Electric potential energy | |
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Common symbols | U_{E} |

SI unit | joule (J) |

Derivations from other quantities | U_{E} = C · V^{2} / 2 |

Articles about |

Electromagnetism |
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**Electric potential energy** is a potential energy (measured in joules) that results from conservative Coulomb forces and is associated with the configuration of a particular set of point charges within a defined system. An *object* may be said to have electric potential energy by virtue of either its own electric charge or its relative position to other electrically charged *objects*.

The term "electric potential energy" is used to describe the potential energy in systems with time-variant electric fields, while the term "electrostatic potential energy" is used to describe the potential energy in systems with time-invariant electric fields.

The electric potential energy of a system of point charges is defined as the work required to assemble this system of charges by bringing them close together, as in the system from an infinite distance. Alternatively, the electric potential energy of any given charge or system of charges is termed as the total work done by an external agent in bringing the charge or the system of charges from infinity to the present configuration **without** undergoing any acceleration.

The electrostatic potential energy, *U*_{E}, of one point charge *q* at position **r** in the presence of an electric field **E** is defined as the negative of the work *W* done by the electrostatic force to bring it from the reference position **r**_{ref}^{[note 1]} to that position **r**.^{[1]}^{[2]}^{: §25-1 }
where **E** is the electrostatic field and d**r'** is the displacement vector in a curve from the reference position **r**_{ref} to the final position **r**.

The electrostatic potential energy can also be defined from the electric potential as follows:

The electrostatic potential energy, *U*_{E}, of one point charge *q* at position **r** in the presence of an electric potential is defined as the product of the charge and the electric potential.
where is the electric potential generated by the charges, which is a function of position **r**.

The SI unit of electric potential energy is joule (named after the English physicist James Prescott Joule). In the CGS system the erg is the unit of energy, being equal to 10^{−7} Joules. Also electronvolts may be used, 1 eV = 1.602×10^{−19} Joules.

The electrostatic potential energy, *U _{E}*, of one point charge

where *r* is the distance between the point charges *q* and *Q*, and *q* and *Q* are the charges (not the absolute values of the charges—i.e., an electron would have a negative value of charge when placed in the formula). The following outline of proof states the derivation from the definition of electric potential energy and Coulomb's law to this formula.

The electrostatic force **F** acting on a charge *q* can be written in terms of the electric field **E** as

By definition, the change in electrostatic potential energy, *U _{E}*, of a point charge

where:

**r**= position in 3d space of the charge*q*, using cartesian coordinates**r**= (*x*,*y*,*z*), taking the position of the*Q*charge at**r**= (0,0,0), the scalar*r*= |**r**| is the norm of the position vector,- d
**s**= differential displacement vector along a path*C*going from**r**_{ref}to**r**, - is the work done by the electrostatic force to bring the charge from the reference position
**r**_{ref}to**r**,

Usually *U _{E}* is set to zero when

When the curl **∇** × **E** is zero, the line integral above does not depend on the specific path *C* chosen but only on its endpoints. This happens in time-invariant electric fields. When talking about electrostatic potential energy, time-invariant electric fields are always assumed so, in this case, the electric field is conservative and Coulomb's law can be used.

Using Coulomb's law, it is known that the electrostatic force **F** and the electric field **E ** created by a discrete point charge *Q* are radially directed from *Q*. By the definition of the position vector **r** and the displacement vector **s**, it follows that **r** and **s** are also radially directed from *Q*. So, **E** and d**s** must be parallel:

Using Coulomb's law, the electric field is given by

and the integral can be easily evaluated:

The electrostatic potential energy, *U _{E}*, of one point charge

where *r _{i}* is the distance between the point charges

The electrostatic potential energy *U*_{E} stored in a system of *N* charges *q*_{1}, *q*_{2}, …, *q*_{N} at positions **r**_{1}, **r**_{2}, …, **r**_{N} respectively, is:

| (1) |

where, for each *i* value, V(**r**_{i}) is the electrostatic potential due to all point charges except the one at **r**_{i},^{[note 2]} and is equal to:
where *r*_{ij} is the distance between *q*_{i} and *q*_{j}.

The electrostatic potential energy *U*_{E} stored in a system of two charges is equal to the electrostatic potential energy of a charge in the electrostatic potential generated by the other. That is to say, if charge q_{1} generates an electrostatic potential V_{1}, which is a function of position **r**, then

Doing the same calculation with respect to the other charge, we obtain

The electrostatic potential energy is mutually shared by and , so the total stored energy is

This can be generalized to say that the electrostatic potential energy *U*_{E} stored in a system of *n* charges *q*_{1}, *q*_{2}, …, *q _{n}* at positions

The electrostatic potential energy of a system containing only one point charge is zero, as there are no other sources of electrostatic force against which an external agent must do work in moving the point charge from infinity to its final location.

A common question arises concerning the interaction of a point charge with its own electrostatic potential. Since this interaction doesn't act to move the point charge itself, it doesn't contribute to the stored energy of the system.

Consider bringing a point charge, *q*, into its final position near a point charge, *Q*_{1}. The electric potential V(**r**) due to *Q*_{1} is

Hence we obtain, the electrostatic potential energy of *q* in the potential of *Q*_{1} as
where *r*_{1} is the separation between the two point charges.

The electrostatic potential energy of a system of three charges should not be confused with the electrostatic potential energy of *Q*_{1} due to two charges *Q*_{2} and *Q*_{3}, because the latter doesn't include the electrostatic potential energy of the system of the two charges *Q*_{2} and *Q*_{3}.

The electrostatic potential energy stored in the system of three charges is:

Using the formula given in (**1**), the electrostatic potential energy of the system of the three charges will then be:

Where is the electric potential in **r**_{1} created by charges *Q*_{2} and *Q*_{3}, is the electric potential in **r**_{2} created by charges *Q*_{1} and *Q*_{3}, and is the electric potential in **r**_{3} created by charges *Q*_{1} and *Q*_{2}. The potentials are:

Where *r*_{ij} is the distance between charge *Q*_{i} and *Q*_{j}.

If we add everything:

Finally, we get that the electrostatic potential energy stored in the system of three charges:

The energy density, or energy per unit volume, , of the electrostatic field of a continuous charge distribution is:

One may take the equation for the electrostatic potential energy of a continuous charge distribution and put it in terms of the electrostatic field.

Since Gauss's law for electrostatic field in differential form states where

- is the electric field vector
- is the total charge density including dipole charges bound in a material
- is the permittivity of free space,

then,

so, now using the following divergence vector identity

we have

using the divergence theorem and taking the area to be at infinity where

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

Some elements in a circuit can convert energy from one form to another. For example, a resistor converts electrical energy to heat. This is known as the Joule effect. A capacitor stores it in its electric field. The total electrostatic potential energy stored in a capacitor is given by
where *C* is the capacitance, *V* is the electric potential difference, and *Q* the charge stored in the capacitor.

One may assemble charges to a capacitor in infinitesimal increments, , such that the amount of work done to assemble each increment to its final location may be expressed as

The total work done to fully charge the capacitor in this way is then where is the total charge on the capacitor. This work is stored as electrostatic potential energy, hence,

Notably, this expression is only valid if , which holds for many-charge systems such as large capacitors having metallic electrodes. For few-charge systems the discrete nature of charge is important. The total energy stored in a few-charge capacitor is which is obtained by a method of charge assembly utilizing the smallest physical charge increment where is the elementary unit of charge and where is the total number of charges in the capacitor.

The total electrostatic potential energy may also be expressed in terms of the electric field in the form

where is the electric displacement field within a dielectric material and integration is over the entire volume of the dielectric.

*(A virtual experiment based on the energy transfert between capacitor plates reveals that an additional term must be taken into account when the electrostatic energy is expressed in terms of the electric field and displacement vectors ^{[3]}.*

*While this extra energy cancels when dealing with insulators, in general it cannot be ignored, as for instance with semiconductors.)*

The total electrostatic potential energy stored within a charged dielectric may also be expressed in terms of a continuous volume charge, , where integration is over the entire volume of the dielectric.

These latter two expressions are valid only for cases when the smallest increment of charge is zero () such as dielectrics in the presence of metallic electrodes or dielectrics containing many charges.

**^**Electromagnetism (2nd edition), I.S. Grant, W.R. Phillips, Manchester Physics Series, 2008 ISBN 0-471-92712-0**^**Halliday, David; Resnick, Robert; Walker, Jearl (1997). "Electric Potential".*Fundamentals of Physics*(5th ed.). John Wiley & Sons. ISBN 0-471-10559-7.**^**Sallese (2016-06-01). "A new constituent of electrostatic energy in semiconductors".*The European Physical Journal B*.**89**(6): 136. arXiv:1510.06708. doi:10.1140/epjb/e2016-60865-4. ISSN 1434-6036. S2CID 120731496.

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