DOMESTIC ELECTRIC CIRCUITS
In our homes, we receive supply of electric power through a main supply which is also called mains. The various other features of domestic electric currents are :
Schematic diagram of common domestic circuits
FEATURE OF DOMESTIC CIRCUIT:
Electric cable or overhead wires
The electric power to a house is supplied either through overhead wires or through underground cables. The cable has three separate insulated wires:
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Live wire (or phase or positive)
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Neutral wire (or negative) and
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Earth wire.
The live wire has usually red insulation cover, neutral wire has black insulation cover and the earth wire has green insulation cover. As per the new International Convention, live wire has brown coloured insulation cover whereas neutral and earth wires have light blue and green (or yellow) insulation covers. The potential difference between the live and neutral wire is 220 V. The neutral and the earth wires are connected together at the local sub-station so that both of them are at zero potential.
Pole Fuse:
Before the electric lines enter a house, the agency supplying electricity, places a fuse (called the pole fuse or company fuse) in the live wire. The current rating of this fuse depends upon the load sanctioned by the agency to that house.
Energy Meter or kWh Meter:
After the company fuse, the cable is connected to the energy meter, which records the electricity consumption of the house in kilowatt, hour (kWh). The earth wire from the meter is locally earthed in the compound of the house.
Distribution Board:
Power lines coming from the electricity meter are taken to the distribution board. It is from the distribution board that the wires go to the different parts of the house through fuses in the board.
WHY IS SERIES ARRANGEMENT NOT USED FOR DOMESTIC CIRCUITS?
In domestic circuits, series arrangement is not used because of the following reasons:
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The total potential difference available (usually 220 volts) is divided between various appliances in the circuit according to their resistances since the current flowing through all the appliances is the same. Thus, each appliance will not get the required potential difference for it to operate properly.
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if one of the appliances is out of order, e.g., if a bulb gets fused or if we switch off one of the appliances all the appliances in the circuit will stop working, as the circuit gets broken.
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All the appliances will work simultaneously whether we want them to work or not, thereby involving a lot of power wastage.
SHORT-CIRCUITING:
In general, short-circuiting occurs when the ends of a circuit are connected by a conductor of very low resistance as compared to that of the circuit. In household connections, short-circuiting occurs when the live (positive) wire and the neutral (negative) wire come in direct contact with each other.
Reasons of short-circuiting
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Damage to the insulation of the power-lines
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A fault in an electric appliance due to which current does not pass through it.
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Over loading
Consequences of short-circuiting
On account of short-circuiting, resistance of the circuit decreases to a very small value and consequently, the current becomes very large. This large current results in heating of live wires, which produces sparking at the point of short-circuiting. This sparking sometimes causes fire in a building. (Apart from short-circuiting, the increase in current in the circuit and consequent heating may also be due to overloading of the circuit).
ELECTRIC FUSE: A SAFETY DEVICE:
An electric fuse is a device, which is used in series to limit the current in an electric circuit so that it easily melts due to overheating when excessive current passes through it. A fuse is a wire of a material with very low melting point.
A fuse is a wire made of an alloy of lead (75%) and tin (25%), which melts at around 200ºC (low melting point). Electric fuse can avoid incidents like electric shock, fire, damage to an electric appliance due to:
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Short-circuiting or
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Overloading (withdrawing current beyond a specified limit) in a circuit.
When a heavy current flows through the circuit, the fuse wire gets heated and melts. Consequently, the circuit is broken and the current stops flowing in it. A few important points regarding a fuse are as follows :
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In household supply, a fuse is always connected in live wire and not in the neutral wire under any circumstance. Though it will melt even when connected with neutral wire, the electric appliance will continue to be in contact with the live wire. Thus, when the electric appliance is touched, it will give shock.
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A fuse is always connected in the beginning of the circuit before any appliance is connected. This is done to protect the appliance from getting damaged.
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Fuses of various current capacities are available. Remember that thicker the fuse wire, the greater is its current capacity.
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A fuse used must be of current capacity (also called current rating) less than the maximum current which a circuit or an appliance can withstand. A fuse of current capacity of 5 A is put in a line meant to supply power to lights (i.e., bulbs) and fans whereas a fuse of 15 A current capacity is meant for a line which operates an electric heater or a geyser, etc.
EARTHING:
Many electric appliances of daily use like electric press, toaster, refrigerator, table fan etc. have a metallic body. If the insulation of any of these appliances melts and makes contact with the metallic casing, the person touching it is likely to receive a severe electric shock. This is due to the reason that the metallic casing will be at the same potential as the applied one. Obviously, the electric current will flow through the body of the person who touches the appliance. To avoid such serious accidents, the metal casing of the electric appliance is earthed. Since the earth does not offer any resistance, the current flows to the earth through the earth wire instead of flowing through the body of the person. Moreover, due to very low resistance (almost nil) offered by the earth wire, the current in the circuit rises to a very high value, thereby melting fuse in that circuit and cutting off its electric supply.
ROLE OF MAGNETISM IN MEDICINE AND ORGANISMS:
In our body, small electric current travels along the nerve cells due to ions. This current produces a very weak magnetic field (about one billionth time weaker than the Earth's magnetic field) in our body. Heart and brain are the two main organs in our body where this magnetic field is quite significant. The magnetic field in our body enables us to obtain the images of its different parts by using a technique called MRI (Magnetic Resonance Imaging). On analysing the images obtained through MRI, we are able to make a medical diagnosis, e.g., location and size of a tumour in brain etc. Thus, magnetism plays an important role in modern medical science.
Apart from this, there are certain organisms, which have the ability to sense Earth's magnetic field and travel from one place to another. For example, some type of fishes are able to detect magnetic field by using special receptors whereas in certain organisms, crystals of magnetite enable to move along the Earth's magnetic field.
