In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. This electric field exerts a force on other electrically charged objects. The concept of an electric field was introduced by Michael Faraday. The electric field is a vector field with SI units of newtons per coulomb (N C−1) or, equivalently, volts per metre (V m−1). The SI base units of the electric field are kg·m·s−3·A−1. The strength of the field at a given point is defined as the force that would be exerted on a positive test charge of +1 coulomb placed at that point; the direction of the field is given by the direction of that force. Electric fields contain electrical energy with energy density proportional to the square of the field intensity. The electric field is to charge as gravitational acceleration is to mass and force density is to volume.
Electric charges produce an electric field that fills the space. To
find the value of the electric field E at a point, we place a very small point charge q at that point and measure
the force F acting on the point charge; the electric field is then given by E = F / q. Note that the electric
field is a vector, since F is a vector.
The electric field produced by a point charge Q at a distance r has
a magnitude E = kQ/r^2. The field is directed radially outward if Q is positive and radially inward if Q is
The electric field produced by a collection of point charges is
equal to the vector sum of the fields produced by the individual point charges.
Between the plates of a charged parallel-plate capacitor, E = σ /
ε0, where σ is the surface charge density on the plates and ε0 is the permittivity of free space.
Electric field lines are always tangent to the electric field at
every point in space. The lines always begin at positive charge and end on negative charge, and their density is
proportional to the field strength.
Under electrostatic conditions, any excess charge, added to a
conductor, must reside on the conductor surface.
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.