Before we look at DC circuits, we need to take a moment to talk about what electricity is. We will begin by looking at atoms. The atom is the basic building block of all matter. Everything in the universe is made up of various combinations of approximately 90 different kinds of atoms that occur in nature. A substance composed of only one type of atom is called an element, while a substance composed of more than one element may be either a compound (the atoms are attached to each other through some kind of chemical bond) or a mixture (the atoms are not chemically bonded). Here are some everyday examples of all three types of substances:
Elements: iron, copper, carbon, sulfur, oxygen
Compounds: water (hydrogen + oxygen), salt (sodium + chlorine)
Mixtures: brass (copper + zinc) air (nitrogen + oxygen + argon + carbon dioxide)
Atoms in turn, are composed primarily of three elementary particles:
protons (positively charged)
electrons (negatively charged)
neutrons (uncharged)
The protons and neutrons are clumped together in the center of the atom. This collection of protons and neutrons is known as the nucleus and its protons give it a positive charge. The electrons orbit the nucleus somewhat like the solar system's planets orbit the sun. The electrons are held in orbit around the nucleus because the positively charged nucleus exerts an electrostatic force on the negatively charged electron. When we remove electrons from an atom, we have electricity. Electricity is simply a collection of charged particles, usually electrons. There are certain cases in which electricity can be composed of ions, which are atoms that have either extra electrons or too few electrons. (an example would be the ionized gas in a neon lamp or a fluorescent lamp). Electricity never consists of collections of protons - they are too tightly bound to the nucleus to be removed without destroying the entire atom.
There are many ways that electrons can be removed from atoms to create electricity, including:
friction (the static electricity we observe after walking across a rug and touching a doorknob)
chemical reactions (batteries)
mechanical motion (alternators and generators)
heat (thermocouples)
light (solar cells)
The electricity created by these processes falls into one of two categories:
Static Electricity - a collection of electrons with no net motion
Electric Current - a collection of electrons all of which are moving in a particular direction.
Of these two types, electric current is the most useful and the entire field of electronics ultimately is concerned with the creation and control of electric currents.
A flow of electric current is the result of electrons being acted on by an electric field, so this is a good time to take a small detour and talk about electric charge and the electric field. A subatomic particle that has an electric charge, such as a proton or neutron, creates and electric field. This electric field extends throughout all space and can affect other charged particles that it encounters. Consider a region of empty space that contains only a single proton. The proton's positive electrical charge gives rise to an electric field that fills the entire space. Now let us add an electron. Because the electron is negatively charged, it is affected by the electric field. The field causes the electron to be attracted to the proton. Therefore:
Unlike charges attract one another
Let us remove the electron and add a proton. The proton carries a positive charge and thus the electric field created by the other proton affects it. In this case, the electric field pushes the second proton away from the first proton. Therefore:
Like charges repel one another
In a real atom, the interaction between protons in the nucleus and electrons in a cloud surrounding the nucleus is very complex because more than just two particles are involved. There are however, certain generalizations that can be made, based on the two above-mentioned rules, along with some principles from quantum physics:
1. The electrons are not distributed randomly in the electron cloud, but rather are grouped into electron shells.
2. Each shell can only hold a specific number of electrons, which is larger for shells more distant from the nucleus.
3. Electrons in the outer shell are held to the nucleus more weakly because of repulsion by the electrons in the inner shells and because the attractive force of the positively charged nucleus decreases as one moves farther away from it.
4. If a shell has only one or two electrons, these are very weakly held and can be easily removed from the atom. As more electrons are added to the shell, they become more difficult to remove, and when the shell is full or nearly full, it is very difficult to remove any electrons.
The chemical and electrical properties of most materials can be explained in terms of these 4 rules. For our purposes, we will define three types of materials
1. Conductor - many electrons are available to move from atom to atom throughout the bulk of this type of material.
2. Semiconductor - only a few electrons are available to move around.
3. Insulator - all electrons are tightly held by their atoms and cannot move.
Now we know that electricity is a collection of free electrons and an electric current is a collection of electrons moving under the influence of an electric field. Now we will define the two basic types of electric current:
Direct Current (DC) - an electric current that always flows in one direction at a constant value.
Alternating Current (AC) - an electric current whose value and direction change with time.
In the remainder of this section, we will concern ourselves with DC. Before we proceed, we need to define some fundamental units of measure for electricity:
Electric
Charge. Symbol: Q. Charge
is quantity of electricity, and can be both positive and negative. Unit:
the coulomb,
abbreviated as C.
Definition: One electron has a charge of -1.6*1019C, so
that charge of -1C is carried by 6.2*1018 electrons.
Voltage (also called electrical potential). The unit is the volt, (named after an Italian scientist, Alessandro Volta)Symbol: V. Because charges of opposite sign attract each other, we must put energy into a system in order to separate charges, for example, removing an electron from an atom. The electrical potential difference or voltage between two points represents the energy that is liberated by allowing charge to move between the points. Unit: the volt (abbreviated V). Definition: A potential difference of 1V exists when 1C of charge has been separated.
Now it is time to look at DC circuits.
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