor domain fields. Neat