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18.4: Electric Field: Concept of a Field Revisited

  • Page ID
    2544
  • [ "article:topic", "authorname:openstax", "Electric Field", "field", "Point charge", "test charge" ]

    Contact forces, such as between a baseball and a bat, are explained on the small scale by the interaction of the charges in atoms and molecules in close proximity. They interact through forces that include the Coulomb force. Action at a distance is a force between objects that are not close enough for their atoms to “touch.” That is, they are separated by more than a few atomic diameters.

    For example, a charged rubber comb attracts neutral bits of paper from a distance via the Coulomb force. It is very useful to think of an object being surrounded in space by a force field. The force field carries the force to another object (called a test object) some distance away.

    Concept of a Field

    A field is a way of conceptualizing and mapping the force that surrounds any object and acts on another object at a distance without apparent physical connection. For example, the gravitational field surrounding the earth (and all other masses) represents the gravitational force that would be experienced if another mass were placed at a given point within the field.

    In the same way, the Coulomb force field surrounding any charge extends throughout space. Using Coulomb’s law, \(F=k|q_{1}q_{2}|/r^{2}\), its magnitude is given by the equation \(F=k|qQ|/r^{2}\), for a point charge (a particle having a charge \(Q\)) acting on a test charge \(q\) at a distance \(r\) (Figure \(\PageIndex{1}\)). Both the magnitude and direction of the Coulomb force field depend on \(Q\) and the test charge \(q\).

    Figure \(\PageIndex{1}\): The Coulomb force field due to a positive charge \(Q\) is shown acting on two different charges. Both charges are the same distance from \(Q\). (a) Since \(q_{1}\) is positive, the force \(F_{1}\) acting on it is repulsive. (b) The charge \(q_{2}\) is negative and greater in magnitude than \(q_{1}\), and so the force \(F_{2}\) acting on it is attractive and stronger than \(F_{1}\). The Coulomb force field is thus not unique at any point in space, because it depends on the test charges \(q_{1}\) and \(q_{2}\) as well as the charge \(Q\).

    To simplify things, we would prefer to have a field that depends only on \(Q\) and not on the test charge \(q\). The electric field is defined in such a manner that it represents only the charge creating it and is unique at every point in space. Specifically, the electric field \(E\) is defined to be the ratio of the Coulomb force to the test charge:

    \[\mathbf{E}=\dfrac{\mathbf{F}}{q},\]

    where \(\mathbf{F}\) is the electrostatic force (or Coulomb force) exerted on a positive test charge \(q\). It is understood that \(\mathbf{E}\) is in the same direction as \(\mathbf{F}\). It is also assumed that \(q\) is so small that it does not alter the charge distribution creating the electric field. The units of electric field are newtons per coulomb (N/C). If the electric field is known, then the electrostatic force on any charge \(q\) is simply obtained by multiplying charge times electric field, or \(\mathbf{F}=q\mathbf{E}\). Consider the electric field due to a point charge \(Q\). According to Coulomb’s law, the force it exerts on a test charge \(q\) is \(F=k|qQ|/r^{2}\). Thus the magnitude of the electric field, \(E\), for a point charge is

    \[E=|\dfrac{F}{q}|=k|\dfrac{qQ}{qr^{2}}|=k\dfrac{|Q|}{r^{2}}.\]

    Since the test charge cancels, we see that

    \[E=k\dfrac{|Q|}{r^{2}}.\]

    The electric field is thus seen to depend only on the charge \(Q\) and the distance \(r\); it is completely independent of the test charge\(q\).

    Example \(\PageIndex{1}\): Calculating the Electric Field of a Point Charge

    Calculate the strength and direction of the electric field \(E\) due to a point charge of 2.00 nC (nano-Coulombs) at a distance of 5.00 mm from the charge.

    Strategy

    We can find the electric field created by a point charge by using the equation \(E=kQ/r^{2}\).

    Solution

    Here \(Q=2.00\times 10^{-9}C\) and \(r=5.00\times 10^{-3}m\). Entering those values into the above equation gives

    \[ \begin{align*} E&=k\dfrac{Q}{r^{2}} \\[5pt] &= (8.99\times 10^{9} N\cdot m^{2}/C^{2})\times \dfrac{(2.00\times 10^{-9}C)}{(5.00\times 10^{-3}m)^{2}} \\[5pt] &= 7.19\times 10^{5} N/C. \end{align*}\]

    Discussion

    This electric field strength is the same at any point 5.00 mm away from the charge \(Q\) that creates the field. It is positive, meaning that it has a direction pointing away from the charge \(Q\).

    Example \(\PageIndex{2}\): Calculating the Force Exerted on a Point Charge by an Electric Field

    What force does the electric field found in the previous example exert on a point charge of\(-0.250\mu C\)?

    Strategy

    Since we know the electric field strength and the charge in the field, the force on that charge can be calculated using the definition of electric field \(\mathbf{E}=\mathbf{F}/q\) rearranged to \(\mathbf{F}=q\mathbf{E}\).

    Solution

    The magnitude of the force on a charge \(q=-.250\mu C\) exerted by a field of strength \(E=7.20\times 10^{5} N/C\) is thus,

    \[ \begin{align*} F &=-qE \\[5pt] &= (0.250\times 10^{-6}C)(7.20\times 10^{5} N/C) \\[5pt] &=0.180N. \end{align*}\]

    Because \(q\) is negative, the force is directed opposite to the direction of the field.

    Discussion

    The force is attractive, as expected for unlike charges. (The field was created by a positive charge and here acts on a negative charge.) The charges in this example are typical of common static electricity, and the modest attractive force obtained is similar to forces experienced in static cling and similar situations.

    PHET EXPLORATIONS: ELECTRIC FIELD OF DREAMS

    Play ball! Add charges to the Field of Dreams and see how they react to the electric field. Turn on a background electric field and adjust the direction and magnitude.

    PhET_Icon.png

    Figure \(\PageIndex{2}\): Electric Field of Dreams

    Summary

    • The electrostatic force field surrounding a charged object extends out into space in all directions.
    • The electrostatic force exerted by a point charge on a test charge at a distance\(r\) depends on the charge of both charges, as well as the distance between the two.
    • The electric field \(\mathbf{E}\) is defined to be \(\mathbf{E}=\dfrac{\mathbf{F}}{q},\) where \(\mathbf{F}\) is the Coulomb or electrostatic force exerted on a small positive test charge \(q\). \(\mathbf{E}\) has units of N/C.
    • The magnitude of the electric field \(\mathbf{E}\) created by a point charge \(Q\) is \(\mathbf{E}=k\dfrac{|Q|}{r^{2}}.\) where \(r\) is the distance from \(Q\). The electric field \(\mathbf{E}\) is a vector and fields due to multiple charges add like vectors.

    Glossary

    field
    a map of the amount and direction of a force acting on other objects, extending out into space
    point charge
    A charged particle, designated Q, generating an electric field
    test charge
    A particle (designated q) with either a positive or negative charge set down within an electric field generated by a point charge

    Contributors

    • Paul Peter Urone (Professor Emeritus at California State University, Sacramento) and Roger Hinrichs (State University of New York, College at Oswego) with Contributing Authors: Kim Dirks (University of Auckland) and Manjula Sharma (University of Sydney). This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).