Properties of a Magnetic Field | Class 7 | Science | Chapter 19 | Maharashtra State Board
Explore the fascinating world of magnetism with us! Dive into the properties of magnetic fields, understand why Earth is a gigantic magnet, and learn about the pioneering work of Michael Faraday. Discover the penetrating ability of magnetic fields and how this principle is applied in metal detectors. Join us for an exciting journey into the heart of magnetism!
Questions & Answers
1. Write the appropriate terms in the blanks:
(a) The alloys called alnico and Nipermag are used for making industrial magnets.
(b) A magnetic field can pass through cardboard and bottle.
(c) The intensity of a magnetic field is indicated by the lines of force.
(d) The real test of a magnet is repulsion.
2. With whom should I pair up?
Column 'A' | Answer | Column 'B' |
Compass | d. A magnetic needle | a. The highest magnetic force |
Door of cupboard | c. A magnet | b. Like poles |
Repulsion | b. Like poles | c. A magnet |
Magnetic pole | a. The highest magnetic force | d. A magnetic needle |
3. Write answers to the following questions:
(a) Distinguish between the two methods of making artificial magnets.
Ans. Two common methods of making artificial magnets:
Electromagnet Method: This method involves using electricity to create a magnetic field. When electricity flows through a wire, a magnetic field is generated around the wire. This magnetic field can be intensified by coiling the wire so that the overlapping magnetic fields reinforce each other. The coil becomes an artificial magnet as long as the electricity is flowing.
Touch or Stroke Method: This is a simple method where a ferromagnetic material (like a paperclip) is rubbed with a permanent magnet in one direction. This aligns the magnetic domains of the material, turning it into a magnet. However, this type of magnet is usually temporary.
(b) Which substances are used for making electromagnets?
Ans. To make an electromagnet, you need a metal core, such as an iron nail or rod, a coil of wire, a power source, such as a battery, and some electrical tape to secure the wire ends. You also need some magnetic items, such as paper clips or iron fillings, to test the strength of your electromagnet.
(c) Write a note on magnetic field.
Ans. A magnetic field is a region around a magnet or a moving electric charge where magnetic forces are exerted. It influences the behaviour of other magnets or magnetic materials placed within its vicinity. Magnetic fields are characterized by field lines that represent the direction and strength of the magnetic force. They play a crucial role in various technologies, from electric motors and generators to medical imaging devices like MRI machines. Earth itself has a magnetic field, essential for navigation, as compass needles align with the Earth's magnetic field lines.
(d) Why is a magnetic needle used in a compass?
Ans. A magnetic needle is used in a compass because it aligns itself with the Earth's magnetic field. The needle points towards the Earth's magnetic North Pole, providing a reliable reference for navigation and helping determine directions such as north, south, east, and west.
(e) Explain with the help of a diagram how the intensity and direction of the magnetic field of a bar magnet can be determined.
Ans.
Intensity: Magnetic field lines emerge from the north pole and enter the south pole. The field is stronger near the poles and weaker as you move away.
The density of field lines indicates intensity, with closely packed lines representing stronger fields.
Direction: Imaginary magnetic field lines form closed loops around the magnet.
The lines always travel from the north pole to the south pole outside the magnet.
Inside the magnet, the lines travel from the south pole to the north pole.
4. Give detailed information about how the merchants of olden times used a magnet while travelling.
Ans. The merchants of olden times used a magnet while travelling in the following way:
They used a piece of magnetite, a natural magnetic mineral, that could point to the north-south direction when hung freely.
They called this stone a lodestone, meaning a leading stone, as it helped them to find their way in unknown regions.
They used this stone as a primitive compass, by either suspending it from a thread or floating it on a piece of wood in water.
This is how the merchants of olden times used a magnet while travelling.
E X T R A
1. Fill in the blanks:
1. The magnetic lines of force always run from the north pole to the south pole.
2. The unit of magnetic field strength is A/M (Amperes per metre).
3. The earth behaves like a giant bar magnet.
4. The Michael Faraday properties of magnetic field was proposed by scientist.
5. The region around a magnet where the magnetic force acts on an object is called the magnetic field.
6. The strength of the magnetic field can be obtained from the number of lines of force.
7. The function of the metal detector is based on electromagnets.
8. The magnetism is temporary in the case of an electromagnet.
Column 'A' | Answer | Column 'B' |
Iron, nickel, cobalt | d. Magnetic metals | a. Electromagnet |
Doorbell magnet | a. Electromagnet | b. Permanent magnet |
Nickel, cobalt, aluminium | f. Alnico | c. Iron, nickel, titanium, aluminium |
Cupboard magnet | b. Permanent magnet | d. Magnetic metals |
Lodestone | d. Magnetic metals | e. Mariner's compass |
Nipemag | c. Iron, nickel, titanium, aluminium | f. Alnico |
3. Write True or False correct and rewrite the false statements:
1. Material alnico is a mixture of aluminium, nickel and iron.
Ans. False. Alnico is an alloy of aluminium, nickel and cobalt.
2. Nipermag is alloy of iron, nickel, cobalt.
Ans. False. Nipermag is an alloy of iron, nickel, aluminium and titanium.
3. The magnetic force is concentrated at the centre of the magnet.
Ans. False. The magnetic force is concentrated at the poles of the magnet.
4. Magnetism of electromagnet is permanent.
Ans. False. Magnetism of electromagnet is temporary.
5. Like poles attract each other and unlike poles repel each other.
Ans. False. Like poles repel each other and unlike poles attract each other.
6. Two poles of a magnet cannot be separated from each other.
Ans. True.
4. Answer in one or two sentences:
1. In which direction does a freely suspended magnet settle?
Ans. A freely suspended magnet always settles in the North-South direction. This is because it aligns itself with the Earth’s magnetic field.
2. Write any two functions of metal detectors.
Ans. Metal detectors are devices that can detect the presence of metal objects in various situations. Some of the functions of metal detectors are:
• To find buried treasure, coins, relics, or other metal items for hobby or archaeological purposes.
• To ensure security and safety by screening people, luggage, or vehicles for weapons, explosives, or contraband.
3. Which magnetic poles attract each other?
Ans. Magnetic poles that attract each other are the North Pole of one magnet and the South Pole of another magnet. Opposite magnetic poles attract, while like poles (North-North or South-South) repel each other.
4. What metals are used for making magnets?
Ans. Magnets are made from alloys of iron, cobalt and nickel.
Nipermag is an alloy made up of iron, nickel, aluminium and titanium whereas alnico is an alloy made up of aluminium, nickel and cobalt.
5. Give scientific reasons:
1. Earth is called as a gigantic magnet.
Ans. Earth is called a gigantic magnet because it has a magnetic field generated by the movement of molten iron and nickel in its outer core, creating a magnetic effect similar to that of a large bar magnet.
2. A bar magnet kept on the cardboard arranges the iron fillings in line.
Ans. The bar magnet arranges iron fillings in line because the fillings align themselves along the magnetic field lines produced by the magnet. The magnetic field exerts a force on the iron fillings, causing them to arrange in the direction of the magnetic field.
6. Write short note on:
1. Functions of metal detector.
Ans. Metal detectors are devices that can detect the presence of metal objects in various situations. Some of the functions of metal detectors are:
• To find buried treasure, coins, relics, or other metal items for hobby or archaeological purposes.
• To ensure security and safety by screening people, luggage, or vehicles for weapons, explosives, or contraband.
2. Write down the characteristics of magnets.
Ans. Characteristics of magnets are as follows.
1. Attraction and Repulsion: Magnets exhibit attraction between their opposite poles (North attracts South, and vice versa) and repulsion between like poles (North repels North, and South repels South).
2. Magnetic Field: Magnets create a magnetic field around them, influencing nearby objects. The field is stronger near the poles and weaker as you move away.
3. Two Poles: Every magnet has at least two poles, a North Pole and a South Pole. Magnetic field lines emerge from the North Pole and enter the South Pole.
4. Magnetization: Materials can be magnetized by exposing them to a magnetic field. This process aligns the magnetic domains within the material, making it behave like a magnet.
5. Retentivity and Coercivity: Magnets can retain their magnetism over time (retentivity) and may require varying degrees of effort to demagnetize them (coercivity).
6. Induction: A magnet can induce magnetism in nearby materials without direct contact, influencing their magnetic properties.
7. Earth as a Magnet: Earth itself acts as a giant magnet with a magnetic field, and a freely suspended magnet aligns itself with the Earth's magnetic field.
8. Applications: Magnets find applications in various technologies, such as electric motors, generators, speakers, and medical devices like MRI machines.
3. Characteristic of magnetic lines of force.
Ans. Characteristic of magnetic lines of force are as follows.
1. Direction: Magnetic lines of force, also known as magnetic field lines, always form closed loops. They extend from the North Pole of a magnet to the South Pole outside the magnet and from the South Pole to the North Pole inside the magnet.
2. Density Indicates Strength: The density of magnetic field lines represents the strength of the magnetic field. Closer lines indicate a stronger magnetic field, while lines spaced farther apart indicate a weaker field.
3. No Intersections: Magnetic field lines never intersect with each other. If they did, it would imply the existence of two different directions for the magnetic field at the point of intersection, which is not possible.
4. Tangent to Field: At any point on a magnetic field line, the line is tangent to the direction of the magnetic field at that point. The arrowhead of the field line points in the direction of the magnetic force that a north pole would experience if placed at that location.
5. Outside to Inside Transition: When a magnetic field line enters a magnet, it goes from the North Pole to the South Pole. Inside the magnet, it continues from the South Pole to the North Pole.
6. Indicator of Magnetic Behaviour: The pattern of magnetic field lines provides a visual representation of the magnetic behaviour of a magnet and helps in understanding how magnetic forces act in the surrounding space.
7. Answer the following questions:
1. Why does freely suspended magnet always settle in the north-south direction?
Ans. A freely suspended magnet always settles in the north-south direction due to its alignment with the Earth's magnetic field. The north pole of the magnet points towards the Earth's magnetic north pole, and the south pole points towards the Earth's magnetic south pole.
2. Which magnetic poles attract each other?
Ans. Opposite magnet poles attract each other.
3. Which pole of a spherical magnet will get attracted towards the south pole of the bar magnet?
Ans. The north pole of the spherical magnet will get attracted towards the south pole of the bar magnet.
4. Why does a bar magnet kept at a distance doesn’t attract pins?
Ans. If a bar magnet kept at a distance doesn't attract pins, it might be because the magnetic field strength diminishes with distance. The force of attraction between the magnet and the pins weakens as the distance increases, and beyond a certain point, the magnetic force is not sufficient to overcome other forces, such as gravity, preventing the pins from being attracted.
5. What is an electromagnet?
Ans. An electromagnet is a temporary magnet created by passing an electric current through a coil of wire, often wrapped around a core material.
8. Answer the following questions:
1. Which direction will a magnetic needle show on the geographic north pole?
Ans. At the geographic North Pole, a magnetic needle would point towards the magnetic south pole. This is because the Earth's magnetic field lines emerge from the magnetic south pole and enter the magnetic north pole, causing a magnetic needle to align itself in the opposite direction.
2. Is magnetic force a vector or a scalar quantity?
Ans. Magnetic force is a vector quantity. A vector quantity is characterized by both magnitude and direction. In the case of magnetic force, the magnitude represents the strength of the force, and the direction indicates the orientation of the force. This vector nature is essential for describing the behaviour of charged particles in magnetic fields, as the force they experience depends on the relative orientation of their velocity and the magnetic field.
3. What is meant by magnetic force?
Ans. Magnetic force is the force exerted on a charged particle moving through a magnetic field. It is one of the fundamental forces in nature, alongside gravitational and electromagnetic forces. The strength and direction of the magnetic force depend on the charge of the particle, its velocity, and the strength and orientation of the magnetic field.
4. How does a magnetic force act without direct contact?
Ans. Magnetic forces act at a distance due to the interaction between magnetic fields and charged particles, such as electrons or protons, without the need for direct physical contact.
5. What is the difference between the gravitational force and magnetic force?
Ans.
Gravitational force | Magnetic force |
It is a universal force of attraction between any two masses in the universe. All objects with mass experience gravitational attraction. | It arises due to the motion of charged particles in a magnetic field. It is associated with the property of magnetism and acts on charged particles. |
It is always attractive and acts between any two masses, regardless of their charge or composition. | It can be attractive or repulsive and acts on charged particles (moving charges) in the presence of a magnetic field. |
It is independent of the charge of the objects involved. | It depends on the charge of the particles and their motion in a magnetic field. |
Acts between any two masses, such as planets, stars, and everyday objects. | Acts on charged particles, like electrons and protons, moving in a magnetic field. It is also associated with magnetic materials. |
Generally weaker than electromagnetic forces but has an infinite range. | Can be stronger than gravitational force in certain situations but has a limited range. |
Always attractive and directed towards the center of mass. | Can be attractive or repulsive, and the direction depends on the motion of the charged particles and the orientation of the magnetic field. |
6. Why is repulsion the real test for identifying a magnet?
Ans. Repulsion is a reliable test for identifying a magnet because it is a unique and characteristic behaviour of magnets. When two magnets repel each other, it confirms the presence of magnetism, distinguishing it from non-magnetic materials.
7. How will you find a magnet from among the various articles given to you?
Ans. To find a magnet among various articles:
1. Use a compass; a deflection indicates magnetism.
2. Test for repulsion between suspected magnets.
3. Sprinkle iron filings; a magnet will attract and align them.
4. Check materials; magnetic metals like iron are likely candidates.
8. Why does the magnetic needle of a compass not settle parallel to the ground but at an angle to it?
Ans. The magnetic needle of a compass does not settle parallel to the ground because it aligns with the Earth's magnetic field, which has both horizontal and vertical components. The angle at which the needle settles is influenced by the inclination or dip of the Earth's magnetic field at that location.
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