Magnetism Online Test 8th Science Lesson 16 Questions in English

Explanation

Magnets are objects of stone, metal or other material which have the property of attracting metals like iron, cobalt and nickel.

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The attracting property of a magnet is called magnetism and it is either natural or induced. The branch of physics which deals with the property of a magnet is also called magnetism.

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The earliest evidence for magnets are found in a region of Asia Minor called Magnesia. It is believed that the Chinese had known the property of magnet even before 200 B.C. They used a magnetic compass for navigation in 1200 A.D.

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Today magnets play an important role in our lives. Magnets are used in refrigerators, computers, car engines, elevators and many other devices.

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Magnets are classified into two types. They are: i. Natural magnets ii. Artificial magnets

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Magnets found in the nature are called natural magnets. They are permanent magnets they will never lose their magnetic power. These magnets are found in different places of the earth in the sandy deposits.

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Lodestone called magnetite (Iron oxide) which is the ore of iron is the strongest natural magnet. Minerals like Pyrrhotite (Iron Sulphide), Ferrite and Coulumbite are also natural magnets.

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There are three types of iron ores. They are Hematite (69% of Iron), Magnetite (72.4% of Iron) and Siderite (48.2% of Iron).

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Magnetite is an oxide ore of iron with the formula Fe3O4. Among these ores, Magnetite has more magnetic property.

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Magnets that are made by people in the laboratory or factory are called artificial magnets. These are also known as man-made magnets, which are stronger than the natural magnets.

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Artificial magnets can be made in various shapes and dimensions. Bar magnets, U-shaped magnets, horseshoe magnets, cylindrical magnets, disc magnets, ring magnets and electromagnets are some examples of artificial magnets.

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Artificial magnets are usually made up of iron, nickel, cobalt, steel, etc. Alloy of the metals Neodynium and Samarium are also used to make artificial magnets.

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Explanation

William Gilbert laid the foundation for magnetism and suggested that the Earth has a giant bar magnet. William Gilbert was born on 24th May 1544. He was the first man who performed the systematic research on the properties of the lodestone (magnetic iron ore) and published his findings in the influential ‘De Magnete’ (The Magnet).

Explanation

The properties of a magnet can be explained under the following headings.

• Attractive property
• Reflective property
• Directive property

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The ends are called poles of a magnet. This shows that the attractive property of a magnet is more at the poles.

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Magnetic poles always exist in pairs. Each broken piece behaves like a separate bar magnet.

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When a magnet is split vertically, the length of the magnet is altered and each piece acts as a magnet. When a magnet is split horizontally the length of the new pieces of magnet remains unaltered and there is no change in their polarity. In both cases the strength of the magnet is reduced.

Explanation

The magnetic poles repel each other that are a north pole repels another north pole and a south pole repels another south pole. If you bring the south pole of the magnet close to the north pole of the suspended magnet you can see that the south pole of the suspended magnet is immediately attracted.

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Unlike poles of a magnet attract each other than the North Pole and the South Pole of a magnet attract each other.

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A freely suspended bar magnet always aligns itself in the geographic north-south direction. The property of a magnet, by which it aligns itself along the geographic north-south direction, when it is freely suspended, is known as the directive property of a magnet.

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The space around the bar magnet where the arrangement of iron filings exists, represents the field of influence of the bar magnet. It is called the magnetic field. Magnetic field is defined as the space around a magnet in which its magnetic effect or influence is observed.

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Magnetic field is measured by the unit tesla or gauss (1 tesla =10,000 gauss).

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The compass needle gets deflected to a large extent, when it is closer to the magnet. When the distance is large, the deflection of the needle is gradually decreased.

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At one particular position there is no deflection because there is no magnetic force at this position. This shows that each magnet exhibits its magnetic influence around its specific region.

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A compass needle, also known as plotting compass or magnetic needle, consists of a tiny pivoted magnet in the form of a pointer, which can rotate freely in the horizontal plane. The ends of the compass needle point approximately towards the geographic north and south direction.

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Materials which are attracted by magnets are called magnetic materials and those materials which are not attracted by magnets are called non-magnetic materials. There are a number of materials that can be attracted by magnets. These can be magnetized to create permanent magnets.

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Magnetic materials can be categorized as magnetically hard or magnetically soft materials. Magnetically soft materials are easily magnetized.

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Magnetically hard materials also can be magnetized but they require a strong magnetic field to be magnetized. It is because materials have different atomic structure and they behave differently when they are placed in a magnetic field.

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Based on their behavior in a magnetic field they can be classified as below.

• Diamagnetic
• Paramagnetic
• Ferromagnetic

Explanation

Diamagnetic materials have the following characteristics.

• When suspended in an external uniform magnetic field they will align themselves perpendicular to the direction of the magnetic field.
• They have a tendency to move away from the stronger part to the weaker part when suspended in a non-uniform magnetic field.
• They get magnetized in a direction opposite to the magnetic field.
• Magnetic character of these substances is not affected by the external temperature.

Explanation

Examples for diamagnetic substances are bismuth, copper, mercury, gold, water, alcohol, air and hydrogen.

Explanation

The following are the characteristics of paramagnetic materials.

• When suspended in an external uniform magnetic field they will align themselves parallel to the direction of the magnetic field.
• They have a tendency to move from the weaker part to the stronger part when suspended in a non-uniform magnetic field.
• They get magnetized in the direction of the field.
• Examples for paramagnetic substances are aluminium, platinum, chromium, oxygen, manganese, solutions of salts of nickel and iron.
• Magnetic character of these substances is affected by the external temperature.

Explanation

The characteristics of ferromagnetic materials are given below.

• When suspended in an external uniform magnetic field they will align themselves parallel to the direction of the magnetic field.
• It has a tendency to move quickly from the weaker part to the stronger part when suspended in a non-uniform magnetic field.
• They get strongly magnetized in the direction of the field.
• Examples for ferromagnetic substances are iron, cobalt, nickel, steel and their alloys.
• Magnetic character of these substances is affected by the external temperature. When they are heated they become Para magnetic.

Explanation

The temperature at which the ferromagnetic material becomes paramagnetic is called the Curie temperature.

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Artificial magnets are produced from magnetic materials. These are generally made by magnetizing iron or steel alloys electrically. These magnets are also produced by stroking a magnetic material with magnetite or with other artificial magnets.

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Depending on their ability to retain their magnetic property, artificial magnets are classified as permanent magnets or temporary magnets.

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Temporary magnets are produced with the help of an external magnetic field. They lose their magnetic property as soon as the external magnetic field is removed. They are made from soft iron. Soft iron behaves as a magnet under the influence of an external magnetic field produced in a coil of wire carrying a current. But, it loses the magnetic properties as soon as the current is stopped in the circuit. Magnets used in electric bells and cranes are the examples of temporary magnets.

Explanation

Magnetization is a process in which a substance is made a permanent or temporary magnet by exposing it to an external magnetic field. This is one of the methods to produce artificial magnets.

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Permanent magnets are artificial magnets that retain their magnetic property even in the absence of an external magnetic field. These magnets are produced from substances like hardened steel and some alloys.

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The most commonly used permanent magnets are made of ALNICO (An alloy of aluminium, nickel and cobalt).

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Magnets used in refrigerator, bar magnet, speaker magnet, fridge magnet and magnetic compass are some familiar examples of a permanent magnet. Neodymium magnets are the strongest and the most powerful magnets on the Earth.

Explanation

The magnetic properties of a magnet will be removed from it by the following ways.

• Placing the magnet idle for a long time.
• Continuous hammering of the magnetic substance.
• Dropping the magnet from a height.
• Heating a magnet to a high temperature.
• Passing a variable current in a coil that encloses the magnet.
• Improper storage of the magnet.

Explanation

The position of the Earth’s magnetic poles is not well defined in the Earth. The south pole of the imaginary magnet inside the Earth is located near the geographic North Pole and the north pole of the earth’s magnet is located near the geographic South Pole. The line joining these magnetic poles is called the magnetic axis.

Explanation

The magnetic axis intersects the geographic North Pole at a point called the north geomagnetic pole or northern magnetic pole. It intersects the geographic South Pole at a point called the south geomagnetic pole or southern magnetic pole. The magnetic axis and the geographical axis (axis of rotation) do not coincide with each other. The magnetic axis of the Earth is inclined at an angle of about 10° to 15° with the geographical axis.

Explanation

The most powerful magnet in the universe is actually a neutron star called magnetar (magnetic neutron star) located in the Milky Way Galaxy. The diameter of the magnetar is 20 kilometer and its mass is 2 to 3 times that of the Sun. Its magnetic field is so enormous and lethal that it is capable of absorbing all the iron atoms from the bloodstream (hemoglobin) of a living body even if it is positioned at a distance of 1000 km from it.

Explanation

The exact cause of the Earth’s magnetism is not known even today. However, some important factors, which may be the cause of the Earth’s magnetism, are as follows.

• Masses of magnetic substances in the Earth
• Radiations from the Sun
• Action of the Moon

Explanation

The Earth’s magnetic field is due to the molten charged metallic fluid inside the Earth’s surface with a core of radius of about 3500 km compared to the Earth’s radius of 6400 km.

Explanation

Pigeons have extraordinary navigational abilities. It enables them to find their way back home even if you take them to a place where they have never been before. The presence of magnetite in their beaks enables them to sense the magnetic field of the Earth. Such a magnetic sense is called magneto-reception.

Explanation

A freely suspended magnetic needle at a point on the Earth comes to rest approximately along the geographical north- south direction. This shows that the Earth behaves like a huge magnetic dipole with its magnetic poles located near its geographical poles.

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The north pole of a magnetic needle approximately points towards the geographic north (NG). Thus, it is appropriate to say that the magnetic north pole of the needle is attracted by the magnetic south pole of the Earth (Sm), which is located at the geographic north NG. Also, the magnetic south pole of the needle is attracted by the magnetic north pole of the Earth (Nm), which is located at the geographic south SG.

Explanation

The magnitude of the magnetic field strength at the Earth’s surface ranges from 25 to 65 micro tesla.

Explanation

We come into contact with magnets often in our daily life. They are used in wide range of devices. Some of the uses of magnets are given below.

• In ancient times the magnet in the form of ‘direction stone’ helped seamen to find the directions during a voyage.
• Nowadays, magnets are used to generate electricity in dynamos.
• Magnets, especially electromagnets are used in our day to day life.
• They are used in electric bells and electric motors.
• They are used in loudspeakers and microphones.
• An extremely powerful electromagnet is used in the fast moving Maglev train to remain floating above the tracks.
• In industries, magnetic conveyor belts are used to sort out magnetic substances from scraps mixed with non-magnetic substances.
• Magnets are used in computer in its storing devices such as hard disks.

Explanation

Maglev train (Magnetic levitation train) has no wheels. It floats above its tracks due to strong magnetic forces applied by computer controlled electromagnets. It is the fastest train in the world. The speed attained by this train is around 500 km/hr.