Let's continue with basic electrical properties and start with electric charge. If an atom contains its usual number of electrons, the atom or groups of similar atoms display no electrical properties. Such an atom or atoms are called electrically neutral. If electrons are lost or captured by the atoms, they become electrically charged. Positively or negatively charged atoms or combinations of atoms are called ions. Charged objects are attracted or repelled by forces that depend on the nature of the charges. Objects with the same charge repel one another, while objects with different charges attract each other. The following animation illustrates the relationship. Give the two balls various charges by dragging a charge into either ball with a mouse and see what happens. Let's talk about 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 subject to a force, F. The relationship between the force and the field strength is given by the following expression: E equals F divided by Q. The force itself is determined from the following equation; F equals E times Q. The force on the charge in an electric field is therefore stronger when the field is stronger and when the charge itself is greater. Electric field is not solely defined by the magnitude of the force on the charge, but by its direction as well. Electric fields are thus portrayed in the form of field line diagrams that indicate the direction of the force as seen at the following images. Note the direction of the force and how it depends on the polarity of the charge. Let's talk about voltage. In current sources, such as batteries or generators, positive and negative charges that exist in all materials are separated from one another by the effect of some energy. One terminal of the source has an excess of electrons, the negative pole, while the other displays a deficit or the positive pole. An electric field exists between these two charges and the system will try to even this imbalance, so the charges flow from one terminal to the other and generate a so-called electric current. If both poles are connected via conductors, the charges seek to even out by passing along these conductors and giving rise to a current. This involves the source, such as a battery, exerting an amount of work, W, on the charge, Q, that has been transported. The voltage, V, of the source is now defined as the quotient of the work and the charge. So, V equals W divided by Q. The unit of electrical voltage is called the volt, or V for short. A voltage can only exist between two points, such as the poles of the electricity source. The meaning of the concept of voltage can be illustrated by an analogy from the world of physics. The electricity source is equated with the water pump. A pump is able to exert an amount of lifting work on each gallon of water it raises up to a certain maximum height. So, for each gallon of water, it provides a certain amount of energy. Let's take a look at the case sketched in the following graphic. Pump P2 has to exert twice the amount of work on each gallon as pump P1 since the height is twice as high. The quotient of the work and the volume is thus twice as large. The analogy with electricity would then suggest that the right-hand pump has double the voltage. That leads us to DC voltage sources. If the polarity of an electrical energy source doesn't change over time, it is called a direct current source. If the magnitude of the voltage is invariable, the source is termed as being fixed voltage source. Let's take a look at the graphics that depicts the circuit diagram symbol for such a source. We all know about galvanic elements. The principle of a galvanic element involves using electrochemical processes to generate a voltage. Such an element contains two materials of different conductivity, such as zinc and carbon, that are used as electrodes immersed in the so-called electrolyte. Elements of this kind are called primary elements. The magnitude of the voltage produced by a galvanic element depends on the material used for the electrodes. Among the examples of such galvanic elements are common household batteries. These are available nowadays in a variety of shapes and forms. A typical voltage output is one and a half volts or some multiple of this number, three volts or nine volts. Let's talk about power supplies. A power supply provides power supplied from the AC network, which has a specified voltage, currently defined in North America is 120 volts. This usually involves using a transformer to step the voltage down to the required voltage for the appliance. In a stabilized power supply, a controller or voltage stabilizer will ensure that the output voltage remains generally constant in spite of changing load or input. Such power supplies are also available in many forms, such as adjustable laboratory power supply shown here or the fixed voltage supply that is often used for toys or music and games equipment. Other forms include plugin power supplies or the power supply in the computer that provides various different but constant voltage outputs. Now, let's talk about electric current. As we discussed earlier, conductors possess large number of free electrons that are able to move between atoms in the atomic framework. If there is no voltage across the conductor, the movement of such electrons is purely arbitrary. So, there is no overall direction of motion or specific destination as shown in the following graphic. If a DC voltage is applied to the conductor, the electrons now flow in a specific direction through the conductor, and electric current flows from one pole of the voltage source to the other. We will start with direction of electron flow. Electrons flow in a conductor in the circuit outside of the voltage source from the negative of the source, where electrons are in surplus, to the positive pole, where there is a relative deficit of electrons. Inside the source, the electrons are forced away from the positive pole towards the negative. Take a look at the graphics. The source ensures that there is always a potential difference between the two poles. The electrons are not actually created by the source. The source merely sets existing free electrons in the conductors of the circuit in motion. Similarly, the electrical devices being powered, that are generally called loads, such as lamp in the circuit that you can see, they do not actually use up electrons. They just use some of the energy that the moving electrons carry. That leads us to direction of conventional current. Before the theory of electrons was developed, even though the terminology positive and negative has been chosen, it was assumed that any carrier of current was actually in excess as the positive pole, and in deficit at the negative, so that current would flow from positive to negative. Despite the more recent knowledge about electricity, the assumptions about the direction of flow had been well-established and it was not practical to stick to the convention that current flows from positive towards negative pole. Thus, the direction of an electric current is by convention in the opposite direction to the electron flow. It is common to speak of conventional current as illustrated in the following figure. To sum it up, electric current flows by convention from positive pole to negative pole in any circuit external to a voltage source.
