Magnetism Book Back Questions 8th Science Lesson 7
8th Science Lesson 7
7] Magnetism
Book Back Questions with Answer and Do You Know Box Content
Do You Know?
There are three types of iron ores. They are: Hematite (69% of Iron), Magnetite (72.4% of Iron) and Siderite (48.2% of Iron). Magnetite is an oxide ore of iron with the formula Fe3O4. Among these ores, magnetite has more magnetic property.
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).
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.
The temperature, at which the ferromagnetic material becomes paramagnetic is called curie temperature.
Magnetisation 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.
Alnico cow magnet is used to attract sharp iron wire and other iron objects that may be ingested by animals while grazing thereby causing damage o their digestive tract.
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 kilometre 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 (haemoglobin) of a living body even if it is positioned at a distance of 1000 km from it.
Earth’s magnet is 20 times more powerful than a fridge magnet.
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 mageneto-reception.
Maglev train (Magnetic levitation train) has no wheels. If 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.
The strip on the back of a credit card/debit card is a magnetic strip, often called a magstripe. The magstripe is made up of tiny iron-based magnetic particles in a thin plastic film. Each particle is really a very tiny bar magnet about 20 millionth of an inch long.
Choose the best answers:
1. A magnet attracts ______________.
(a) Wooden materials
(b) Any metal
(c) Copper
(d) Iron and steel
2. One of the following is an example for a permanent magnet.
(a) Electromagnet
(b) Mumetal
(c) Soft iron
(d) Neodymium
3. The south pole of a bar magnet and the north pole of a U-shaped magnet will _____________.
(a) Attract each other
(b) Repel each other
(c) Neither attract nor repel each other
(d) None of the above
4. The shape of the Earth’s magnetic field resembles that of an imaginary _____________.
(a) U-shaped magnet
(b) Straight conductor carrying current
(c) Solenoid coil
(d) Bar magnet
5. MRI stands for _______________.
(a) Magnetic Resonance Imaging
(b) Magnetic Running Image
(c) Magnetic Radio Imaging
(d) Magnetic Radar Imaging
6. A compass is used for _____________.
(a) Plotting magnetic lines
(b) Detection of magnetic field
(c) Navigation
(d) All of these
Fill in the blanks:
1. The magnetic strength is ____________ at the poles.
2. A magnet has _____________ magnetic poles.
3. Magnets are used in ___________ for generating electricity.
4. _____________ are used to lift heavy iron pieces.
5. A freely suspended bar magnet is always pointing along the ___________ north-south direction.
Match the following:
1. Magnetite – Magnetic lines
2. A tiny pivoted magnet – Natural magnet
3. Cobalt – Compass box
4. Closed curves – Ferromagnetic material
5. Bismuth – Diamagnetic material
Consider the statements given below and choose the correct option
1. Assertion: Iron fillings are concentrated more at the magnetic poles.
Reason: The magnets are so sharp.
(a) Both assertion and reason are true and reason is the correct explanation of the assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Both assertion and reason are false.
2. Assertion: The Earth’s magnetic field is due to iron present in its core.
Reason: At a high temperature a magnet loses its magnetic property or magnetism.
(a) Both assertion and reason are true and reason is the correct explanation of the assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Both assertion and reason are false.
Answers:
Choose the best answers:
1. iron and steel 2. Neodymium 3. attract each other 4. bar magnet 5. Magnetic Resonance Imaging 6. All of these
Fill in the blanks:
1. Maximum 2. Two 3. Dynamos 4. Electromagnets 5. Geographic
Match the following:
1.Magnetite – Natural magnet
2.A tiny pivoted magnet – Compass box
3.Cobalt – Ferromagnetic material
4.Closed curves – Magnetic lines
5.Bismuth – Diamagnetic material
Consider the statements given below and choose the correct option:
1.Both assertion and reason are true, but reason is not the correct explanation of assertion.
2.Assertion is false, but reason is true