Kerala Syllabus Class 8 Basic Science: Chapter 11 Magnetism and Electricity - Questions and Answers
Std 8: Physics: Chapter 11: Magnetism and Electricity: Questions and Answers
♦ Some situations in which magnets are used are given below. Add more situations.
• MRI scanning
• Headphones
• Loud speakers
• Electric motors and generators
• Dynamo
• Refrigerator doors
• Induction cooktops
• Doorbells and buzzers
• Toys
• Compass navigation
♦ Natural magnets
Natural magnets are the magnets that are obtained directly from nature. An example of this is lodestone.
♦ Artificial magnets
Artificial magnets are made using metal alloys like Alnico, ceramic or Ferrite Magnets
• Alnico Magnets- These are alloys made from aluminium (Al), nickel (Ni), cobalt (Co) and iron (Fe).
• Ceramic/Ferrite Magnets – These are made by mixing iron oxide with carbonates of elements like barium or strontium.
♦ Some familiar magnets are given below. Complete the table.
Similarly, bring the south pole, observe the changes and complete the table.
| Activity | Observations |
|---|---|
| When the north poles are brought close to each other | Repels |
| When the north poles and the south poles are brought close to each other | Attracts |
| When the South Poles are brought close to each other | Repels |
♦ What conclusion do you reach from the above activities?
Conclusion:
• Like poles of magnets repel
• Unlike poles attract
♦ Look at the picture.
It is due to the repulsion of the magnets on the board and on the pencil.
♦ What will happen if you bring another magnet near one side of the top magnet?
The pencil will move
♦ Do you know if this special property of magnets is used in any technology?
Maglev trains, or magnetic levitation trains, are a type of train that runs without wheels. These trains use powerful electromagnets located both on the underside of the train and along the track. Due to the attraction and repulsion of these magnets, the train can suspend above the track and can move very fast.
♦ Other uses of Magnetic Levitation:
• Maglev launch system
• Maglev conveyors
• Magnetic bearing
♦ Magnetic levitation
Magnetic levitation is the phenomenon where an object is made to float in the air using magnetic force. It happens when the magnetic repulsion or attraction balances the force of gravity.
♦ Suspend a bar magnet freely, using a string. In which direction does it align?
A freely suspended bar magnet will always align in the north-south direction of the Earth. This directional property of a magnet is used in a magnetic compass.
♦ Magnetic Compass
A compass is a device with a small magnetic needle that can rotate freely. By looking at the marked directions on the compass, we can easily understand other directions like east and west, too.
♦ Why does a magnetic compass always show the north-south direction?
It is due to the influence of Earth’s magnetic field. The Earth acts like a big magnet, and it has a north pole and a south pole. The north pole of the suspended magnet is attracted towards the south pole of the earth magnet, and the south pole is attracted towards the north pole of the earth magnet.
♦ How are the Earth's magnetic poles formed?
• The Earth’s magnetic poles are formed by the movement of large amounts of molten iron and nickel in its inner core.
• The magnetic north pole of the Earth is located near the geographical south pole, and the magnetic south pole is near the geographical north pole.
♦ How to make an artificial magnet?
Place a hacksaw blade on a table. Imagine one end of it as A and the other end as B. Take a bar magnet and rub its N pole from end A of the blade in the direction shown in the figure. Repeat this process several times without changing the ends. If rubbed as shown in the picture, the hacksaw blade will transform into a magnet, with end A becoming the North pole and end B becoming the South pole.
Bring both ends of each broken piece near a compass and observe. Record your conclusions in the science diary.
• If a magnet is broken into several pieces, each piece has two poles. According to the theory of magnetism, each molecule in a magnet has a north pole and a south pole. This makes each piece a perfect magnet with two poles.
• No matter how small a magnet is, it will always have two poles. A magnet with only one pole does not exist.
♦ Magnetic field
The region around a magnet where its force can be felt is called the magnetic field.
♦ Magnetic field lines
Magnetic field lines are imaginary lines used to represent the direction and strength of a magnetic field. They are closed loops. Outside the magnet, the direction of the magnetic field lines is from the north pole to the south pole.
♦ Characteristics of Magnetic Field Lines
What will happen to the magnetic field lines when two magnetic poles come close to each other? Try drawing the magnetic field lines in this situation and see how they look like.
• Figure 1: When two north poles come close to each other
• Figure 2: When the north and south poles come close to each other
♦ In the picture, the magnetic poles are placed close to each other in different ways. Can you observe how the magnetic field lines change in each situation?
• Magnetic field lines never intersect each other.
• When like poles of two magnets come close, the magnetic field lines bend away from each other.
• When unlike poles come close, the magnetic field lines go from the north pole of one magnet to the south pole of the other.
♦ Observe the magnetic field lines of a bar magnet and a U-magnet given.
♦ What will happen to the magnetic flux if the size of the flat surface increases?
Decrease
♦ Magnetic flux density
The number of magnetic field lines passing normal through a unit area is the magnetic flux density of that region.
♦ Is the flux density higher at the poles of a magnet or at other places? Write an Experiment.
with coconut oil. Add iron filings into the oil and stir it well. Carefully place the test tube with the magnet into the bottle. Try to fix the test tube at the bottom of the bottle. Close the mouth of the bottle securely. What do you observe? Don’t you see the iron filings inside the bottle arrange itself under the influence of the magnet? Where do the iron filings stick the most?
Observation:
Iron fillings are concentrated at the poles and are seen least at other parts.
The magnetic field strength is stronger at the poles of the magnet, and it becomes weaker as we move away from the poles.
♦ Is the flux density higher at the poles of a magnet or at other places?
Magnetic flux density is higher in places where the magnetic field strength
is more. The poles of a magnet have the highest magnetic flux density.
♦ Magnetic Induction
The phenomenon of a magnetic substance acquiring magnetism due to the presence of a magnet is known as Magnetic Induction.
♦ What is induced magnetism?
The magnetism acquired by the magnetic substance is the Induced Magnetism.
♦ Give one practical example of induced magnetism.
When a steel paperclip is brought near a magnet, it becomes temporarily magnetised and can attract another paperclip. This is due to induced magnetism.
♦ Why does a magnetic material exhibit induced magnetism when placed under the influence of a magnetic field?
When a magnetic material comes under the influence of a magnetic field, the magnetic properties of the atoms inside it get temporarily arranged. This is the reason for induced magnetism.
♦ What are the uses of magnetic induction?
• Magnetic lock
• Door of the refrigerator
• To separate magnetic materials from a mixture
♦ Magnetic Induction in Soft Iron and Steel
Experiment
Place a piece of soft iron on one of the poles of a bar magnet. Doesn't the soft iron become magnetised? Now check whether the soft iron attracts pins.
Carefully remove the soft iron piece from the magnet and observe what happens to the pins.
Observation:
Soft iron gets magnetized easily. But when the magnet is removed, it quickly loses its magnetism.
Steel takes more time to get magnetised. However, even after the magnet is removed, it does not lose its magnetism quickly.
♦ Define susceptibility and retentivity.
Susceptibility is the ability of magnetic materials to get magnetised due to the influence of an external magnetic field.
Retentivity is the ability to retain the magnetism.
♦ Let us compare the properties of soft iron and steel.
• Soft iron - Greater susceptibility, Low retentivity, Used to make a strong temporary magnet.
• Steel - Low susceptibility, Greater retentivity, Used to make a permanent magnet.
♦ Which of these has greater susceptibility?
Soft iron
♦ Which of these has greater retentivity?
Steel
♦ Activity
Observation:
Iron fillings do not stick in the gap of the nut
More iron filings are distributed over the nut.
Inference:
Soft iron has a higher ability than air to allow magnetic field lines to pass through it.
♦ What is Permeability?
Permeability is the ability of a substance to pass the magnetic field lines through it.
♦ Why do compass needles not show direction when placed inside a box made of soft iron?
If a compass needle is placed inside a box made of soft iron, it cannot deflect or show direction. It is because soft iron has higher permeability and the field lines of the needle pass through it, and the iron case becomes a magnet. Then the needle is attracted towards the case, and it cannot deflect.
♦ What are electromagnets?
When electricity flows, the soft iron becomes a magnet. Such magnets are called electromagnets.
♦ Take a soft iron nail and wind insulated copper wire around it. Connect the ends of the wire to a cell. Check whether the electromagnet thus made attracts paper clips.
| Experiment | Observation |
|---|---|
| When electricity flows, the soft iron piece and the paper clips | attracts |
| After removing the cell, the soft iron piece and the paper clips | does not attract |
♦ Electromagnets
When electricity flows, the soft iron becomes a magnet. Such magnets are called electromagnets.
♦ Write down the different ways to increase the strength of an electromagnet.
• Increase the number of turns
• Increase the cross-sectional area of the soft iron inside the coil
• Increase the intensity of the current
♦ Let’s write examples of devices that use electromagnets :
• Electric bells
• Loudspeakers
• Microphone
• Generator
• Motor
♦ Compare electromagnets and permanent magnets. Complete the table.
| Electromagnets | Permanent magnets |
|---|---|
| • Poles can be changed • Magnetic strength can be increased. • When there is no current flow, it will loss magnetic property • It can be magnetised when it is needed • Magnetic property is temporary | • The gained magnetism can be retained for a long time. • Magnetic strength cannot be increased. • Poles cannot be changed • Magnetic property is constant |
Let's Assess
1. A student is trying to make a device to find direction using a magnetic needle. For this, he places the magnetic needle inside a box made of iron.
a) Will this device work properly?
b) Explain your answer.
c) What changes should be made to make this device work properly?
Answer:
a) no
b) If a compass needle is placed inside a box made of soft iron, it cannot deflect or show direction. It is because soft iron has higher permeability and the field lines of the needle pass through it, and the iron case becomes a magnet. Then the needle is attracted towards the case, and it cannot deflect.
c) The case should be made with non-magnetic substances.
2. AB is a bar magnet shown in the figure below. An iron rod CD is placed near its B pole.
Answer: C - north (N), D - south (S)
• Which property of magnets does this phenomenon demonstrate?
Answer: Magnetic induction
3. A bar magnet and a U magnet are shown in figures (a, b) each having two iron nails hanging from them.
b. Explain the reason clearly.
Answer:
a.
b.
4. You are given a soft iron piece, a steel piece of the same size, insulated copper wire and a battery:
a. Suggest a method to make a powerful permanent magnet.
b. Suggest a method to make a temporary magnet.
Answer:
(a) To make a permanent magnet, take the steel piece and wind it with insulated copper wire with large number of turns and pass high-intensity current from battery.
(b) To make a temporary magnet, use a soft iron piece.
5. In an experiment, a plastic car with an iron piece inside it is made to run on a wooden table by sliding a strong magnet below it.
a) The experiment failed when a steel table was used. What is the reason for this?
b) If an aluminium table is used instead of steel, what will happen? Why?
Answer:
(a) Steel has higher permeability than wood. So magnetic flux lines spread into the table, making it a magnet. It will attract the iron piece which is inside the car. So it cannot move.
(b) Aluminium has low permeability. So the car can move.
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