In physics, electrical phenomena can be described by the fundamental concept of electric charge: it is a property of matter that can be quantified. There are two types of charge: we call one kind of charge positive and the other negative. Through experiment, we find that like-charged objects repel and opposite-charged objects attract.
Hence, we find that there is a force between these charges that is directly proportional to the magnitude of the charge of the objects and inversely proportional to distance between them. If one object has a certain charge we will call q1, and another has charge q2, then we can express the force from q1 on q2 by:
Where r is the distance between the charges, and ε0 is a universal constant. See Physical constants.
The unit of charge is called a Coulomb.
Note that this formula, called Coulomb's Law is only fully accurate when the charges are static or moving slowly. When charges start moving quickly, the Electric field undergoes a transformation described by Einstein's Theory of Relativity, and Magnetic fields are produced that effect the amount of force on q1 and q2.
In electrical engineering, a positive current is a current flowing from a positive voltage to a negative voltage.
Natural forms of electricity: lightning.
Many animals are sensitivie to electric fields, some (eg sharks) more than others (eg people). A few generate their own electric fields, such as the electric eel.