Defence Exam Notes: Electricity

Electricity has a significant place in today's society. For a variety of uses in homes, schools, hospitals, industries and so on, it is a controllable and convenient form of energy.

Electricity is related to magnetism, both being part of the electromagnetism phenomenon as Maxwell's equations explain it. Various other phenomena are related to electricity, including lightning, static electricity, electric heating, electrical discharges.

An electric charge produces an electric field, which can be positive or negative. The electric charging action is an electrical current and generates a magnetic field.

ELECTRIC CURRENT AND CIRCUIT

If electrical charge flows through a conductor (for example, through a metal wire), we say the conductor has electrical current. In a torch, we know that the cells (or a battery, when properly placed) provide a flickering flow of charges or an electric current through the torch bulb. We have seen that the torch gives light only when its switch is on. A switch establishes a conducting link between the cell and the bulb. An electrical circuit is called a continuous and closed path for passage of an electric current.

Electric current is expressed in unit time by the amount of charge that flows through a given area. In other words, it is the rate of flow of electric charges is known as electric current. Electrons represent the flow of charges in circuits requiring metallic wires. At the time of first observation of the electricity phenomenon, however, electrons were not known. Thus, the flow of positive charges was considered to be electrical current and the direction of flow of positive charges was taken as the direction of electrical current. In an electric circuit, conventionally, the direction of electrical current is taken as opposed to the direction of electrons flow, which is negative charges.

In a circuit, an instrument called an ammeter tracks electrical current. In a circuit in which the current is to be measured it is always tied in series.

ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE

The gravity, of course, has no part to play in the movement of charges in a conducting metallic wire; the electrons travel only if there is an electrical pressure difference which is called the potential difference along the conductor. A battery, consisting of one or more electric cells, can produce that potential difference. The chemical action within a cell produces the potential difference across the cell's terminals, even if no current is drawn from it. When the cell is connected to a conductive circuit element, the potential difference sets the conductive charges in motion and produces an electrical current. To maintain the current in a given electric circuit, the cell must spend its stored chemical energy in it.

In an electric circuit carrying some current, we describe the electrical potential difference between two points as the work performed to transfer a unit charge from one point to another.

OHM’S LAW

In 1827, a German physicist Georg Simon Ohm (1787–1854) discovered the relationship between the current I, flowing across terminals in a metal wire and its potential difference. The potential difference, V, is directly proportional to the current that flows through it through the ends of a given metallic wire in an electric circuit, provided its temperature remains the same. This is known as Ohm’s law. In other words – V ∝ I

Or V/I = constant

V/I = R

Or V = IR

R is a constant at a given temperature for the metallic wire given, and is called its resistance. It is a conductor 's property to withstand the movement of charges through it. Its SI unit is ohm, represented by the Greek letter Ω. According to Ohm’s law,

R = V/I

If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω. That is,

1 ohm = 1 Volt/1 ampere

Some materials provide an easy path for flowing electric current while the others hinder the flow. In an electrical circuit, the motion of electrons constitutes an electric current. However, the electrons inside a conductor are not completely free to move. They are constrained by the attraction of the atoms they pass through. Thus, its resistance retards the movement of electrons through a conductor. A good conductor is a component of a given size which offers low resistance. A conductor whose resistance is appreciable is called a resistor. A poor conductor is a part of equal size that offers greater resistance. A similar-size insulator offers even greater resistance.

HEATING EFFECT OF ELECTRIC CURRENT

A battery or a cell is an electric power source. The chemical reaction inside the cell generates the potential difference between its two terminals, which sets the electrons in motion to flow the current through a resistor or resistor system connected to the batteries. The source has to keep expending its energy to keep up the current. A part of the source energy in keeping the current can be consumed into useful work (such as rotating an electric fan's blades). Rest of the source energy can be expended in heat to increase the gadget 's temperature. On the other hand, if the electrical circuit is purely resistive, that is, a configuration of resistors only connected to a battery; in the form of heat, the source energy is continually dissipated altogether. That is known as the electric current heating effect. This effect is used in devices like electric heaters, electric iron etc. This is called the heating law of Joule. The law implies that heat produced in a resistor is

directly proportional to the square of current for a given resistance,
directly proportional to resistance for a given current, and
directly proportional to the time for which the current flows through the resistor.

Practical Applications of Heating Effect of Electric Current

In a conductor heat generation is an obvious result of electric current. It is undesirable in many cases, since it converts useful electrical energy into heat. The inevitable heating in electric circuits will increase the temperature of the components and alter their properties. Electric current heating effect has a lot of useful applications though. Some of the familiar Joule-based heating devices are the electric laundry iron, electric toaster, electric oven, electric kettle, and electric heater. As in an electric bulb, the electric heating is often used to produce electricity. The filament here has to retain as much of the heat generated as possible, so it gets very hot and emits light. It must not melt at such an elevated temperature. For the production of bulb filaments, a strong metal with a high melting point such as tungsten (melting point 3380 ° C) is used. The filament should be thermally shielded as much as possible, using the insulating support, etc. In general, the bulbs are filled with chemically inactive nitrogen and argon gases to extend filament life. Most of the power consumed by the filament appears as heat, but a small fraction of it is radiated in the form of light.

Another popular form of heating by Joule is the fuse that is used in electric circuits. By preventing the flow of any unduly high electric current it protects circuits and appliances. The fuse and the device are placed in series. It consists of a piece of wire made of a metal or an alloy with a sufficient melting point, such as aluminium , copper, iron, lead etc. Where a current greater than the specified value flows through the circuit, the fuse wire temperature increases. This melts the fuse wire and the circuit breaks. The fuse wire is normally enclosed with metal ends in a porcelain case, or similar material. Domestic-use fuses are marked as 1 A, 2 A, 3 A, 5 A, 10 A, etc. A current of (1000/220) A, i.e. 4.54 A, will flow in the circuit for an electric iron which consumes 1 kW of electric power when operated at 220 V. In this case, the use of a 5 A fuse is necessary.

ELECTRIC POWER

The rate of doing work is power. This is also the rate of consumption of energy. The SI unit of electric power is watt (W). It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. Thus,

1 W = 1 volt × 1 ampere = 1 V A

The 'watt' unit is fairly small. Therefore we use a much larger unit named 'kilowatt' in actual practice. It is 1000 watts equivalent. As electrical energy is the combination of power and time, hence the unit of electrical energy is watt-hour (W h). One-watt hour is the energy consumed when it uses 1 watt of power for 1 hour. The commercial unit of electric energy is kilowatt-hour (kW h), commonly known as ‘unit’.

1 kW h = 1000 watt × 3600 second

= 3.6 × 106 watt second

= 3.6 × 106 joule (J)