Science textbook solutions for Class 7 science Chapter 19 Properties of a Magnetic Field


Important points to remember :
  1. Magnets are made from alloys of iron, cobalt and nickel.  Nipermag, an alloy of iron,  nickel,  aluminium  and titanium  is used to  make  magnets. 
  2. The scientist William Gilbert gave a scientific explanation, based on experiment, of the observation that a freely  suspended magnet  always settles  in the north-south direction only.


Question 1: Write the appropriate term 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 water.

(c) The intensity of a magnetic field is indicated by the lines of magnetic force passing perpendicularly through a unit area.

(d) The real test of a magnet is repulsion.


Question 2: With whom should I pair up?
Group 'A'                            Group 'B'

a) Compass                       1. The highest magnetic force

b) Door of a cupboard    2. Like poles

c) Repulsion                      3. A magnet

d) Magnetic pole              4. A magnetic needle

Answer :
a) Compass - 4. A magnetic needle

b) Door of a cupboard - 3. A magnet

c) Repulsion - 2.Like poles

d) Magnetic pole - 1.The highest magnetic force.


Question 3: Write answers to the following questions:

a) Distinguish between the two methods of making artificial magnets.
Answer : Artificial magnets are man-made. There are two types of magnets:

I. Temporary magnet: Aren’t always magnetic in nature but can be turned magnetic, depending on the need.

II. Permanent magnets: The magnetic strength of a permanent magnet never deteriorates.
Example: Fridge magnets. An artificial magnet can either be made by single touch method or by using electricity.

b) Which substances are used for making electromagnets?
Answer : A magnet made of an insulated wire coiled around an iron core (or any magnetic material such as iron, steel, nickel, cobalt) with electric current flowing through it to produce magnetism. The electric current magnetizes the core material.

c) Write a note on 'magnetic field'.
Answer : A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. The effects of magnetic fields are commonly seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets.

d) Why is a magnetic needle used in a compass?
Answer: The magnetic needles used in compasses are Magnetic in nature because the Earth has a strong magnetic dipole due to its dynamo effect which creates the magnetic field around the earth's surface.

e) Explain with the help of a diagram how the intensity and direction of the magnetic field of a bar magnet can be determined.
Answer : The lines of magnetic field from a bar magnet form closed lines. By convention, the field direction is taken to be outward from the North pole and in to the South pole of the magnet. As can be visualized with the magnetic field lines, the magnetic field is strongest inside the magnetic material.


Question 4. Give  detailed information about how the merchants of olden times  used a magnet while travelling.
Answer: In the past, it was known that a piece of magnetite (stones having magnetic properties), when hung freely, points in the North-South direction. Thus, these stones were used by merchants of olden times to find the direction while travelling through unknown regions.

# Use your brain power 
1. Is magnetic force a vector  or a scalar quantity ?
Answer : Magnetic force is a vector quantity. - Magnetic force is the force of attraction/repulsion that arises between electrically charged particles because of their motion. - Its a vector quantity as it has both magnitude & direction. - Physical quantities having only magnitude are called scalars.

2. What is meant  by magnetic  force ? 
Answer : Magnetic force, attraction or repulsion that arises between electrically charged particles because of their motion.

3. How  does a magnetic force act without direct contact ?
Answer :  Magnetic forces are non contact forces; they pull or push on objects without touching them. Magnets are only attracted to a few 'magnetic' metals and not all matter. Magnets are attracted to and repel other magnets.

4. What is the difference between gravitational force and magnetic  force ?
Answer. Gravity is relatively weaker, is a result of mass and acts on all things that have mass. Magnetic force is relatively stronger, is a result of interaction of photons(charged particles) and acts on things that contain iron(ferrous) or have a charge on them.

5. Why is repulsion the real test for identifying  a magnet ? 
Answer : Repulsion is the surest test of the magnets, because a magnetic substance also has attractive property. Repulsion is the surest test because any substance made of iron,nickel, cobalt will also have attraction but won't have repulsion.

6. How will you find a magnet from among the various articles  given to you ?
Answer : magnet has a property to attract iron. we will use this property to find magnet from the given articles. we can use iron particles or iron objects. from the given articles magnet will attract the iron particles / objects.

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