Current Electricity

Electric current is different from static electricity and it involves the flow of electric charge. The flow of electrons in solid conductors, and the flow of ions and electrons in liquids constitute the current. To maintain continuous flow of current in a circuit, it is essential to have an electromotive force that can be provided by a cell or a generator.

Electrical Resistance

Conductors such as a metallic wire offer some obstruction when electric current flows through it. This character of conductors to offer obstruction is called its electrical resistance. The resistance (R) of a wire of a given material depends on its length (l) and area of cross-section (a).

R= ρ (l/a); ρ is a constant called the resistivity of the material of the wire.

With increasing temperature, resistivity of a good conductor increases whereas resistivity of a semiconductor decreases.

Electric Cell

In cells, chemical energy is converted into electrical energy. Cells are of two types viz. primary and secondary. Examples of primary cells include torches, radios, etc. The constituents of a dry cell are a negative electrode, a positive electrode and an electrolyte. The negative electrode is made with zinc as it is the outer shell of the cell. A carbon rod surrounded by a mixture of carbon and manganese dioxide acts as the positive electrode. The electrolyte is a mixture of ammonium chloride and zinc chloride in the form of a paste. A dry cell produces about 1.5 volt.
Lead cells which are used for ignition and lighting on motor car are secondary cells. Secondary cells are used as storage cells or accumulators. Due to their low internal resistance, secondary cells are capable of giving large currents. They can be recharged after they discharged. They are used in emergency lights in hospitals and other buildings.

Car Battery

In a car battery, there is a combination of 6 lead-acid secondary cells each with 2.04 volts. These cells use lead plates as electrodes and sulphuric acid as the electrolyte. The car battery provides large currents for a short time as large currents are required to start the engine. After engine started, the alternator provides power to the car.

Effects of Electric Current

Magnetic Effect

When current flows through a wire, a magnetic field is developed around it. If a current-carrying wire is brought near to a bar of iron, it gets magnetised and when the current flow is stopped, the iron bar loses its magnetism. Electromagnets produced in this way have lot of applications. They are used for lifting and transporting steel plates, scrap iron etc. They are also used in electric bells, telephone receivers, etc.

Chemical Effect, Electrolysis

When electric current passes through a solution, it results in decomposition of the solution into negative and positive ions. Positive ions are collected at the negative electrode i.e. cathode and negative ions are collected at the positive electrode i.e. anode. This process is called electrolysis. This process is widely used in electroplating.

Heating Effect

When electric charge flows through a conductor, it results in heating of the conductor i.e. electric energy is converted into heat energy. The heating effect is used in a wide variety of appliances such as geyser, room heater, etc. These appliances use coils of nichrome, which are heated with flow of current. In an electric iron, the heating element is placed between two thin sheets of mica, which is highly insulating and can withstand high temperatures.

Motor Effect

If a current-carrying conductor is placed at right angles to a magnetic field, a force acts on the conductor. When a current-carrying rectangular coil is placed in a magnetic field, the coil starts rotating as a couple acts on the coil. This is the mechanism used in an electric motor i.e. in an electric motor, electric energy is converted into mechanical energy. Electric motors are used in electric fans, washing machines etc. In loudspeakers, energy is transferred from electric current into mechanical energy of vibration.

Electric Generator (Dynamo)

The electric generator works on the principle identical to that of an electric motor. In a generator, the armature is rotated in the magnetic field and an emf is generated in it due to electromagnetic induction. Thus a generator converts mechanical energy into electrical energy. With a minor change in construction, a generator can produce alternating emf or direct emf.  The corresponding currents produced are called alternating Current (ac) and direct Current (dc).

Inverter

An inverter converts DC to AC. The inverters for home and office purpose are designed to convert DC from a battery to AC, and also to charge the battery. If there is a power failure, the inverter automatically switches on the AC, converted from the battery’s DC. After the mains supply is restored, the inverter automatically switches to a mode where it starts charging the battery.

Power Generation and Transmission

Power stations generate electricity of 11 kilovolts (kV) and it is stepped up to 132 kV for transmission to main substations. In a high voltage power transmission, there is little power-loss in the transmission cables. In the main substations, the voltage is stepped down to 33 kV.This voltage is further stepped down and consumers are supplied at 220V. As the voltage is alternating, 220 is the effective value of the voltage.The frequency of a.c. is 50 Hz (cycle per second).

Domestic Electric Installation

Electricity is supplied to houses by using two cables, the “live” cable and the “neutral” cable. A third cable is also used for safety purposes. It is called the “earth” and is connected to the earth terminal provided in the building. From the meter installed in a house, connections are made to the distribution board through a main fuse and a main switch.

Fuse

A fuse is a short piece of wire that has a low melting point. It is generally made of a tin-lead alloy. Fuse melts and breaks whenever there is short circuiting, overloading, voltage fluctuation, etc. This will protect the electrical appliances and also prevents fire accidents.Fuses are always connected in the live wire. Nowadays, miniature circuit breakers (M CBs) are replacing fuses.

A circuit breaker automatically protects an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, a circuit breaker need not be replaced. It needs to be reset manually to resume normal operation.

Earth

The earth wire is used for safeguarding of electrical appliances against shocks.

Flexible Cables

All electrical appliances comes with three-core flexible cables. The cables insulations are coloured red or brown (for live connection), black or light blue (for neutral connection), and green or yellow (for earth connection).

Plugs, Sockets and Switches

In a three pin plug, one pin is longer and thicker, and the other two are similar. The longer pin is used for earthing and it is connected to the green wire of the appliance. The other pins are connected to the red (or brown) and the black (or blue) wires. As the earth pin is longer, an appliance is always first earthed before it is connected to the live circuit. In a socket, the lower right hole is used for the live connection and the left hole is for the neutral connection, and the top bigger hole is for the earth.All switches in a house are connected to the live wires. If they are connected in the neutral wire, the sockets would remain live even if the switches are in off position.

Electric Light

Incandescent Lamp or Filament Lamp

In electric lamps, electrical energy is converted to light energy. In electric lamps, a tungsten filament is connected between two lead-in wires. The tungsten filament is heated with passage of current and emits light.The reason for use of tungsten is it has a high melting point of 3,400°C. The electric lamp also contains a small quantity of argon (an inert gas) to prevent evaporation of tungsten. Air is not used as it would oxidise the tungsten.

Fluorescent Tubes

A fluorescent tube consists of mercury vapours at low pressure. When electricity flows through the tube, the mercury vapours emit invisible ultraviolet rays. These ultraviolet rays fallon the fluorescent coating on the inside of the tube and emit visible light. In a fluorescent tube very little heat is produced, so almost all the electrical energy is converted to light energy. The fluorescent tubes are cheaper and efficient.

Compact Fluorescent Lamps

In incandescent light bulbs, lot of electricity is wasted in the form of heat. A CFL (compact fluorescent lamp) is a miniature fluorescent tube that works 4 to 6 times more efficient than an incandescent bulb. A 15W fluorescent bulb can produce the same amount of light as a 60W incandescent bulb. Mercury used in the fluorescent lamps is a hazardous substance. Most light sources including fluorescent bulbs emit a small amount of UV, but it is far less than the amount produced by natural daylight.

Cost of Electricity

Electricity consumption is measured in the unit kWh. From the power rating of electrical appliances, we can calculate the consumption of electrical energy. By knowing the rate per unit, one can work out the cost of consumption.

Working of Some Electronic Devices

TV Remote Control

A TV or music system remote control contains a chip (an integrated circuit) and other components, such as a diode, a transistor, capacitor, etc. When a key is pressed, remote control translates it into infra-red signals which are received by the electronic circuit in the TV, and the desired operation is performed.

Cordless Phone

Cordless phones are directly plugged into an existing telephone socket that essentially serves as a wireless extension to the existing phone wiring. A cordless phone has two parts viz. a base unit and a hand set. The hand set can communicate with a number of frequencies (channels) in the 46-48 MHz bands. Based on the quality, a cordless phone can permit mobility (range) up to 100 m.