How to Calculate Electric Potential Energy

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When you first undertake a study of the motion of particles in electric fields, there's a solid chance that you have already learned something about gravity and gravitational fields.

As it happens, many of the important relationships and equations governing particles with mass have counterparts in the world of electrostatic interactions, making for a smooth transition.

You have perhaps learned that energy of a particle of constant mass and velocity v is the sum of kinetic energy EK, which is found using the relationship mv2/2, and gravitational potential energy EP, found using the product mgh where g is the acceleration owing to gravity and h is the vertical distance.

As you'll see, finding the electric potential energy of a charged particle involves some analogous mathematics.

Electric Fields, Explained

A charged particle Q establishes an electric field E that can be visualized as a series of lines radiating symmetrically outward in all directions from the particle. This field imparts a force F on other charged particles q. The magnitude of the force is governed by Coulomb's constant k and the distance between the charges:

F = \frac{kQq}{r^2}

k has a magnitude of 9 × 109 N m2/ C2, where C stands for Coulomb, the fundamental unit of charge in physics. Recall that positively charged particles attract negatively charged particles while like charges repel.

You can see that the force decreases with the inverse square of increasing distance, not merely "with distance," in which case the r would have no exponent.

The force can also be written F = qE, or alternatively, the electric field can be expressed as E = F/q.

Relationships Between Gravity and Electric Fields

A massive object such as a star or planet with mass M establishes a gravitational field that can be visualized in the same manner as an electric field. This field imparts a force F on other objects with mass m in a manner that decreases in magnitude with the square of the distance r between them:

F = \frac{GMm}{r^2}

where G is the universal gravitational constant.

The analogy between these equations and those in the previous section are evident.

Electric Potential Energy Equation

The formula of electrostatic potential energy, written U for charged particles, accounts for both the magnitude and polarity of the charges and their separation:

U = \frac{kQq}{r}

If you recall that work (which has units of energy) is force times distance, this explains why this equation differs from the force equation only by an "r" in the denominator. Multiplying the former by distance r gives the latter.

Electric Potential Between Two Charges

At this point you may be wondering why there has been so much talk of charges and electric fields, but no mention of voltage. This quantity, V, is simply electric potential energy per unit charge.

Electric potential difference represents the work that would have to be done against the electric field to move a particle q against the direction implied by the field. That is, if E is generated by a positively charged particle Q, V is the work necessary per unit charge to move a positively charged particle the distance r between them, and also to move a negatively charged particle with the same charge magnitude a distance r away from Q.

Electric Potential Energy Example

A particle q with a charge of +4.0 nanocoulombs (1 nC = 10 –9 Coulombs) is a distance of r = 50 cm (i.e. 0.5 m) away from a charge of –8.0 nC. What is its potential energy?

\begin{aligned} U &= \frac{kQq}{r} \\ &= \frac{(9 × 10^9 \;\text{N} \;\text{m}^2/\text{C}^2)×(+8.0 × 10^{-9} \;\text{C})×(–4.0 × 10^{-9} \;\text{C})}{0.5 \;\text{m} } \\ &= 5.76 × 10^{-7} \;\text{J} \end{aligned}

The negative sign results from the charges being opposite and therefore attracting each other. The amount of work that must be done to result in a given change in potential energy has the same magnitude but the opposite direction, and in this case positive work must be done to separate the charges (much like lifting an object against gravity).

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

Photo Credits

  • 1000 powers image by Kostyantyn Ivanyshen from Fotolia.com

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