Electric Field

The forces exerted on one another by electric charges are related to an electric field that surrounds any charged body. The magnitude of this field is given by the electric field strength E. If a charge Q is present within an electric field (resulting from another different charge), it is subjected to a force F. The relationship between the force and the field strength is given by the following expression:


The force itself thus obeys the following equation:

The force on a charge in an electric field is therefore stronger when the field is stronger and when the charge is itself is greater.

Electric field is not solely defined by the magnitude of the force on the charge, however, but by its direction. Electric fields are thus portrayed in the form of field line diagrams, that indicate the direction of the force. The form of an electric field is given by the geometric shapes of the charges that give rise to the field and by their position with respect to one another. The field lines indicate at any point in the field in which direction the electrical force will act on another charge. The following graphic shows an electric field around a positive point charge (left). The field lines radiate in straight lines from the charge. The direction of the lines is indicated by arrows. The arrows indicate the direction that a positive point charge (the smaller points in the graphic) would move if free to do so.  Field lines always radiate from a positive charge (or infinity). The density of the field lines (how close they are together) shows the strength of the electric field, so that in this case the field strength decreases with increasing distance from the point charge. 

If positive and negative charges are evenly distributed on two metal plates positioned parallel to one another (as is the case with a plate capacitor as we shall see later), the field lines between the plates are parallel as shown in the following graphic. The field lines emerge from the positively charged plate and end at the negatively charged plate. Since the density of lines inside a plate capacitor is always the same, the electric field strength E between the plates is also the same.  Such an electric field is described as being uniform

Note: field lines also run outside the capacitor but these are curved and are not shown here since we shall not be studying them in detail. 

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