Category : 7th Class
Electricity and Light
All electrical devices such as torches, fans, washing machines, music systems, etc. work on electricity. These devices work when electric current flows through them. The flow of electricity is called current. Current flows through an electric device when voltage is supplied across the device. The amount of current that flows depends on the amount of voltage supplied.
The unit of measurement of voltage is volts (V). Voltage is supplied through electric cells and main electric supply.
An electric circuit is a path through which electric current flows. An electric circuit includes a source of electricity, conductors and a device that uses electricity. The source of electricity can be a cell, the conductor can be a wire and a device that uses electricity can be an electric bulb.
The electric circuit can be diagrammatically represented with the help of a circuit diagram. A circuit diagram represents circuit using symbols for its components.
The electrical resistance of a material is the measure of hindrance, the material creates in the flow of electric current through it. The high the resistance of a material, less the amount of current flows through it.
In addition to material, the resistance also depends on the thickness and length of the material used. A thin wire has a higher resistance than a thick wire. Similarly a long wire has a higher resistance than a short wire.
Material such as metals have low electrical resistance. These materials allow electricity to flow through them and are called good conductors or simply conductors.
Material such as wood, rubber and plastic have high electrical resistance. These materials do not conduct electricity. These are called bad conductors or insulators.
Heating Effect of Current
Heat is produced when current flows through a conductor. The heat produced depends upon the following factors:
Disadvantages of Heat Production
When current passes through a conductor, heat is produced due to the conversion of some amount of electrical energy to heat energy. This conversion of electrical energy to heat energy is a waste of energy. The heat produced can cause damage to electrical components or can cause fires. Fuse is an electrical device used as a safety measure that opens an electric circuit when excessive current flows through it.
Uses of Heating Effect
Electric bulbs work on the heating effect of electrical current. The bulb has a high resistance and high melting point tungsten filament. When current passes through the filament, it becomes hot and starts glowing.
Similarly electric heaters, toasters, etc. use a heating element made of a material called nichrome. This element becomes very hot when current flows through it.
Magnetic Effect of Current
A current carrying conductor behaves like a magnet. It produces a magnetic field around it. A number of devices has been developed to utilize this effect of electricity.
An electromagnet is a magnet produced by passing electric current through a coil that is wound around a soft iron core. The strength of electromagnet depends on the number of turns of wire and amount of current passing through it. The electromagnets are not permanent magnets. The magnetism lasts only during the time for which current passes through its coil.
The uses of electromagnets are:
Light enables us to see objects. We see objects because they reflect light. When the light reflected from an object reaches our eyes, we see the object.
Light travels in a straight line. This is called rectilinear propagation.
When light falls on an object, some portion of it is bounced off or reflected and the rest either passes through or absorbed by the object. Smooth and polished surfaces reflect more light.
Reflection of Light by a Mirror
The angle at which ray of light is reflected from a mirror is dependent upon the angle at which the ray of light falls on it. The ray of light falling on a surface is called the incident ray and the ray of light reflected from the surface is called the reflected ray. The incident ray and reflected ray make equal angles with a normal ray i.e. \[\angle i=\angle r\]
There are mainly two types of curved surfaces. If the reflecting surface is on the inside then such a mirror is called a concave mirror and if the reflecting surface is on outside, it is called a convex mirror.
Refraction of Light
When light travels from one medium to another, the rays of light bends, this is called refraction of light.
A lens is a piece of transparent glass bound by two spherical surfaces. Lenses are two types:
Convex lens: The lens which is thicker in the middle than at the edges. It is a converging lens.
Concave lens: The lens which is thinner in the middle than at the edges. It is a diverging lens.
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