Terms Used in Electricity
People involved with radios talk about volts, amps, and ohms. These terms have specific meanings and describe the electrical properties involved. Before we talk about these terms, let us look an the analogy of a water system. Water flows through pipes, being pushed by pressure provided by ones town or by pumps owned by home owners. The pipes cause friction between the pipes and the water, and this friction opposes the flow of water.
Similarly, electrical systems have voltage, current, and resistance. There is a direct analogy between electrical systems and water systems. Think of voltage as electrical pressure, measured in volts, that "pushes" current through wires. Think of current as the movement, measured in amperes or amps, of electrons through wires. Think of resistance, measured in ohms, as the characteristic of wires that oppose the movement of the electrons. These terms, volts, amps, and ohms comprise the basic relationships in electrical circuits
Batteries
I think everyone is familiar with batteries that are used in numerous appliances in their home. Older flashlights use the large D-cell batteries or C-cell batteries, and newer flashlights use AA or AAA batteries. These batteries are the same except for the amount of power they provide before they need to be thrown away, recycled, or recharged. These batteries provide 1.5 volts. Technically, these objects are cells not batteries, but we call them "batteries". The larger the battery the more power it provides. Most people are also familiar with the small 9-volt rectangular batteries used in many home fire alarms. Alkaline batteries should be thrown away when discharged. Lithium batteries should be recycled since lithium is a hazardous material. NI-CAD or NIMH batteries should be recharged by the user, but the two types should not be mixed.
From left to right: C, AAA, D, AA, 9-volt
Parallel and Series Connections
Batteries can be connected together in two ways to give more power or more voltage. The + ends of the batteries can be connected together, and the - ends of the batteries can be connected together. This forms a connection that is known as a parallel connection, and the batteries still gives 1.5 volts (or 9 volts) but provides more power. Another way is to to connect the + end of one battery to the - end of another battery. This form of connection is known as a series connection, and the voltage provided is the sum of the voltages of individual batteries, but the power obtained is equal to that of the smallest battery used in the connection. There is a custom among many manufactures of electrical appliances that the - end of a battery goes against a spring that is inside the socket, but be careful in following this convention, because not all manufactures follow the convention. Before you insert a battery into a socket, look for a + or - at one end of the socket.
The batteries discussed in previous paragraphs create circuits known as DC, or direct current circuits. In DC circuits current always flows in one direction. Since current is the movement of electrons and electrons have negative charges, current flows from the - end of batteries to the + end of batteries. However, it is customary to think of current as the flow of electrons from the + to the - ends of batteries, and this convention is used in this page.
Ohm's Law
The relationship between the voltage, current, and resistance of a DC circuit was first discovered by a German scientist, Georg Simon Ohm, and is known as Ohm's Law. This relationship is depicted by the following triangle.
Cover the variable you wish to determine, and the triangle gives the value of that variable in terms of the other two variables. For example, V = I R, where V is the voltage in volts across the circuit (some authors use the letter E), and I R is the value of the current I in amps multiplied by the value of the resistance of the circuit R in ohms. Similarly, the current in amps flowing through the circuit is I=V/R where V/R is the voltage divided by the resistance, and the resistance of the circuit is R=V/I.
Schematic Diagrams
People working with electrical circuits use diagrams, known as schematic diagrams, to describe the nature of the circuits. Notice that the elements of the circuit are shown by specific symbols. Here are two diagrams, having as elements a source of voltage (VB) and two resistors (R1 and R2) connected in parallel and in series. The total current flowing in the circuit is IT. In the parallel connection, the current flowing through each resistor is I1 and I2. In the series circuit, the total current flows through both resistors. If values are given for VB, R1, and R2, the values of IT, I1, and I2 could be calculated via Ohm's Law. To simplify calculations, the wires connecting the voltage-source and the resistors are assumed to have zero resistance.
DC vs. AC for Lights
The Electric light bulb was invented by Thomas Edison, and he used DC power sources. Gas lights had been used to light buildings in Manhattan, NY, but they were more expensive and dangerous than light bulbs, and they were replaced by light bulbs.
Engineers discovered that moving water could be used to turn turbines to produce electricity, and they began using the Niagara River at Niagara Falls to produce electricity. Large tunnels diverted water from the river to turbines. Soon, Niagara, NY was lit by light bulbs. Then a problem arose. More electricity was being produced than was being used by the city, and engineers began looking at the feasibility of transmitting power to other cities. Of course, Thomas Edison and his colleagues planned on using DC for this transmission. The basic problem they encountered was that the resistance in the long transmissions consumed much of the power being generated, and relatively little power was left to light the distant cities.
An engineer named Nikola Telsa realized that less power would be consumed by the transmission lines if AC were used, and a serious political battle ensued between Edison and Telsa. Telsa won the battle, and cities began using AC power to light their streets and buildings.
Turbines used at Niagara Falls
AC Circuit ParametersWhen you plug an appliance into a wall power socket, you are using AC to power that appliance. In AC, the current periodically reverses direction; hence the name, alternating current or AC. In the United States, AC power is used in almost all buildings, and AC circuits typically use 117 volts, and the reversal of current-flow occurs 120 times per second.
Pictures of AC signals are typically used in which the amount or amplitude of current or voltage is shown as a graph versus time. These graphs look similar to the one in the following picture.
AC circuits have voltage, current, and resistance as do DC circuits. In addition, though, AC circuits, due to the change in value of the signals, have additional parameters of inductance and capacitance. Ohm's Law can be used with AC circuits if the resistance of the circuit includes the effect of inductance and capacitance. However, inductance and capacitance are not discussed in this page.
AC signals are called sine waves due to the mathematical shape of the curve. Notice the voltage or current starts at a zero value and increases to a peak and then decreases to zero. The voltage or current then reverses and the value increases to a negative-peak and then decreases back to zero. The change in voltage or current shown in the picture is called a cycle, and in the United States the cycle is repeated 60 times per second. That is, AC circuits reverse or pulsate at a frequency of 60 Hertz or 60 Hz. In some countries, the AC reverses at a frequency of 50 Hz. The repetition of AC signals is named Hertz in honor of Heinrich Rudolf Hertz who was the first scientist to prove that radio signals, known as electromagnetic waves, did exist.
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