Electrical Energy

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The electric potential energy U_E of a system is the potential energy associated with the conservative force Coulomb forces between charged particles in a physical system, where the reference potential energy is usually chosen to be zero for particles at infinite separation. It can be defined as the amount of work one must apply to (massless) charged particles to bring them from infinite separation to some finite proximity configuration. This is also equal to the negative of the work of the Coulomb forces that the particles exert on each other during the quasistatic process move:

U_E = W_{app} = - W_\infty. :W_{app} is the work required to bring the system to a certain finite proximity configuration. "app" stands for applied, because this is work that must be applied to the system (or be supplied by another form of energy contained by the system) to configure it :W_\infty is the work done by electrostatic inter-particle Coulomb forces during the move from infinity

Disambiguation Sometimes people refer to the potential energy of a charge in an electric field. This actually refers to the potential energy of the system containing the charge and the other particles that created the electric field.

Furthermore, to calculate the work required to bring a charged particle into the vicinity of other particles, it is sufficient to know only the field generated by the other particles and the charge of the particle being moved. The field of the moving particle and the individual charges of the other particles do not need to be known.

Finally, it must be stressed that, even though this article talks about moving particles, the Coulomb force law on which this discussion is based only holds in the case of electrostatic systems. Therefore, any movement would have to be a quasistatic process.

Units The International System of Units unit of electrical energy is the joule. In the context of use of electrical energy for lighting, heating, motors, and other applications, larger units such as the watt-hour, equivalent to 3.6 million joules, are used.

Properties Since Coulomb forces are conservative, change in electric potential energy is independent of the path the system takes between two configurations. Correspondingly, the work done by the forces is also path independent and gives the change in electric potential energy:

U_f-U_i = W_{app} = - W_c :U_f is the electric potential energy at the final state :U_i is the electric potential energy at the initial state :W_{app} is the work required to bring the system to the final state from the initial state :W_c is the work done by inter-particle Coulomb forces during the move

The usual choice of datum is U_i = 0 at infinite particle separation, as stated in the opening.

Due to the proportionality of the Coulomb force with the charges of the particles on which it acts, the potential energy of a charged particle in a given electric field is also proportional to the charge of the particle. So, the potential energy may be normalized by the charge of the particle on which the field acts to give a quantity called the electric potential, which is only a function of the strength of the electric field (and the particle location):

V = \frac {U_E}{q} = \frac {W_{app-->{q} = - \frac {W_\infty}{q} :V is the electric potential of the field :q is the charge of the particle in the field

The above equation is the definition of electric potential for a system (with a datum of zero potential energy at infinite particle separation). It is very important to note that the electric field under consideration is not the total field, but the field due only to all particles except the one at the location for which the electric potential energy (and thus electric potential) is calculated.

Potential energy stored in a configuration of discrete charges The potential energy between two charges is equal to the potential energy of one charge in the electric field of the other. That is to say, if q_1 generates a scalar electric potential field V_1(\mathbf r), which is a function of position \mathbf r, then U_E = q_2 V_1(\mathbf r_2). Also, a similar development gives U_E = q_1 V_2(\mathbf r_1).

This can be generalized to give an expression for a group of N charges, q_i at positions \mathbf r_i:

U_E = \frac{1}{2}\sum_i^N q_i V(\mathbf r_i) :V(\mathbf r_i) refers to the electric field due to all particles except the one at \mathbf r_i

Note: The factor of one half accounts for the 'double counting' of charge pairs. For example, consider the case of just two charges.

Alternatively, the factor of one half may be dropped if the sum is only performed once per particle pair. This is done in the examples below to cut down on the math.

One charged particle The electric potential energy of a system containing only one point charge is zero, as no energy is required to move the charge particle from infinity to its location.

Two charged particles Consider bringing a second charge into position:

U_\mathrm{E} = k_e \frac{q_1 q_2}{r}

where ke is Coulomb's constant q1, q2 are the two charges r is the distance between the two particles

The electric potential energy will be negative if the charges have opposite sign and positive if the charges have the same sign. This simply means that potential energy is lost by a system of opposite charges moving together, which can be explained as 'opposite charges attract'.

Three or more charged particles For 3 or more point charges, the electric potential energy of the system may be calculated by bringing individual charges into position 1 after another, and taking the sum of energy required to bring the additional charge into position.

U_\mathrm{E} = k_e \left({\frac{q_1 q_2}{r_{12-->} + {\frac{q_1 q_3}{r_{13-->} + {\frac{q_2 q_3}{r_{23-->} + ...\right)

where

ke is Coulomb's constant q1, q2, ..., are the charges rmn is the distance between two particles, m and n (e.g. r12).

Potential energy of a uniform charge distribution The previous equation can again be generalized to give an expression of the potential energy of a uniform charge distribution.

U = \frac{1}{2}\int \limits_{\text{all space--> \rho(r)V(r)d^3r

where: \rho(r) is the charge density of the distribution. V(r) is the electric potential at position r.

Energy stored in an electric field One may take the equation for the potential energy of a uniform charge distribution and put it in terms of the electric field.

Since

\mathbf{\nabla}\cdot\mathbf{E} = \frac{\rho}{\epsilon_o}

where \epsilon_o is the permittivity of the medium E is the electric field vector.

then,

U = \frac{1}{2}\int \limits_{\text{all space--> \rho(r)V(r)d^3r

= \frac{1}{2}\int \limits_{\text{all space--> \epsilon_o(\mathbf{\nabla}\cdot{\mathbf{E-->)V(r)d^3r

so, now using the following divergence vector identity

\nabla\cdot(\vec{A}\vec{B}) = (\nabla\cdot\vec{A})\vec{B} + \vec{A}\cdot(\nabla\vec{B}) = (\nabla\cdot\vec{A})\vec{B} + (\vec{A}\cdot\nabla)\vec{B}

we have

U = \frac{\epsilon_o}{2}\int \limits_{\text{all space--> \mathbf{\nabla}\cdot(\mathbf{E}V) d^3r - \frac{\epsilon_o}{2}\int \limits_{\text{all space--> (\mathbf{\nabla}V)\cdot\mathbf{E} d^3r

using the divergence theorem and taking the area to be at infinity where V(\infty) = 0

U = \frac{\epsilon_o}{2}\int V\mathbf{E}\cdot dA - \frac{\epsilon_o}{2}\int \limits_{\text{all space--> (-\mathbf{E})\cdot\mathbf{E} d^3r

= \int \limits_{\text{all space--> \frac{1}{2}\epsilon_o\left|{\mathbf{E-->\right|^2 d^3r

So, the energy density, or energy per unit volume of the electric field is:

u_e = \frac{1}{2} \epsilon_o \left|E\right|^2

See also

References