If we keep the same resistance in a circuit but vary the voltage, the current will vary. The circuit in Fig. 2-1 demonstrates this idea. The applied voltage V can be varied from 0 -12 V, as an example. The bulb has a 12-V filament, which requires this much voltage for its normal current to light with normal intensity. The meter I indicates the amount of current in the circuit for the bulb.
With 12 V applied, the bulb lights, indicating normal current. When V is reduced to 10 V, there is less light because of less I. As V decreases, the bulb becomes dimmer. For zero volts applied there is no current and the bulb cannot light. In summary, the changing brilliance of the bulb shows the current is varying with the changes in applied voltage.
For the general case of any V and R, Ohm's Law is
where I is the amount of current through the resistance R connected across the source of potential difference V. With volts as the practical unit for V and ohms for R, the amount of current I is in amperes. Therefore,
This formula says to simply divide the voltage across R by the ohms of resistance between the two points of potential difference to calculate the amperes of current through R.
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2/09/2011
The Current I = V x R
Ohm's Law
This note is explain how the amount of current I in a circuit depends on its resistance R and the applied voltage. Specifically, I = V/R, determined in 1828 by the experiments of George Simon Ohm. If we know any two of the factors V, I, and R, we can calculate the third. As an example, with 6V applied accros an R of 2 ohm, the I is 6/2 = 3 A. Ohm's law also determines the amount of electric power in the circuit. The amount of power consumed P in a circuit is equal to the voltage times the current (P = V x I). The power used by the R above is therefore equal 6 x 3 = 18 W. This relations apply to both DC and AC circuit.
Important terms in this note are:
volt
ampere
ohm
watt
joule
milli
micro
kilo
mega
linear graph
power
voltampere characteristic
Read More......
Important terms in this note are:
volt
ampere
ohm
watt
joule
milli
micro
kilo
mega
linear graph
power
voltampere characteristic
12/25/2010
Charges of Opposite Polarity Attract
If two small charged bodies of light weight are mounted so that they are free to move easily and are placed close to each other, one can be attracted to the other when the two charges have opposite polarity. In terms of electrons and protons, they tend to be attracted to each other by the force of attraction between opposite charges. Furthermore, the weight of an electron is only about 1/1840 weight of a proton. As a result, the force of attraction tends to make electrons move to protons.
Read More......Negative and Positive Polarities
Historically, the negative polarity has been assigned to the static charge produced on rubber, amber, and resinous materials in general. Positive polarity refers to the static charge produced on glass and other vitreous materials. On this basis, the electrons in all atoms are basic particles of negative charge because their polarity is the same as the charge on rubber. Protons have positive charge because the polarity is the same as the charge on glass.
Read More......The Coulumb Unit of Charge
If you rub a head rubber pen or comb on a sheet of paper, the rubber will attract a corner of the paper if it is free to move easily. The paper and rubber then give evidence of a static electric charge. The work of rubbing resulted in separating electrons and protons to produce a charge of excess electrons on the surface of the rubber and a charge of excess protons on the paper.
Because paper and rubber are dielectric materials, they hold their extra electrons or protons. As a result, the paper and rubber are no longer neutral, but each has an electric charge. The resultant electric charges provide the force of attraction between the rubber and the paper. This mechanical force of attraction or repulsion between charges is the fundamental method by which electricity makes itself evidence.
Any charge is an example of static electricity because the electrons or protons are not in motion. There are many examples. When you walk across a wool rug, your body becomes charged with an excess of electrons. Similarly, silk, fur, and glass can be rubbed to produce a static charge. This effect is more evident in dry weather, because a moist dielectric does not hold its charge so well. Also, plastic materials can be charged easily, which is why thin, light weight plastics seem to stick to everything.
The charge of many billions of electrons or protons is necessary for common applications of electricity. Therefore, it is convenient to define a practical unit called the coulomb (C) as equal to the charge of 6.25 x 10^18 electrons or protons stored in a dielectric. The analysis of static charge and their forces is called electrostatics.
The symbol for electric charge is Q or q, standing for quantity. For instance, a charge of 6.25 x 10^18 electrons is stated as Q = 1 C. This unit is named after Charles A. Coulomb (1736-1806), a French physicist, who measured the force between charges.
Particles in the Nucleus
A stable nucleus, which is not radioactive, contains protons and neutrons. A neutron is electrically neutral without any net charge. Its mass is almost the same as a proton.
A proton has the positive charge of a hydrogen nucleus. The charge is the same amount as that of an orbital electron but of opposite polarity. There are no electrons in the nucleus. Table 3 lists the charge and mass for these three basic particles in all atoms.
Subshells
Although not shown in the illustrations, all the shells except K are divided into subshells. This subdivision accounts for different types of orbits in the same shell. For instance, electrons in one subshell may have elliptical orbits, while other electrons in the same main shell have circular orbits. The subshells indicate magnetic properties of the atom.
Read More......1/07/2010
Electron Valence
This value is the number of electrons in an incomplete outermost shell. Copper, for instance, has a valence of 1 because there is 1 electron in the last shell, after the inner shells have been completed with their stable number. Similarly, hydrogen has a valence of 1, and carbon has a valence of 4. The number of outer electrons is considered positive valence, as these electrons are in addition to the stable shells.
Except for H and He, the goal of valence is 8 for all the atoms, as each tends to form the stable structure of 8 electrons in the outside ring. For this reason, valence can also be considered as the number of electron s in the outside ring needed to make 8. This value is the negative valence. As examples, the valence of copper can be considered +1 or -7; carbon has the valence of ±4. The inert gases have a valence of 0, as they all have a complete stable outer shell of 8 electrons.
The valence indicates how easily the atom can gain or lose electrons. For instance, atoms with a valence of +1 can lose this 1 outside electron, especially to atoms with a valence of +7 or -1, which need 1 electron to complete the outside shell with 8 electrons.
9/25/2009
Orbital Rings
This value is the number of electrons in an incomplete outermost shell. Copper, for instance, has a valence of 1 because there is 1 electron in the last shell, after the inner shells have been completed with their stable number. Similarly, hydrogen has a valence of 1, and carbon has a valence of 4. The number of outer electrons is considered positive valence, as these electrons are in addition to the stable shells.
Except for H and He, the goal of valence is 8 for all the atoms, as each tends to form the stable structure of 8 electrons in the outside ring. For this reason, valence can also be considered as the number of electron s in the outside ring needed to make 8. This value is the negative valence. As examples, the valence of copper can be considered +1 or -7; carbon has the valence of ±4. The inert gases have a valence of 0, as they all have a complete stable outer shell of 8 electrons.
The valence indicates how easily the atom can gain or lose electrons. For instance, atoms with a valence of +1 can lose this 1 outside electron, especially to atoms with a valence of +7 or -1, which need 1 electron to complete the outside shell with 8 electrons.
9/21/2009
Atomic Number
This gives the number of protons or electrons required in the atom for each element. For the hydrogen atom in Fig. 1, the atomic number is 1, which means the nucleus has 1 proton balanced by 1 orbital electrons. Similarly, the carbon atom in Fig. 2 with atomic number 6 has 6 protons in the nucleus and 6 orbital electrons. Also, the copper atom has 29 electrons because its atomic number is 29.
Structure of the Atom
Although nobody has ever seen an atom, its hypothetical structure fits experimental evidence that has been measured very exactly. The size and electric charge of the invisible particles in the atom are indicated by how much they are deflected by known forces. Our present planetary model of the atom was proposed by Niels Bohr in 1913. His contribution was joining the new ideas of the nuclear atom developed by Lord Rutherford (1871-1937) with the quantum theory of radiation developed by Max Planck (1858-1947) and Albert Einstein (1879-1955).
The nucleus contains protons for all the positive charge in the atom. The number of protons in the nucleus is equal to the number of planetary electrons. Thus, the positive and negative charges are balanced, as the proton and electron have equal and opposite charges. The orbits for the planetary electrons are also called shells or energy levels.
Molecules and Compounds
A group of two or more atoms forms a molecule. For instance, two atoms of hydrogen (H) form a hydrogen molecule (H2). When hydrogen unites chemically with oxygen, the result is water (H2O), which is compound. A compound, then consists of two or more elements. The molecule is the smallest unit of a compound with the same chemical characteristic. We can have molecules for either elements or compounds. However, atoms exist only for the elements.
5/31/2009
Elements
The combination of electrons and protons forming stable atomic structures result in different kinds of elementary substance having specific characteristics. A few examples are the elements hydrogen, oxygen, carbon, copper, and iron. An element is defined as substance that cannot be decomposed any further by chemical action. The atom is the smallest particle of an element that still has the same characteristics as the element. Atom itself is a Greek word meaning a particle too small to be subdivided. As an example of the fact that atoms are to small to be visible, a particle of carbon the size of a pinpoint contains many billions of atoms. The electrons and protons within the atom are even smaller.
Table lists some more examples of elements. These are just a few out of a total of 106. Notice how the elements are grouped. The metals listed across the top row are all good conductors of electricity. Each has an atomic structure with an unstable outside ring that allows many free electrons.
The semiconductors have 4 electrons in the outermost ring. This means they neither gain or lose electrons but share them with similar atoms. The reason is that 4 is exactly halfway to the stable condition of 8 electrons in the outside ring.
The inert gases have a complete outside ring of 8 electrons, which makes them chemically inactive. Remember that 8 electrons in the outside ring is a stable structure. An example is neon.
Conductors, Insulators, and Semiconductors
When electrons can move easily from atom to atom in a material, it is a conductor. In general, all the metals are good conductors, with silver the best and copper second. Their atomic structure allows free movement of the outermost orbital electrons. Copper wire is generally used for practical conductors because it costs much less than silver. The purpose of using conductors is to allow electric current to flow with minimum opposition.
The wire conductor is used only as a means of delivering current produced by the voltage source to a device that needs the current in order to function. As an example, a bulb lights only when current is made to low through the filament.
A material with atoms in which the electrons tend to stay in their own orbits is an insulator because it cannot conduct electricity very easily. However, the insulators are able to hold or store electricity better than the conductors. An insulating material, such as glass, plastic, rubber, paper, air, or mica, is also called a dielectric, meaning it can store electric charge.
Insulators can be useful when it is necessary to prevent current flow. In addition, for applications requiring the storage of electric charge, as in capacitors, a dielectric material must be used because a good conductor cannot store any charge.
Carbon can considered a semiconductor, conducting less than the metal conductors but more the insulators. In the same group are germanium and silicon, which are commonly used for transistors and other semiconductor components.
5/26/2009
Electrons and Protons in the Atom
Although there are many number of posibble methods by which electrons and protons might be grouped, they assemble in specific combinations that result in a stable arrengement. Each stable combination of electrons and protons makes one particular type of atom. Figure 1 illustrates the electron and proton structure of one atom of the gas hydrogen. This atom consists of a central mass called the nucleus and 1 electron outside. The proton in nucleus makes it the massive and stable part of the atom because a proton is 1840 times heavier than electron.
In figure 1, the 1 electron in the hydrogen atom is shown in an orbital ring around the nucleus. In order to account for the electrical stability of atom, we can consider the electron as spinning around the nucleus, as planets revolve around the sun. Then the electrical force attracting the electrons in toward the nucleus is balanced by the mechanical force outward on the rotating electron. As a result, the electron stays in its orbit around the nucleus.
In an atom that has more electrons and protons than hydrogen, all the protons are in the nucleus, while all the electrons are in one or more outside rings. For example, the carbon atom illustrated figure 2a has 6 protons in two outside rings. The total number of electrons in the outside rings must equal the number of protons in the nucleus in a neutral atom.
The distribution of electrons in the orbital rings determines the atom’s electrical stability. Especially important is the number of electrons in the ring farthest from the nucleus. This outermost ring requires 8 electrons for stability, except when there is only one ring, which has a maximum of 2 electrons.
In the carbon atom in fig. 2a, with 6 electrons, there are just 2 electrons in the first ring because 2 is its maximum number. The remaining 4 electrons are in the second ring, which can have a maximum of 8 electrons.
As another example, the copper atom in Fig. 2b has only 1 electron in the last ring, which can include 8 electrons. Therefore, the outside ring of the copper atom is less stable than the outside ring of the carbon atom.
When there are many atoms close together in a copper wire, the outermost orbital electrons are not sure which atoms they belong to. They can migrate easily from ane atom to another at random. Such electrons that can move freely from one atom to the next are often called free electrons. This freedom accounts for the ability of copper to conduct electricity very easily. It is the movement of free electrons that provides electric current in a metal conductor.
The net effect in the wire itself without any applied voltage, however, is zero because of the random motion of the free electrons. When voltage is applied, it forces all the free electrons to move in the same direction to produce electron flow, which is an electric current.
4/30/2009
Negative and Positive Polarities
We see the effects of electricity in a battery, static charge, lightning, radio, television, and many other applications. What do they all have in common that is electrical in nature? The answer is basic particles of electric charge with opposite polarities. All the materials we know, including solids, liquids, and gases, contain two basic particles of electric charge; the electron and proton. An electron is the smallest amount of electric charge having the characteristic called negative polarity. The proton is a basic particle with positive polarity.
Actually, the negative and positive polarities indicate two opposite characteristic that seem to be fundamental in all physical applications. Just magnets have north and south poles. Electric charges have the opposite polarities labeled negative and positive. The opposing characteristics provide a method of balancing one against the other to explain different physical effects.
It is the arrangement of electrons and protons as basic particles of electricity that determines the electrical characteristic of all substances. As an example, the paper has electrons and protons in it. There is no evidence of electricity, though, because the number of electrons equals the number of protons. In that case the opposite electrical forces cancel, making the paper electrically neutral. The neutral condition means that opposing forces are exactly balanced, without any net effect either way.
When we want to use the electrical forces the associated with the negative and positive charges in all matter, work must be done to separate the electrons and protons. Changing the balance of forces produces evidence of electricity. A battery, for instance, can do electrical work because its chemical energy separates electric charges to produce an excess of protons at its positive terminal. With separate and opposite charges at the two terminals, electric energy can be supplied to a circuit connected to the battery.
4/01/2009
Electricity
Electricity is an invisible force that can produce heat, light, motion, and many other physical effects. The force is an attraction or repulsion between electric charges. More specifically, electricity can be explained in terms of electric charge, current, voltage, and resistance. The corresponding electrical units are the coulomb for measuring charge, the ampere for current, voltage for potential difference, and the ohm for resistance. A basic element of electricity is the electric circuit. A circuit is a closed path that allows for the movement of charges. Current is the name given to the movement of charges. The study of electricity involves the behavior of charges, current, and voltage with the components that make up the circuit.
3/19/2009
Digital electronics
We see the digits 0 to 9 on an electronic calculator or digital watch, but digital electronics has a much broader meaning. The circuits for digital applications operate with pulses of voltage or current, as shown in figure (a). A pulse waveform is either completely ON or OFF because of the sudden changes in amplitude. In between values have no function. Note that the ON and OFF states can also be labeled HIGH or LOW, or 1 and 0 in binary notation, which uses only two digits. Effectively, the digital pulses correspond to the action of switching circuits that are either ON or OFF.
Actually, all the possible variations in types of electronic circuits can be divided into just two types-digital circuits that recognize pulses when they are HIGH or LOW, and analog circuits that use all values in the waveform. The applications of digital electronics, including calculators, computers, processing, and data communications, possibly the biggest branch of electronics. Many other applications, including radio and television, use both analog and digital circuits. Analog-to-digital (A/D) converters in change the signal from one form to the other.
Electric power
These applications are in the generation and distribution of 60-Hz ac power, as the source of energy for electrical equipment. Included are lighting, heating, motors, and generators.
Communications electronics
This field includes AM radio, FM radio with stereo, and television with color. The equipment is divided between transmitters and receivers. Also, transmitters can be divided between radio-frequency equipment to produce the carrier wave radiated from the antenna and the audio and video equipment in the studio that supplies the modulating signal with the desired information. high-fidelity audio equipment can be considered with radio receivers. The receiver itself has audio amplifiers to drive the loudspeakers that reproduce the sound. Satellite communications is also a transmit-receive system using electromagnetic radio waves. The satellite just happens to be orbiting around the earth at a height of above 22,000 miles in order to have a tremendous field of view. Actually, the satellite is a relay station for transmitter and receiver earth stations.
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