Electrostatics

Objects are made of atoms, which in turn are made of electrons, protons, and neutrons. Protons have an intrinsic property called positive charge. Neutrons don’t contain any charge, and electrons have a property called negative charge. An excess of electrons will cause an object to be negatively charged, while an excess of protons will create positively charged objects. Electrostatics deal with electric charges that aren’t moving through circuit components, hence, their name.1 Electric charges follow a simple rule: like charges repel and opposite charges attract. Two positively charged particles will try to get as far away from each other as possible, while a positively charged particle and a negatively charged particle will try to get as close as possible. The reason that big objects like trees and animals don’t behave like charged particles is because they contain so many billions of protons and electrons, that an extra few here or there won’t really make much of a difference. So even though they might have a slight electric charge, that charge would be much too small, relatively speaking, to detect. Microscopic objects, like atoms, more commonly carry a measurable electric charge, because they have so few protons and electrons that an extra electron, for example, would make a big difference. Conductors allow charge to easily move through them.1 Insulators do not let charges move easily but hold them in place where they are. Electrons and protons are real particles that can move from place to place, or transfer from object to object by contact, but the net charge of the system always stays the same (conservation of charge). The unit of charge is the Coulomb, abbreviated C. One proton has a charge of 1.6 × 10−19 coulombs. One electron has a charge of –1.67 × 10–19 C.

An electric field provides a force on a charged particle. Electric potential, also called voltage, provides energy to a charged particle. Electric field is a vector, and electric potential is a scalar. An electric field is a property of a region of space that applies a force to charged objects in that region of space. A charged particle in an electric field will experience an electric force. Unlike a gravitational field, an electric field can either push or pull a charged particle, depending on the charge of the particle. Electric field is a vector; so, electric fields are always drawn as arrows. Every point in an electric field has a certain value called the electric field value, or E, and this value tells you how strongly the electric field at that point would affect a charge. The units of E are Newtons/Coulomb, abbreviated N/C. The force felt by a charged particle in an electric field is described by a simple equation:

f=qe

particle, q, multiplied by the electric field value, E. The direction of the force on a positive charge is in the same direction as the electric field; the direction of the force on a negative charge is opposite the electric field. To find the magnitude of the force, we plug in just the magnitude of the charge and the electric field, no negative signs are used. To find the direction of the force, use the reasoning presented earlier (positive charges are forced in the direction of the E field, negative charges opposite the E field). Electric potential refers to the potential energy provided by an electric field per unit charge. It is also known as the voltage. Electric potential is a scalar quantity.1 The units of electric potential are volts. 1 volt is equal to 1 J/C.

We can use the term “zero of potential” when we talk about voltage. We cannot solve a problem that involves voltage unless we know where the zero of potential is. Often, the zero of electric potential is termed ground. Unless it is otherwise specified, the zero of electric potential is assumed to be very far away. This means that if you have two charged particles and you move them farther and farther from each another, ultimately, once they’re infinitely far away from each other, they won’t be able to feel each other’s presence. The electrical potential energy of a charged particle is given by this equation:

ΔU E q ΔV

Here, q is the charge on the particle, and ΔV is the difference in potential voltage. It is important to note that electric potential and electric field are not the same thing. Electric field lines point in the direction that a positive charge will be forced, which means that a charged particle, when placed in an electric field, will be pushed from left to right or right to left. So, just as an object in Earth’s gravitational field has greater potential energy when it is higher off the ground (think “mgh”), our charged particle will have the greatest electrical potential energy when it is farthest from where it wants to get to. Very often the distances between charges in a group of charges are much smaller than the distance from the group to some point of interest (for example, a point where the electric field is to be calculated). In such situations, the system of charges is smeared out, or continuous. That is, the system of closely spaced charges is equivalent to a total charge that is continuously distributed along some line, over some surface, or throughout some volume.

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