Current Electricity And Magnetism
The rate of flow of electrons or charges through an electric circuit in per unit time is called current electricity. Its SI unit is Ampere (A).
If one coulomb charge (6.67×1018 electrons) flow through a point of a circuit or conductor in unit time, is called one ampere current.
So, 1A = 1C/1S
Direct current (D.C.):
If the polarity (magnitude and direction) of current does not change with time, is called direct current. In D.C., current always flows in the same direction. Dry cell, Battery and DC genetors produce the direct current.
Alternating current (AC):
If the polarity (magnitude and direction) of current changes with time, is called alternating current. Generator or dynamo produces the alternating current.
Effects of current electricity:
Electrical encrgy can be transformed into different forms of energy. This is called effects of current electricity. For example:- Heating effect, Lighting effect, Chemical effect, Magnetic eflect, etc.
When an electric current is passed through some electrical appliances having high resistance wire, they change electrical energy into heat energy. This is called the heating effect of an electric current. Electric iron, immersion rod, kettle, etc. are some such electrical appliances.
The wire used in the electrical heating appliances to convert electrical energy into heat energy is called heating element. It has high meeting point and resistivity.
Nichrome wire is usually used as a heating element. Nichrome is an alloy of nickel (60%) and chromium (40%) metals.
Characteristics of nichrome wire:
i) Its resistance is very high and it produces more heat in comparatively short time.
ii) This docs not react with oxygen of air even at a temperature of 900C. So, heating element made up of nichrome is more durable than others.
When an electric current is passed through some electrical appliances having high resistance wire, they change electrical energy into light energy. This is called the lighting effect of an electric current. Lighting effect of an electric bulb is used with the help of filament lamp, fluorescent lamps and other lighting devices.
A coil made up of thin wire is used inside the electric bulb. It is called filament. Filament is generally made of tungsten metal.
Causes of using tungsten metal as a filament:
i) Tungsten has high resistance.
ii) It has a high melting point i.e. 3400° C.
iii) As electricity passes through the filament, it becomes very hot at nearly 2900’C and radiates bright light.
Full Form of C.F.L.:
The full form of CFL is Compact Fluorescent Lamp.
Magnetic effect of current:
When an electric current is passed through a conductor, it behaves like a magnet. This is called magnetic effect of electric current.
The temporary magnet which is made by using electric current is called electromagnet.
Characteristics of an electromagnet:
i) It is a temporary magnel.
ii) We can change its shape and size according to the requirement.
iii) We can change its strength according to the requirement.
The strength of electromagnet can be increased by:
i) Increasing the number of turns of insulated wire around the magnetic material.
ii) Increasing the magnitude of current through the insulated wire.
iii) Inserting a soft iron core in the solenoid increase the strength of the magnet.
The cylindrical form of insulated copper coil having a large number of closed turms use make electromagnet is called solenoid.
Uses of electromagnet:
i) Electric magnets are widely used in electrical appliances such as Radio, TV, loudspeaker and electric bell.
ii) They are used in industries for lifting and transporting steel plates, scrap iron, girders etc.
iii) They are used in separation of magnetic bodies from garbage.
The phenomenon in which current is induced by the change in magnetic flux by a conducting wire is called electromagnetic induction.
The total magnetic lines of force passing perpendicularly from an area is called magnetic flux.
Generator or Dynamo:
Machine used to convert mechanical energy (kinetic energy) in to electrical energy are called generators or dynamos. They are based on the principle of electromagnetic induction. Usually dynamo is used to induce current in less amount and generator is used to induce large scale.
In a generator/dynamo, nmagnitude of induced e.m.f. can be inereased by the following methods:
i) By increasing the number of turns in the coil.
ii) By increasing the strength of magnetic field.
iii) By winding coil in a more solt iron core.
iv) By increasing the speed of dynamo i.e. speeds of rotation of the magnet near the coil.
v) By decreasing the distance between the coil and the magnet.
When current flows through a wire placed in a magnetic field, a movement is developed in the wire. This movement is called motor effect.
The machine used to convert electrical energy into mechanical energy (kinetic energy) is called electric motor. It works on the principle of motor effect. It is used in domestic appliances such as electric fans, water pumps, retrigerators, washing machines, vacuum cleaners, etc.
When an altemating e.m.f. is applied to the primary coil of a rectangular seft iron core made of laminated sheets, a changing current flowing in it produces an alternating magnetic flux in it. This causes to change magnetic flux linked with the secondary coil. It is called principle of mutual induction.
A transformer is a device to convert low A.C. voltage in to high A.C. voltage and vice-versa. A device used for incrcasing or decreasing the magnitude of e.m.f. or voltage is called transformed. Transformer is based on the principle of mutual induction or electromagnetic induction.
It is based on the following two principles:
i) In transformer, input energy is equal to output energy.
ii) Voltage is directly proportional to the number of turns of wire.
Types of transformer:
(a) Step-up transformer:
The transformer which converts low AC voltage to high AC voltage is called step-up transformer. It increases the magnitude of alternating e.m.f.. It is used in power house.
(b) Step-down transformer:
The transformer which converts high AC voltage to low AC voltage is called step-down transformer. It decreases the magnitude of alternating e.m.f.
Uses of transformer:
Transformers are used in voltage regulntors for computers, television, air conditioner, record player, trolley buses, etc. They are used for doorbells, welding purpose and in electrical furnaces. The most important use of the transformer is to transmit alternating current over long distances.
Safety measures in using electricity:
i) Use wires of high quality, proper amperage and good insulating material.
ii) Cover all naked wires and joints with insulating tape.
iii) All connections at plugs, switches, sackets must be tight.
iv) Replace any defective plugs, switches and sockets.
v) Never touch any part of the circuit without putting on rubber shoes or rubber gloves.
vi) Use fuse (now-a- days MCB) of proper rating and material.
vii) All electrical appliances must be properly carthed.
viii) Connect switches and fuse to live wires.
ix) Put out the main switch in case of short circuiting and fire.
x) Do not use water as fire extinguishers in case of fire due to electricity.
A.C. circuit is used in the places like industries, factories, houses, etc, is called a domestic circuit.
Earthing is the process of connecting the metallic trody of an electric appliance to the earth through a conducting wire.
The energy consumed by an electric appliance of po wer one kilowatt for one hour is called one kilowatt-hour. It is custamarily called unit only.
‘1kWh -1 unit.’
Short-circuiting is the condition in which the neutral wire and phase wire come in contact to each other directly. The total resistance in the circuit becomes low and a very large current flows in the eircuit. This is known as short-circuiting.
Fuse is a safety measure to protect a circuit firom excessive heating, It is a short m tal wire, having low melting point, made of an alloy of tin (63%) and lead (37%). It is connected in series with the circuit.
Function of fuse:
Fuse saves the circuit from a possible risk, during over loading and short-circuiting.
Miniature Circuit Breaker (MCB):
An automatic switch that stops the flow of electric current in a suddenly overloaded or otherwise abnormally stressed electric circuit. A circuit breaker is an automatically operated electrical switch designed to protect and electrical circuit damage caused by overload or short circuit.