In physics, electromotive force, or most commonly emf (seldom capitalized), or (occasionally) electromotance is 'that which tends to cause current (actual electrons and ions) to flow.More formally, emf is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals. The electric potential difference is created by separating positive and negative charges, thereby generating an electric field.The created electrical potential difference drives current flow if a circuit is attached to the source of emf. When current flows, however, the voltage across the terminals of the source of emf is no longer the open-circuit value, due to voltage drops inside the device due to its internal resistance.
A changing (in time) magnetic field induces an electric field. So
if a magnetic field exists in space and it varies with time, then an electric field will necessarily exist in
The value of the induced electric field is obtained by noting that
the integral of E.ds around any closed loop is equal to minus the derivative of the magnetic flux with respect to
time; this is nothing but a restatement of Faradays law.
Two examples that illustrate how to evaluate the induced electric
field are given in the lecture. In the first example a magnetic field, uniform in space, is slowly turned off.
The other example shows how an AC generator works.
Induced Electric Field
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.