We all know that poynting vector points in the direction of the wave propagation and its value of power supplied to a unit area. Homework statement in free space, the electric field of a plane em wave is given by. U v a t u c c e2 e bk and the direction is reasonable. Poynting vector gives the rate of energy transfer per unit area or the energy that a wave carries per unit time per unit area is given by the poynting vector. The flow of energy is described by the energy density flow vector, known as the poynting vector s. The poynting theorem is in the nature of a statement of the conservation of energy for a configuration consisting of electric and magnetic fields acting on charges. There is an energy density associated with both the electric field e and the magnetic field b. In physics, the poynting vector represents the directional energy flux the energy transfer per unit area per unit time of an electromagnetic field.
Poyntings theorem definition of poyntings theorem by. For the extraordinary wave except for those special cases where ndk is parallel or perpendicular to the optical axis z, as shown in fig. Oliver heaviside and nikolay umov independently coinvented the. H wm2 44 equation 44 is known as the poyting vector. However, there have been several problems with this vector, and various attempts to resolve these problems have been carried out. The poynting vector p may be interpreted as the intensity of em energy flow at a given point, that is the energy per second crossing a unit area whose normal is oriented in the direction of the vector e.
It ties together mechanical kinetic energy and the energy stored in electromagnetic fields, thereby justifying the formulas. The poynting vector appears in poyntings theorem see that article for the derivation, an energyconservation law. No book on problems can claim to exhaust the variety in the limited space. Let e and b be electric and magnetic fields external to the. Poynting s theorem is analogous to the workenergy theorem in classical mechanics, and mathematically similar to the continuity. Here rate of energy transfer p is perpendicular to both e and h. On the surface at r a, the first term in brackets is constant, so the integration on s amounts to a multiplication by the area. In general, the timeaveraged value of a periodic function with period t is given by. The derivation and explanation of the poynting theorem. Average complex poynting vector finally, we have computed the complex poynting vector with the time dependence s 1 2. Calculate the power energy per unit time transported down the cables of ex. In an electromagnetic wave, the magnitude of poynting vector fluctuates rapidly with time.
It is named after its discoverer john henry poynting who first derived it in 1884 2 oliver heaviside also discovered it independently and in the more general form that. In electrodynamics, poynting s theorem is a statement of conservation of energy for the electromagnetic field, in the form of a partial differential equation developed by british physicist john henry poynting. John henry poynting first derived this vector in 1884 and hence it was named after him. The current distribution represented by the vector j can be considered as made up of various changes qi moving with velocity. Poyntings theorem is analogous to the workenergy theorem in classical mechanics, and mathematically similar to the continuity equation, because it relates the. So while deriving poynting theorm, the energy flown out of the system. In electrodynamics, poyntings 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. One of these uses the work done on a charge by an external. Poyntings theorem is the statement of local conservation of energy in classical electrodynamics. Note that the expression for the poynting vector shown above has the implicit meaning that. The rate of energy transport per unit area is described by the vector. Poynting theorem states that the net power flowing out of a given volume v is equal to.
In a book i am studying, the poynting vector is defined as. Questions tagged poynting vector ask question the poynting. Newest poyntingvector questions physics stack exchange. Energy density and the poynting vector overview and. Jun 10, 2016 in electrodynamics, poyntings theorem is a statement of conservation of energy of the electromagnetic field. Often, however, often we are more interested in the timeaveraged value of this quantity. Electromagnetic waves carry energy as they travel through empty space. S is referred to as the poynting vector, discovered in 1883 byjohn poynting and independently byoliverheaviside. Poynting s theorem is the statement of local conservation of energy in classical electrodynamics. Poyntings theorem definition is a statement in electromagnetic theory. In this paper, these problems are put forth in order and then the physical meaning of the poynting vector is elucidated based on the concept clarified in the previous paper. Since it represents the rate of energy transfer per unit area, its unit is wm2. Consider first a single particle of charge q traveling with a velocity vector v.
John henry poynting 18521914 the power per unit area in a beam is. Nov 22, 2015 okay so i found an equation for the average energy of a plane wave. November, 2018 the poynting vector is an invaluable tool for analysing electromagnetic problems. The very same expression can be obtained directly from the phasors by the following rule. I2 0 cr 2a2 0 re 2trcer thus the poynting vector and the direction of energy ow point radially out of the capacitor.
But avoid asking for help, clarification, or responding to other answers. Poynting vector is also called as instantaneous energy flux density. Poynting vector and poynting theorem when electromagnetic wave travels in space, it carries energy and energy density is always associated with electric fields and magnetic fields. It is named after english physicist john henry poynting, who introduced it in 1884. The poynting vector s is defined as to be equal to the cross product 1. To introduce the displacement current term that maxwell added to amperes law 2. The poynting vector, named after john henry poynting, is used in order to demonstrate the energy flux density of an em field. Poynting vector wave intensity i time average over one or more cycle 12 then emax22 and bmax22 i av u av c e max b max 2 0 define vector with magnitude power per unit area js. The poynting vector is given by s 1 0 e trcb q a 0 e 0i 0 r 2a e trcez e.
An electromagnetic wave in empty space the electric field, the magnetic field, and the kvector are. San j derivation and explanation of the poynting theorem the poynting theorem is in the nature of a statement of the conservation of energy for a configuration consisting of electric and magnetic fields acting on charges. Hence, for a lossless region, the timeaverage power flow is in the direction of the phase vector. H due to the way it was derived, the poynting vector only has strict meaning in approximately linear, lossless materials. Physical meaning of the poynting vector and the resolution.
The displacement current and poynting vector objectives 1. In electrodynamics, poyntings theorem is a statement of conservation of energy of the electromagnetic field. Elementary calculus, vector calculus and algebra are the prerequisites. Throughout this derivation, we will start from basic principles, introduce the poynting vector, and convert the theorem into the differential form, where the expression of conservation of energy is easiest to see. The above equation 10 represents the poynting theorem. Poynting vector and irradiance the quantum nature of light photon energy and photon momentum. Chapter maxwells equations and electromagnetic waves. Mccall blackett laboratory, imperial college, prince consort road, london sw7 2az, united kingdom. The poynting flux implied by 19 and 20 is this poynting vector has no component normal to the top and bottom surfaces of the volume. Therefore, a practical and useful quantity, the average over one cycle intensity, is used. He was the developer and eponym of the poynting vector, which describes the direction and magnitude of electromagnetic energy flow and is used in the poynting theorem, a statement about energy conservation for electric and magnetic fields. U v u a c t so the energy per unit time per unit area.
It is named after its inventor john henry poynting. Thanks for contributing an answer to physics stack exchange. Momentum of electromagnetic radiation let us reinterpret the poynting vector from a quantum perspective. The poynting vector is parallel to k too, but it describes the energy propagation power per square meter or intensity, resp. It is named after english physicist john henry poynting, who introduced it in 1884 the poynting vector s is defined as to be equal to the cross product 1. We have a coaxial cable with inner radius aand outer. The rate of energy travelled through per unit area i. The poynting vector represents the direction of propagation of an electromagnetic wave as well as the energy flux density, or intensity since an electromagnetic wave is composed of an electric field e. Poynting vector and poynting theorem engineering physics. Gate ece videoselectromagneticspoynting vector problem. Does the poynting vector always represent electromagnetic.
Poynting vector is parallel to the wave vector ndk, leading to spseu 0. Mar 30, 2020 the poynting vector is parallel to k too, but it describes the energy propagation power per square meter or intensity, resp. An attempt is made to include the important types of problems at the undergraduate level. Since e, h, and k form a righthanded system, the power ows in the direction of wave propagation.
571 1485 389 683 377 1265 250 90 678 1050 952 1021 1061 1288 1123 1519 416 941 886 598 1488 1172 218 1444 825 920 810 825 486 274 1242 678 992 821 1079 535 776 314