8.4 Static Electricity
The static means rest and electricity means charges. The static electricity is the study of charges at rest. When an object is rubbed with another object, the electrons from one object transfer to the other.
The object which transfers the electrons gets positive charge and the object on which electrons are transferred gets negative change.
This charge remains at rest on the objects so it is called static electricity. These charges are transferred when the object get in contact to another object. You can observe static electricity in everyday life.
You have ever noticed that in summer when you touch some metal object you get shock. Why this happens? Actually, when you walk on carpet then due to friction, a charge is generated in your body. When you touch metal object, this charge suddenly moves in metal object and you get a shock.
When we comb our dry hairs, it gets charged due to the friction between hairs and comb. If we bring the comb near pieces of paper, the papers stick with comb. This is due to static electricity.
Static electricity can be seen through the following activity.
Charges and their Properties
The Property of a body that causes it to attract or repeal another body is called a charge.There are two types of charges:
Positive Charge
Negative Charge
The charge has following properties.
Charges can be positive or negative.
Same charges repel each other.
Different charges attract each other.
Different types of electricity
Magnetic Field
Electromagnet
Electromagnetic device
Earth magnetism
Magnetic compass
8.5 Magnetic Field
A magnet is a piece of metal which can attract the other metals. Magnet, attracts usually iron, cobalt and nickel.A magnet has two poles, north pole and south pole. Similar poles (like north pole and north pole) repel each other. Opposite poles attract each others.
The region or space around a magnet in which it pushes or pulls other magnet or metal is known as magnetic field. The magnetic field is stronger at poles (north and south poles) and weaker away from the magnet.
Bar magnet and its lines of forces
A bar magnet is a metallic bar having its own magnetic field. Its magnetic field
(magnetic lines of forces) can be seen by following activity.
Types of Magnet
There are two types of magnet, permanent magnet and temporary magnet. A adjective magnet has its own magnetic field.
The strength of permanent magnet does not change. While,The temporary magnet does not have its own magnetic field. We can change its strength. Example of temporary magnet is electromagnet.
8.6 Electromagnet
The current and magnetism have close relation with each other. When current passes through a conductor, it will become a temporary magnet called electromagnet.
The strength of this magnet can be changed by changing the amount of current flowing through it. If current stops flowing, then magnet will demagnetize. It only remains magnet until the current flows through it. We can make an electromagnet.
8.7 Electromagnetic devices
Many things around us are made up of metals like nickle or iron. Electromagnet can attract the iron and nickel.
Due to this property, electromagnets can be used to perform different works in different appliances. The devices that use electromagnet are electric bell, crane, etc.
Electric Crane
Electric crane is used to lift heavy loads in iron and engine industry. It consists of a crane with electromagnet at load end (lifting end).
When the crane is brought near the object (to be lifted) the current through electromagnet is switched on. The electromagnet gets magnetized and attracts the object.
Then the object is moved to the destination. After reaching the destinations the current is switched off. The electromagnet loses its magnetism. The object in this way is separated from electromagnet.
Electromagnet bell
Electric bell consists of a drum, electromagnet and a hammer. When button is pressed, the current through U-shaped electromagnet starts flowing. This magnet attracts hammer.
The hammer moves and strikes the drum. When hammer moves, the circuit breaks so that after striking the drum, hammer returns to its original position.
When hammer returns to its original position the circuit again is completed. In this way, this process repeated until the button it released.
Reciever of Telephone
The earpiece of Telephone has electromagnet. This electromagnet converts the electric signal into sound.
Electric Motors
Electric motors are used in many appliances like fan, washing machines and water pump etc. In motor, the current flow through electromagnet causes rotation.
8.8 Earth's magnetism
Earth behaves like a magnet. It has two geographical poles. We can prove the earth magnetism through the following activity.
Causes of Earth magnet
The Earth's crust consists of liquid rocks. These rocks contain the metals. These metals exist in liquid from. When Earth rotates around axis, these metals move and cause a generations of current and magnetic field. The strength of Earth magnetic field is stronger at poles)
8.9 Effects of Earth magnetic field
The Earth magnetic field is very useful.
It is used to locate the directions.
It repels the harmful radiations and prevents the life.
Birds (migratory birds) use Earth magnetism to locate the direction.
8.10 Magnetic compass
A compass is a device And It's used to Detect of directions. Because the suspended magnet always pointing in a north-south direction. This property is used in making a magnetic compass.
Compass consists of a small magnet that can move (rotate) freely. This small magnet pivoted on a fixed point and fixed in a circular dial. Four directions are representing on the compass.
It is used to locate the direction. It is also used to detect the magnetic field around a current carrying conductor. The magnetic compass can be made by following Activity.
