Electricity : class of physical phenomena resulting from the existence of charge and from the interaction of charges. When a charge is stationary or static, it produces forces on objects in regions where it is present, and when it is in motion, it produces magnetic effects. Electric and magnetic effects are caused by the relative position and movement of positively and negatively charged particles of matter. So far as electrical effects are concerned, these particles are either neutral, positive, or negative. Electricity is concerned with the positively charged particles, such as protons, that repel one another and the negatively charged particles, such as electrons, that also repel one another . Negative and positive particles, however, attract each other. This behavior may be summarized as follows: Like charges repel, and unlike charges attract.


II. Electrostatics


A common manifestation of electricity is the repulsive or attractive force between two stationary bodies. Electrically, they exert an equal force on one another. The electric charge on each body may be measured in statcoulombs or the electrostatic unit of charge, abbreviated to esu .
 The force between particles bearing charges q₁ and q₂ can be calculated by Coulomb's law——that is, the force is proportional to the product of charges, divided by the square of the distance that separates them. The constant of proportionality k is called the dielectric constant and it is equal to . The constant is called the permittivity, and its value depends on the medium surrounding the charges. This law is named after the French physicist Charles Augustin de Coulomb, who developed the equation.

Every electrically charged particle is surrounded by a field of force. This field may be represented by lines of force showing the direction of electrical stresses within the field. To move a charged particle from one point in the field to another requires work. The amount of energy needed to perform such work on a particle bearing a unit charge is known as the potential difference between these two points. The difference is usually measured in volts. The earth, a large conductor, which may be assumed to be substantially uniform electrically, is commonly used as the zero reference level for potential energy. Thus the potential of a positively charged body is said to be a certain number of volts above the potential of the earth, and the potential of a negatively charged body is said to be a certain number of volts below the potential of the earth.