ELECTROMOTIVE FORCE (EMF)

Current needs a “push” to get its flow started. In a battery, this push is produced by the difference in charge at the two opposing terminals. The electrons’ urge to flow from the negative terminal to the positive terminal will get a current started. Whenever a positive charge and a negative charge are positioned close to each other, electromotive force exists. Electromotive force, often abbreviated EMF, is a force that tends to move electricity.

The term potential is also used to describe EMF. That is, in a battery, the potential or ability to generate current is present. All you have to do is connect wires to the battery to give  the electrons a path to flow on.

          When free electrons are dislodged from atoms, electrical energy is released. Chemical reaction, friction heat and electromagnetic induction will cause electrons to move from one atom to another. Whenever energy in any form  is released, a force called electromotive (e. m. f.) is developed.

         If the force exerts its effort always in one direction, it is called direct; and if the force changes its direction of exertion periodically, it is called alternating. The chemical reaction in a dry cell, heat and friction are sources of a unidirectional force. Electromagnetic induction produces an alternating force. The direction of force depends on the direction in which the field is cut. Whenever an e. m. f. is developed, there is also a field of energy called an electrostatic field, which can be detected by an electroscope and measured by an electrometer.

ELECTROMOTIVE FORCE AND RESISTANCE

      The electromotive force is the very force that moves the electrons from one point in an electric circuit towards another. In case this e. m. f. is direct, the current is direct. On the other hand, were the electromotive force alternating, the current would be alternating, too. The e. m. f. is measurable and it is the volt that is the unit used for measuring it. A current is unable to flow in a circuit consisting of metallic wires alone. A source of an e. m. f. should be provided as well. The source under consideration may be a cell or a battery, a generator, a thermocouple or a photocell, etc.

      In addition to the electromotive force and the potential difference reference should be made to another important factor that greatly influences electrical flow, namely, resistance. All substances offer a certain amount of opposition, that is to say resistance, to the passage of current. This resistance may be high or low depending on the type of circuit and the material employed. Glass and rubber offer a very high resistance and, hence, they are considered as good

insulators. All substances do allow the passage of some current provided the

potential difference is high enough.

      Certain factors can greatly influence the resistance of an electric circuit. They are the size of the wire, its length, and type. In short, the thinner or longer the wire, the greater is the resistance offered.

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Electromotive force

What is Electromotive Force?

Ohm's Law

Ohm’s law states that the current I flowing in the circuit is proportional to the applied potential difference V. The constant of proportionality is defined as the resistance R. If V and I are measured in volts and amperes, respectively, R is measured in ohms. Microscopically resistance is associated with the impedance to flow of charge carries offered by the material. For example, in metallic conductor the charge carries are electrons moving in a polycrystalline material in which their journey is impeded by collisions with imperfections in the local crystal lattice, such as impurity atoms, vacancies, and dislocations. In these collisions the carries lose energy to the crystal lattice, and thus Joule heat is liberated in the conductor, which rises in temperature.

One of Ohm's major contributions was the establishment of a definite relationship between voltage, resistance and current in a closed circuit. A circuit consists of a voltage source and a complete path for current. Ohm stated this relationship as follows: Current is directly proportional to voltage and inversely proportional to resistance.

      As a formula, it appeals like this:

                                           Voltage (in volts)

     Current (in amperes)

                                          Resistance (in ohms)

 This formula is commonly known as Ohm's Law.

     About 1817 Ohm discovered that a simple correlation exists between resistance, current and voltage. That is: the current that flows in the circuit is directly proportional to the voltage and inversely proportional to the resistance. A current is measured in amperes, a voltage, or potential difference is measured in volts. A resistance is measured in ohms.

RESISTANCE

  

Resistance is the opposition that a circuit or substance creates against the flow of electri-cal current. Referring to our water system example again, you can compare resistance to the diameter of the pipe. The narrower the pipe, the less water  that can flow through the pipe.

The only way  to get more water to flow through a narrow pipe would be to increase the water pressure. In the same way, a very thin wire offers more resistance to the flow of electricity than does a thick wire. The only way to increase the flow of electricity in a thin wire would be to apply more voltage to it.

Every electrical circuit contains at least some resistance. This is because no substance is a perfect conductor. For example, silver metal is an excellent conductor, but it’s not perfect — it will always put up some resistance to the flow of electricity through it.

The magnitude of electrical resistance in a circuit is measured in ohms. One ohm is equal to the resistance of a circuit in which one volt of EMF is applied to produce one ampere of  current. The abbreviation for the ohm is the Greek symbol omega.

CONDUCTORS, RESISTORS, AND INSULATORS

One of the first scientists to observe  the flow of electrical current  through various materials was Georg Ohm. In the early 1800s, Ohm performed experiments with electric current. He found that when current was applied to different materials, some materials carried the current readily and others didn’t. He found that different materials put up different degrees of resistance to the flow of electricity.

For example, he noticed that silver and copper readily permitted the passage of almost all of the applied electricity; therefore, he called these metals conductors. Some materials carried almost no electricity, even when a very high voltage was applied. Ohm called these materials insulators. Examples of insulators are glass, mica, porcelain, paper, plastic, and rubber. Other materials passed some (but not all) of the current, so these were called resistors. Carbon is an example of a resistor.

Using your knowledge about the structure of atoms, you can now explain the results of Ohm’s experiments. You learned that some atoms are constructed with free electrons in  their outermost orbits. A material in which electrons can be moved easily from one atom  to another by an outside force is a good conductor of electricity.

In comparison, other materials are made of atoms in which the electrons are very tightly  bound to their orbits.  In these atoms, it’s very difficult to remove electrons from their orbits, so the material is a poor conductor of electricity. If the electrons in an atom can’t  be moved from their orbits at all, the material is an insulator. If at least some of the electrons can be moved, the material is a resistor.

FIXED RESISTORS

We’ve already discussed how some materials resist  electrical flow. Now, we’ll examine how resistive materials can be used to make resistors. Resistors are devices that resist or oppose the flow of electrons through a circuit. Resistors are commonly used to protect circuits and control the flow of electricity through them. However, a resistor doesn’t completely prevent the flow of  electricity. This principle has many practical applications in industry.

The most common type of resistor is the carbon resistor, also called a fixed resistor.  A carbon resistor has a body made of resistive carbon surrounded by a composite sealing material. A stiff wire lead protrudes out of each end of the resistor. A series of colored bands mark the resistor. The color bands identify the resistor’s value as measured in ohms.

Most carbon resistors tend to have a very high resistance to electrical current. While carbon resistor may have a value as small as one-tenth of an ohm, values in the hundreds,  thousands, or even millions of ohms are more common. Carbon resistors are used in electronic circuits to control the flow of current, or to lower the voltage delivered to an electronic device (such as an integrated circuit).

Another type of resistor is the wirewound resistor. A wirewound resistor consists of a porcelain or ceramic tube that’s wrapped with a length of wire. Nichrome, a  type of wire that’s a poor conductor, is often used.

Wirewound resistors generally have lower resistance values than carbon resistors. Typical wirewound resistors have resistance values of tens, hundreds, or thousands of  ohms. The resistance value is largely determined by the length and thickness of the wire  used in the resistor, and how well the wire conducts electricity. These resistors are often used in the high-power circuits found in heaters and DC motor controls.

ELECTRICAL UNITS

Before going further with our study of electricity, it may be helpful to look at some of the standard units, notations, and  symbols used in electrical work. In the fields of electricity and electronics, a variety of terms are used to describe quantities in electrical compo-nents and circuits. Some of these units are volts, amps, coulombs, ohms, and watts. All these terms are listed in Table 3.

SERIES AND PARALLEL CIRCUITS

Electrical components can be connected together in two basic ways: in series or in parallel. In both types of circuits, several electrical components are connected together. In a series circuit, the current has only one electrica lpath to follow. The current flows through each of the components, one after another, in a straight line, and remains the same through all of the series components.

In contrast, in a parallel circuit, the current has two or more electrical paths to follow. The electrons flow from the negative battery terminal, branch out into the two or more electrical paths, flow through all the components in the circuit, and return to the positive battery terminal. The current “splits” among the parallel components, and adds together to form  the total current flowing through  the parallel combination.

When a string of lights is connected in a series arrangement, if one light bulb is removed or burns out, the entire string will fail to light.

When light bulbs are connected in a parallel arrangement, if one light bulb is removed or burns out, the

CIRCUIT DRAWINGS

In this section, we’ll examine and compare several different types of circuits.  In order to do this, you’ll need to examine a number of schematic diagrams. Schematic diagrams (or simply, “schematics”) are simplified drawings of electrical circuits that show all the electrical components of a circuit including the power source, conductors, loads, switches, resistors, and so on.

In your study of electricity and in actual electrical work, schematic diagrams are very  important. All electrical or electronic components are identified on schematic diagrams by standard symbols. You should try to memorize these symbols so that you can recognize them whenever you see them.

Political System of Great Britain

Great Britain is a parliamentary monarchy. The King is formally at the head of the state. Everything is done in Ring’s name. But in fact the country is ruled by the Parliament consisting of two chambers (houses): the House of Commons and the House of Lords. The House of Commons is the lower House and the House of Lords is the upper one. The Prime-Minister is head of the Government.

The members of the House of Lords are not elected by the population. The head of the House of Lords is the Lord Chancellor. Near the throne of Lord Chancellor stands a special seat called the Woolsack. The Woolsack symbolizes the wealth of Great Britain.This tradition dates back to the time when Great Britain became rich for its textile and wool industry.

The members of the House of Commons are elected for a period of 5 years. The House of Commons makes the laws for the country. The members of the House of Commons belong to the different political parties and the Government of the country is formed by the party which has the greatest number of members elected to Parliament.

The United Kingdom is a constitutional monarchy. That means it has a monarch at the head of the state. But the King’s power is not absolute. The monarch reigns with the support of Parliament. The was the saying "The Queen reigns but doesn’t rule". In practice, legislation is dominated by the Prime Minister and the Cabi­net, who initiate virtually all proposed bills and who are politically re­sponsible for the administration of the law and the affairs of the na­tion. The power of the Crown to veto legislation has not been exercised in over 280 years.

When the Parliament begins its work the Ring makes a speech in the House of Lords. This ceremony is significant and picturesque.