9.3 Energy conservation

Energy conservation is a cornerstone of electromagnetism, dictating how energy flows and transforms in electromagnetic systems. It's crucial for understanding the behavior of electric and magnetic fields, as well as the propagation of electromagnetic waves.

Poynting's theorem, derived from Maxwell's equations, provides a mathematical framework for energy conservation. It relates the rate of change of electromagnetic energy density to the flow of energy through space and the work done by fields on charges and currents.

Energy conservation in electromagnetism

• Fundamental principle stating that energy cannot be created or destroyed, only converted from one form to another
• Applies to electromagnetic systems where energy is stored in electric and magnetic fields and can be transferred through electromagnetic waves
• Understanding energy conservation is crucial for analyzing and designing electromagnetic devices and systems

Derivation of Poynting's theorem

Top images from around the web for Derivation of Poynting's theorem

• 

  Poynting vector - Wikipedia View original

  Is this image relevant?

• 

  Maxwell equations - Knowino View original

  Is this image relevant?

• 

  Poynting vector - Wikipedia View original

  Is this image relevant?

• 

  Poynting vector - Wikipedia View original

  Is this image relevant?

• 

  Maxwell equations - Knowino View original

  Is this image relevant?

1 of 3

Top images from around the web for Derivation of Poynting's theorem

• 

  Poynting vector - Wikipedia View original

  Is this image relevant?

• 

  Maxwell equations - Knowino View original

  Is this image relevant?

• 

  Poynting vector - Wikipedia View original

  Is this image relevant?

• 

  Poynting vector - Wikipedia View original

  Is this image relevant?

• 

  Maxwell equations - Knowino View original

  Is this image relevant?

1 of 3

• Combines Maxwell's equations to derive a continuity equation for electromagnetic energy
• Starts with Faraday's law and Ampère's law, then uses vector identities to manipulate the equations
• Arrives at the Poynting theorem, which relates the rate of change of electromagnetic energy density to the divergence of the Poynting vector and the work done by the electromagnetic fields

Physical interpretation

• Poynting's theorem states that the rate of change of electromagnetic energy density in a volume is equal to the negative divergence of the Poynting vector minus the work done by the fields on the charges and currents
• The Poynting vector represents the direction and magnitude of electromagnetic energy flow
• The work done by the fields on the charges and currents represents the conversion of electromagnetic energy into other forms (heat, kinetic energy, etc.)

Electromagnetic energy density

• The amount of energy stored per unit volume in the electromagnetic fields
• Consists of two components: electric field energy density and magnetic field energy density
• Total electromagnetic energy density is the sum of the electric and magnetic field energy densities

Electric field energy density

• The energy stored per unit volume in the electric field
• Given by the formula $u_E = \frac{1}{2}\varepsilon_0 E^2$, where $\varepsilon_0$ is the permittivity of free space and $E$ is the electric field strength
• Depends on the square of the electric field strength, so stronger electric fields store more energy

Magnetic field energy density

• The energy stored per unit volume in the magnetic field
• Given by the formula $u_B = \frac{1}{2\mu_0} B^2$, where $\mu_0$ is the permeability of free space and $B$ is the magnetic field strength
• Depends on the square of the magnetic field strength, so stronger magnetic fields store more energy

Electromagnetic energy flux

• The rate at which electromagnetic energy flows through a surface per unit area
• Represented by the Poynting vector, which is a cross product of the electric and magnetic field vectors
• The direction of the Poynting vector indicates the direction of energy flow, and its magnitude represents the intensity of the energy flux

Poynting vector

• Defined as $\vec{S} = \vec{E} \times \vec{H}$, where $\vec{E}$ is the electric field vector and $\vec{H}$ is the magnetic field vector
• Units of watts per square meter (W/m²)
• Represents the instantaneous power density and direction of electromagnetic energy flow

Direction and magnitude of energy flow

• The direction of the Poynting vector is perpendicular to both the electric and magnetic field vectors
• The magnitude of the Poynting vector represents the intensity of the energy flux
• In electromagnetic waves, the Poynting vector is always perpendicular to the direction of wave propagation

Energy conservation in electromagnetic waves

• Electromagnetic waves carry energy as they propagate through space
• The energy density and energy flux of electromagnetic waves are related to the electric and magnetic field amplitudes
• Energy conservation requires that the total energy in an electromagnetic wave remains constant as it propagates, unless energy is dissipated or absorbed

Energy density of electromagnetic waves

• For plane electromagnetic waves, the electric and magnetic field energy densities are equal
• The total energy density is given by $u = u_E + u_B = \varepsilon_0 E^2 = \frac{1}{\mu_0} B^2$
• The energy density oscillates in time and space as the wave propagates

Energy flux of electromagnetic waves

• The energy flux of an electromagnetic wave is given by the time-averaged Poynting vector
• For plane waves, the magnitude of the time-averaged Poynting vector is $\langle S \rangle = \frac{1}{2}\sqrt{\frac{\varepsilon_0}{\mu_0}}E_0^2$, where $E_0$ is the peak electric field amplitude
• The energy flux is constant for lossless media and decreases as the wave propagates through dissipative media

Applications of energy conservation

• Energy conservation principles are applied in various electromagnetic systems and devices
• Understanding energy flow and dissipation is crucial for optimizing the performance and efficiency of these systems
• Examples include waveguides, transmission lines, antennas, and electromagnetic shielding

Waveguides and transmission lines

• Structures that guide electromagnetic waves from one point to another
• Energy conservation dictates that the power input to the waveguide or transmission line must equal the power output plus any losses
• Losses can occur due to ohmic dissipation in the conductors or dielectric losses in the insulating materials

Antennas and radiation

• Antennas convert guided electromagnetic energy into free-space radiation and vice versa
• The radiated power from an antenna is determined by the Poynting vector integrated over a closed surface surrounding the antenna
• Energy conservation requires that the input power to the antenna equals the radiated power plus any losses in the antenna structure

Electromagnetic shielding

• Technique used to reduce the transmission of electromagnetic energy through a barrier
• Shielding materials reflect or absorb electromagnetic waves, reducing the energy flux through the shield
• Energy conservation dictates that the incident energy must equal the sum of the reflected, absorbed, and transmitted energy

Energy dissipation in electromagnetic systems

• Electromagnetic energy can be dissipated through various mechanisms, leading to losses in the system
• Two main types of losses are ohmic losses and radiative losses
• Minimizing energy dissipation is essential for improving the efficiency and performance of electromagnetic devices

Ohmic losses

• Occur due to the resistance of conductors in electromagnetic systems
• Caused by the collision of electrons with the lattice structure of the conductor, converting electromagnetic energy into heat
• The power dissipated due to ohmic losses is given by $P = I^2 R$, where $I$ is the current and $R$ is the resistance

Radiative losses

• Occur when electromagnetic energy is unintentionally radiated from a system
• Can be caused by discontinuities, bends, or other irregularities in the structure of waveguides, transmission lines, or antennas
• Radiative losses reduce the energy available for the intended purpose and can cause interference with other systems

Energy storage in electromagnetic fields

• Electromagnetic energy can be stored in the electric and magnetic fields associated with capacitors and inductors
• Energy storage is essential for various applications, such as energy harvesting, power conditioning, and signal processing
• The stored energy can be released when needed, providing a source of power or influencing the behavior of the system

Capacitive energy storage

• Capacitors store energy in the electric field between two conducting plates
• The energy stored in a capacitor is given by $W = \frac{1}{2}CV^2$, where $C$ is the capacitance and $V$ is the voltage across the plates
• The stored energy is proportional to the square of the voltage, so higher voltages lead to greater energy storage

Inductive energy storage

• Inductors store energy in the magnetic field generated by the current flowing through the inductor
• The energy stored in an inductor is given by $W = \frac{1}{2}LI^2$, where $L$ is the inductance and $I$ is the current through the inductor
• The stored energy is proportional to the square of the current, so higher currents lead to greater energy storage

Boundary conditions and energy conservation

• Electromagnetic energy flow and storage are influenced by the boundary conditions at the interfaces between different media
• Boundary conditions determine the reflection, transmission, and absorption of electromagnetic waves at interfaces
• Energy conservation must be satisfied at the boundaries to ensure a consistent and physically meaningful solution

Continuity of energy flux at boundaries

• The normal component of the Poynting vector must be continuous across a boundary between two media
• This ensures that the energy flowing into the boundary from one medium equals the energy flowing out of the boundary into the other medium
• Any discontinuity in the normal component of the Poynting vector would imply a non-physical accumulation or depletion of energy at the boundary

Energy reflection and transmission at interfaces

• When an electromagnetic wave encounters an interface between two media, some of the energy is reflected, and some is transmitted
• The relative amounts of reflected and transmitted energy depend on the properties of the media and the angle of incidence
• Energy conservation requires that the sum of the reflected and transmitted energy equals the incident energy, assuming no absorption at the interface