Samacheer Kalvi Class 9 Science Solutions Chapter 5 Magnetism and Electromagnetism

I. Choose the correct answer:

 

Question 1. Which of the following converts electrical energy into mechanical energy?
(a) Motor
(b) Battery
(c) Generator
(d) Switch
Answer: (a) Motor
In simple words: A motor changes electricity into movement. It takes in electrical power and uses it to make things turn or move.

🎯 Exam Tip: Remember that motors *use* electricity to move, while generators *make* electricity from movement.

 

Question 2. Transformer works on \( \dots \)
(a) AC only
(b) DC only
(c) Both AC and DC
Answer: (a) AC only
In simple words: Transformers only work with alternating current (AC), not direct current (DC). They need the current to change direction to work.

🎯 Exam Tip: Key concept: Transformers rely on changing magnetic fields, which AC naturally provides, but DC does not.

 

Question 3. The part of the AC generator that passes the current from the armature coil to the external circuit is
(a) field magnet
(b) split rings
(c) slip rings
(d) brushes
Answer: (d) brushes
In simple words: In an AC generator, the brushes touch the spinning parts (slip rings) and help carry the electricity from inside the generator to the wires outside.

🎯 Exam Tip: Remember that brushes make contact with the rotating slip rings to transfer current to the static external circuit.

 

Question 4. The unit of magnetic flux density is
(a) Weber
(b) weber/metre
(c) weber/metre\(^2\)
(d) weber. metre\(^2\)
Answer: (c) weber/metre\(^2\)
In simple words: Magnetic flux density tells us how strong a magnetic field is in a certain area. Its unit, weber per square meter, shows how much magnetic flow goes through each square meter.

🎯 Exam Tip: Don't confuse magnetic flux (Weber) with magnetic flux density (Weber per square metre, or Tesla).

II. Fill in the blanks :

 

Question 1. The SI Unit of magnetic field induction is \( \dots \)
Answer: Tesla
In simple words: Magnetic field induction is a way to measure the strength of a magnetic field. Its standard unit is Tesla.

🎯 Exam Tip: Recall that magnetic field induction and magnetic flux density share the same SI unit, Tesla.

 

Question 2. Devices which is used to convert high alternating current to low alternating current is \( \dots \)
Answer: transformers
In simple words: Transformers are special devices that can change high alternating current into a lower alternating current, or vice versa, to suit different needs.

🎯 Exam Tip: Remember that transformers work only with alternating current (AC) to change voltage levels efficiently.

 

Question 3. An electric motor converts \( \dots \) energy into mechanical energy.
Answer: electrical
In simple words: An electric motor uses electrical power and changes it into physical movement, like making a fan spin or a toy car move.

🎯 Exam Tip: Electric motors are a prime example of energy conversion from electrical to mechanical, following the principle of electromagnetism.

 

Question 4. A device for producing electric current is \( \dots \)
Answer: generator
In simple words: A generator is a machine that makes electricity. It uses motion or other forms of energy to create an electric current.

🎯 Exam Tip: Generators work on the principle of electromagnetic induction, creating current when a conductor moves in a magnetic field.

III. Match the following :

 

Question 1. Match the following:

Answer: 1-b, 2-d, 3-a, 4-e, 5-c
In simple words: This match connects different items and ideas in science. For example, iron is a magnetic material, while wood is not. Oersted is linked to current and magnetism, Faraday to electromagnetic induction, and induction itself to electric generators.

🎯 Exam Tip: When matching, focus on the core concept or discovery associated with each term to ensure accurate pairings.

IV. State whether true or false. If false, correct the statement

 

Question 1. A generator converts mechanical energy into electrical energy.
Answer: True
In simple words: Generators take energy from movement, like spinning a wheel, and turn it into electricity.

🎯 Exam Tip: Remember that generators convert mechanical energy into electrical energy, while motors do the opposite.

 

Question 2. Magnetic field lines always repel each other and do not intersect.
Answer: True
In simple words: Magnetic field lines never cross paths and always push away from each other. They spread out without touching.

🎯 Exam Tip: A key property of magnetic field lines is that they never intersect, otherwise it would imply two directions for the magnetic field at one point.

 

Question 3. Fleming's Left-hand rule is also known as the Dynamo rule.
Answer: False.
Correct statement: Fleming's Left-hand rule is also known as the motor rule.
In simple words: Fleming's Left-hand rule helps us understand how motors work, showing the direction of force on a current-carrying wire in a magnetic field. It is not called the dynamo rule.

🎯 Exam Tip: Distinguish between Fleming's Left-hand rule (for motors, direction of force) and Right-hand rule (for generators/dynamos, direction of induced current).

 

Question 4. The speed of rotation of an electric motor can be increased by decreasing the area of the coil.
Answer: False.
Correct statement: The speed of rotation of an electric motor can be increased by increasing the area of the coil.
In simple words: To make an electric motor spin faster, you should make the coil larger, not smaller. A larger coil area helps it spin quicker.

🎯 Exam Tip: Motor speed is directly influenced by the strength of the magnetic field, the current, and the effective area of the coil.

 

Question 5. A transformer can step up the direct current.
Answer: False.
Correct statement: A transformer can step up alternating current (AC).
In simple words: Transformers only work with alternating current (AC) to change its voltage. They cannot change direct current (DC).

🎯 Exam Tip: Transformers work based on electromagnetic induction, which requires a changing magnetic field, hence they operate only with AC.

 

Question 6. In a step-down transformer the number of turns in the primary coil is greater than that of the number of turns in the secondary coil.
Answer: True.
In simple words: In a step-down transformer, the first coil (primary) has more loops of wire than the second coil (secondary). This helps to lower the voltage.

🎯 Exam Tip: For step-down transformers, \( N_P > N_S \) and \( V_P > V_S \), meaning more turns in primary and higher primary voltage.

V. Answer in brief:

 

Question 1. State Fleming's Left Hand Rule.
Answer: Fleming's Left Hand Rule states that when you stretch your left hand's three fingers so they are all at right angles to each other, the forefinger points to the direction of the magnetic field, the middle finger points to the direction of the current, and then your thumb will show the direction of the force or movement of the conductor. This rule is crucial for understanding how electric motors produce motion.
In simple words: Fleming's Left Hand Rule uses your left thumb, forefinger, and middle finger spread out at right angles. The forefinger shows the magnetic field direction, the middle finger shows current direction, and your thumb shows the direction of the force or movement.

🎯 Exam Tip: Practice the hand gesture for Fleming's Left-Hand Rule to correctly identify the direction of force, magnetic field, or current.

 

Question 2. Define magnetic flux density.
Answer: Magnetic flux density is defined as the number of magnetic field lines that cross a unit area, measured when that area is positioned perpendicularly to the direction of the field lines. It quantifies the strength of a magnetic field in a given region. Its SI unit is Weber per square meter \( (\text{Wb/m}^2) \).
In simple words: Magnetic flux density is a measure of how many magnetic field lines pass through a specific area, held straight to the lines. Its unit is Weber per square meter.

🎯 Exam Tip: Magnetic flux density indicates the strength of the magnetic field; a higher density means a stronger field.

 

Question 3. List the main parts of an electric motor.
Answer: The main parts of an electric motor are:
1. Shaft
2. Primary and Secondary windings
3. Ball bearings
4. Armature
5. Stator
6. Commutator
7. Brushes
8. Terminals
Each part plays a crucial role in converting electrical energy into mechanical energy.
In simple words: An electric motor has several key parts: a shaft that rotates, windings for current, ball bearings to reduce friction, an armature that spins, a stator that stays still, a commutator to switch current direction, brushes for contact, and terminals for connections.

🎯 Exam Tip: Be familiar with the function of each part of an electric motor, especially the armature, commutator, and brushes.

 

Question 4. Draw and label the diagram of an AC generator.
Answer:

In simple words: An AC generator drawing shows how it makes electricity. It has magnets, a spinning coil, slip rings, and brushes to collect the current. The diagram highlights the main parts that work together to create alternating current.

🎯 Exam Tip: Practice drawing and labelling the key components of an AC generator, including the slip rings and brushes, to understand its operation.

 

Question 5. State the advantages of AC over DC.
Answer: Alternating Current (AC) offers several advantages over Direct Current (DC). Firstly, the voltage of AC can be easily changed (stepped up or down) using transformers, which is not possible with DC. This allows AC to be transmitted over long distances at high voltages with minimal energy loss. Secondly, generating AC is generally simpler and more efficient than generating DC. Thirdly, AC can be readily converted into DC when needed, for specific applications. Finally, AC is crucial for producing electromagnetic induction, which is used in many useful devices.
In simple words: Alternating current (AC) has several benefits: its voltage can be easily changed with transformers, making it suitable for long-distance transmission with less energy loss. It's also easier to generate and can be converted to DC, and it's essential for electromagnetic induction.

🎯 Exam Tip: Focus on the practical advantages of AC in power transmission and voltage transformation, which are key reasons for its widespread use.

 

Question 6. Differentiate step up and step down transformer.
Answer:

Step-up transformers increase voltage and have more turns in the secondary coil, used in power stations. Step-down transformers decrease voltage and have fewer turns in the secondary coil, found in chargers. Both types are essential for managing electrical power effectively.
In simple words: Step-up transformers increase voltage and have more turns in the secondary coil, used in power stations. Step-down transformers decrease voltage and have fewer turns in the secondary coil, found in chargers. They both change voltage levels.

🎯 Exam Tip: Clearly remember the relationship between voltage, number of turns, and the purpose (step-up or step-down) for each type of transformer.

 

Question 7. A portable radio has a built-in transformer so that it can work from the mains instead of batteries. Is this a step up or step down transformer? Give reason.
Answer: A portable radio uses a step-down transformer. The reason is that mains electricity (from a wall socket) provides a high voltage, typically 220-240V. However, a portable radio needs a much lower voltage to operate safely and correctly, usually around 3-12V. A step-down transformer reduces the high mains voltage to the lower voltage required by the radio.
In simple words: A portable radio uses a step-down transformer. This is because the mains electricity has high voltage, and the radio needs a much lower voltage to operate safely without batteries.

🎯 Exam Tip: Always consider the required voltage for a device; if it's lower than the supply, a step-down transformer is needed.

 

Question 8. State Faraday's laws of electromagnetic induction.
Answer: Faraday's laws of electromagnetic induction describe how a changing magnetic field can create an electric current:
First law: This law states that whenever there is a change in the magnetic flux linked with a coil, an electric current (and an induced potential difference) is produced in the coil. This induced current and potential difference will only last as long as the magnetic flux is changing.
Second law: This law states that the magnitude (size) of the induced current (or induced electromotive force, EMF) is directly proportional to the rate at which the magnetic flux linked with the coil changes. This means a faster change in magnetic flux will produce a larger induced current.
In simple words: Faraday's laws explain how electricity can be made using magnets. The first law says that if the magnetic field around a coil changes, electricity will flow through it. The second law states that the faster this magnetic change happens, the more electricity will be produced.

🎯 Exam Tip: Faraday's laws are fundamental to understanding how generators and many electrical devices work, linking changing magnetic flux to induced current.

VI. Answer in detail :

 

Question 1. Explain the principle, construction, and working of a dc motor.
Answer:

Principle: An electric motor works on the principle that a current-carrying conductor, when placed within a magnetic field, experiences a force. This force causes the conductor to move. The direction of this force is determined by Fleming's Left-hand Rule. This fundamental interaction is how electrical energy is converted into mechanical energy.
Construction: An electric motor consists of several key parts:

• Armature: This is a rectangular coil (ABCD) with many turns of insulated copper wire wound around a soft iron core. The armature is placed between the poles of a strong field magnet. It is designed to rotate around an axis that is perpendicular to the magnetic field lines.
• Split rings (Commutator): This component consists of a cylindrical metal ring that is divided into two halves, \( S_1 \) and \( S_2 \). As the armature coil rotates, these split rings also rotate with it around the same axis. The main function of the split ring commutator is to reverse the direction of the current in the coil after every half rotation. This ensures continuous rotation in one direction.
• Carbon brushes: These are two graphite or flexible metal rods, \( B_1 \) and \( B_2 \), that maintain sliding contact with the split rings \( S_1 \) and \( S_2 \) alternately. They are stationary.
• Battery: A battery or a few cells are connected to the brushes, providing the electric current that flows through the armature coil via the brushes and split rings.
• Shaft and Terminals: The shaft supports the rotating parts, and terminals provide external connections.

Working: When current flows from the battery through the brushes, split rings, and into the armature coil, each segment of the coil in the magnetic field experiences a force. For instance, if current flows towards B in segment AB and in the opposite direction in segment CD, then according to Fleming's left-hand rule, forces will act on these segments in opposite directions. These opposite forces create a turning effect, causing the coil to rotate. As the coil rotates and the split rings maintain contact with the brushes, the direction of the current in the coil segments is reversed after every half rotation. This continuous reversal of current ensures that the forces on the coil sides always act in a way that keeps the coil rotating in the same direction, leading to continuous mechanical movement.
In simple words: A DC motor works because a current-carrying wire in a magnetic field feels a force, making it move. Fleming's left-hand rule helps find this force direction. It is built with a spinning coil between magnets, split rings to switch current, and brushes for contact, all powered by a battery. When current flows, the coil spins non-stop because the forces keep pushing it in the same direction.

🎯 Exam Tip: For a DC motor, understand how the interaction of magnetic field and current produces force, leading to rotation, and the role of the commutator in maintaining continuous rotation.

 

Question 2. Explain two types of transformer.
Answer: Transformers are electrical devices that change alternating voltage levels from one value to another without changing its frequency. They work on the principle of mutual induction. There are two main types:
1. Step-up transformer: This type of transformer is used to increase (step up) a low alternating voltage to a higher alternating voltage. In a step-up transformer, the secondary coil (output side) has a greater number of turns of wire \( (N_S) \) compared to the primary coil (input side) \( (N_P) \), meaning \( N_S > N_P \). Consequently, the output voltage \( (V_S) \) is higher than the input voltage \( (V_P) \), so \( V_S > V_P \). They are commonly used in power stations to increase the voltage for long-distance power transmission, which reduces energy loss.
2. Step-down transformer: This type of transformer is used to decrease (step down) a high alternating voltage to a lower alternating voltage. In a step-down transformer, the secondary coil has fewer turns of wire \( (N_S) \) compared to the primary coil \( (N_P) \), meaning \( N_S < N_P \). As a result, the output voltage \( (V_S) \) is lower than the input voltage \( (V_P) \), so \( V_S < V_P \). These transformers are widely used in everyday electronics like phone chargers and laptop power adapters to safely reduce the high mains voltage to the lower voltage required by the devices.
In simple words: A transformer is a device that changes the voltage of alternating current. There are two types: a step-up transformer increases low voltage to high voltage by having more loops in its second coil. A step-down transformer does the opposite, changing high voltage to low voltage, and has fewer loops in its second coil.

🎯 Exam Tip: Clearly differentiate step-up and step-down transformers by their voltage transformation (increasing vs. decreasing) and the relative number of turns in their primary and secondary coils.

 

Question 3. Draw a neat diagram of an AC generator and explain its working.
Answer:

An alternating current (AC) generator, as shown in the diagram, generates electricity based on the principle of electromagnetic induction. Its main components are:

• A rotating rectangular coil (ABCD), known as the armature, which is placed between the two poles of a strong permanent magnet.
• The two ends of this coil are connected to two separate slip rings, \( S_1 \) and \( S_2 \). The inner surfaces of these rings are insulated.
• Two conducting stationary brushes, \( B_1 \) and \( B_2 \), make contact with the slip rings \( S_1 \) and \( S_2 \) respectively.
• The slip rings \( S_1 \) and \( S_2 \) are internally attached to an axle, which can be mechanically rotated from outside to spin the coil inside the magnetic field.
• The outer ends of the two brushes are connected to an external circuit, usually containing a load like a galvanometer.

Working:
When the armature coil is rotated mechanically within the magnetic field, the magnetic flux linked with the coil continuously changes. This change in magnetic flux induces an electric current in the coil, as per Faraday's laws of electromagnetic induction.
The direction of this induced current is determined by Fleming's Right-Hand Rule. For example, during the first half of rotation, the current might flow along ABCD in the coil, causing it to flow from brush \( B_2 \) to \( B_1 \) in the outer circuit.
During the second half of rotation, the direction of the current in the coil reverses, flowing along DCBA. Consequently, in the outer circuit, the current flows from brush \( B_1 \) to \( B_2 \).
As the coil continues to rotate, the direction of the induced current in the external circuit keeps changing every half rotation. This periodic reversal of current is what produces alternating current (AC).
In simple words: An AC generator creates electricity by spinning a coil in a magnetic field. As the coil turns, the magnetic field changes through it, causing an electric current to flow. The current's direction changes every half turn, producing alternating current, which is collected by slip rings and brushes.

🎯 Exam Tip: Master the diagram of an AC generator and be able to describe how the rotation of the coil induces a continuously changing alternating current.

Intext Activities

 

Activity 1
Put a magnet on a table and place some paper clips nearby. If you push the magnet slowly towards the paper clips, there will be a point at which the paper clips jump across and stick to the magnet. What do you understand from this?
Answer: This activity shows that magnets have an invisible field around them, called a magnetic field. Within this field, magnets can pull in (attract) magnetic materials like paper clips without touching them directly. The clips jump to the magnet when they enter this field. This proves that magnets have an invisible force, which can attract certain objects from a distance.
In simple words: When a magnet gets close to paper clips, they jump and stick. This tells us magnets have a hidden force field around them that pulls certain things.

🎯 Exam Tip: Activities like this demonstrate the existence of a magnetic field and the attractive force magnets exert on ferromagnetic materials.

Aim: To study the property of a magnet.

Materials required : Strong bar magnet, paper clips.

Procedure: Case (i)

• Put a bar magnet on a table and place some paper clips nearby.
• If you push the magnet slowly towards the paper clips, there will be a point at which the paper clips jump across and stick to the magnet. What do you understand from this?

Conclusion:

• From the above activity, we can conclude that magnets have an invisible field all around them which attracts magnetic materials.
• In this space we can feel the force of attraction or repulsion due to the magnet.
• Thus magnetic field is the region around the magnet where its magnetic influence can be felt. It is called a magnetic field.

 

Activity 2
Take a cardboard and thread a wire perpendicular through it. Connect the wire such that current flows up the wire. Switch on the circuit. Let the current flow. Place a magnetic compass on the cardboard and mark the position. Now move the magnet and mark the new position. If you join all the points you will find that it is a circle. Reverse the direction of the current, you will find the magnetic circles are clockwise.
Aim: To observe the magnetic field around a straight conductor carrying current.

Materials Required:

• Thick copper wire (conductor)
• Connecting wires
• Battery
• Key
• White cardboard
• Magnetic compass

Procedure:

• Fix the white cardboard horizontally.
• Make a small hole at its center.
• Pass the thick copper wire through the hole, making it perpendicular to the cardboard.
• Connect the ends of the thick copper wire to a battery and a key using connecting wires.
• Switch on the circuit to allow current to flow.
• Place a magnetic compass on the cardboard.
• Mark the S and N points of the compass as X and Y on the cardboard.
• Move the compass so its S end touches Y.
• Next, mark the N end as Z. Move the compass so its S end touches Z. Repeat these steps.
• Join all the marked points. Observe what happens.
• Keep the compass away from the center and follow the pattern.

Observation:

• It is observed that the pattern formed is a circle.
• Another magnetic line can be drawn, and these magnetic lines are concentric circles. They will also be found to be anti-clockwise.
• When the direction of the current is reversed, the direction of the magnetic lines of force becomes clockwise.

Activity - 3

Create Your Own Electromagnet

Aim:

To create your own electromagnet.

Materials Required:

Iron nail, insulated copper wire, battery, and paper clips.

Procedure:

• Take an iron nail and wind ten or more loops of insulated copper wire around it. Make sure the diameter of the loop is not more than a centimeter in width.
• Connect the ends of the wire to a key and a battery.
• Switch on the key and bring some paper clips close to the nail.
• Switch off the key. Observe what happens.

Observation:

When the key is switched on, the electromagnet attracts the paper clips because current flows through the coil wound around the iron nail. When the key is switched off, the paper clips detach from the iron nail.

Conclusion:

An electromagnet is a temporary magnet that acts like a magnet only when an electric current is present.

9th Science Guide Magnetism and Electromagnetism Additional Important Questions And Answers

Choose The Correct Answer:

 

Question 1. The direction of the magnetic field around a straight conductor carrying current can be determined by ____________.
(a) Fleming's left-hand rule
(b) Lenz's law
(c) Right-hand thumb rule
(d) Fleming's right-hand rule
Answer: (a) Fleming's left-hand rule
In simple words: Fleming's left-hand rule helps us find the direction of the magnetic field when current flows through a straight wire. This rule links the direction of current with the resulting magnetic field.

🎯 Exam Tip: Remember to distinguish between Fleming's Left-Hand Rule (for force/motion) and Right-Hand Thumb Rule (for magnetic field direction around a current-carrying wire).

 

Question 2. The magnetic field produced due to a circular wire at its centre is ____________.
(a) at 45° to the plane of the wire
(b) at 60° to the plane of the wire
(c) in the plane of the wire
(d) perpendicular to the plane of the wire
Answer: (d) perpendicular to the plane of the wire
In simple words: For a circular loop carrying current, the magnetic field lines become straight and point directly out of or into the center of the loop. This means they are at a 90-degree angle to the flat surface of the wire circle.

🎯 Exam Tip: Visualizing the magnetic field lines as straight lines passing through the center of the loop helps understand its perpendicular nature.

 

Question 3. A magnetic field exerts no force on ____________.
(a) stationary electric charge
(b) a magnet
(c) an electric charge moving perpendicular to its direction
(d) an unmagnetized iron bar.
Answer: (a) stationary electric charge
In simple words: A magnetic field only pushes or pulls on charges that are moving, not on charges that are still. It does not affect a charge that is not moving at all.

🎯 Exam Tip: Recall that the Lorentz force, which describes the force on a charge in a magnetic field, depends on the charge's velocity.

 

Question 4. At the centre of a magnet, the magnetism is ____________.
(a) zero
(b) same as the poles
(c) maximum
(d) minimum
Answer: (a) zero
In simple words: The magnetic strength of a magnet is strongest at its ends (the poles) and weakest in the middle. At the very center, there is no magnetic force pulling or pushing.

🎯 Exam Tip: Magnetic field lines are densest at the poles, indicating maximum strength, and spread out in the middle, showing less influence.

 

Question 5. Induced current flows through a coil ____________.
(a) more than the period during which flux changes through it.
(b) less than the period during which flux changes through it
(c) only for the period during which flux changes through it
(d) None of the options
Answer: (c) only for the period during which flux changes through it
In simple words: An induced current only flows in a coil when the magnetic field passing through it is changing. As soon as the magnetic field stops changing, the induced current also stops.

🎯 Exam Tip: This is a core concept of electromagnetic induction, stating that a changing magnetic flux is essential for inducing current.

 

Question 6. Which of the following instruments works by electromagnetic induction?
(a) dynamo
(b) moving coil galvanometer
(c) telephone receiver
(d) simple motor
Answer: (a) dynamo
In simple words: A dynamo (which is another name for an electric generator) creates electricity by using movement to change magnetic fields. This process is called electromagnetic induction.

🎯 Exam Tip: Dynamos and generators use electromagnetic induction to convert mechanical energy into electrical energy.

 

Question 7. For making a strong electromagnet, the material of the core should be ____________.
(a) brass
(b) laminated steel strips
(c) soft iron
(d) steel
Answer: (c) soft iron
In simple words: Soft iron is the best material for making electromagnets because it can be easily magnetized when current flows and easily demagnetized when the current is turned off. This quick change makes it very useful for electromagnets.

🎯 Exam Tip: Soft iron is chosen for temporary magnets (electromagnets) due to its high magnetic permeability and low retentivity.

 

Question 8. Magnetic field lines determine ____________.
(a) the shape of the magnetic field
(b) only the direction of the magnetic field
(c) only the relative strength of the magnetic field
(d) both the direction and the relative strength of the magnetic field
Answer: (d) both the direction and the relative strength of the magnetic field
In simple words: Magnetic field lines show us two things: the direction in which a north pole would move (direction) and how strong the magnetic force is in different places (strength). Where the lines are closer, the field is stronger.

🎯 Exam Tip: The tangent at any point on a magnetic field line gives the direction, and the density of lines indicates the strength of the field.

 

Question 9. A magnetic needle is kept in a non-uniform magnetic field. It experiences ____________.
(a) a force and a torque
(b) a force but not a torque
(c) a torque but not a force
(d) neither a force nor a torque
Answer: (a) a force and a torque
In simple words: In a non-uniform magnetic field, the forces on the two poles of the needle are not equal or opposite. This causes both a twisting effect (torque) and a pushing/pulling effect (force) on the needle.

🎯 Exam Tip: In a uniform magnetic field, a needle only experiences torque; in a non-uniform field, it experiences both force and torque.

 

Question 10. The most suitable material for making permanent magnets is ____________.
(a) copper
(b) aluminum
(c) iron
(d) steel
Answer: (d) steel
In simple words: Steel is the best material for making permanent magnets because it stays magnetized for a long time after being exposed to a magnetic field. This is due to its strong ability to hold its magnetism.

🎯 Exam Tip: Materials with high retentivity and coercivity, like steel, are ideal for making permanent magnets that retain their magnetism for extended periods.

 

Question 11. An electric generator converts ____________.
(a) electrical energy into mechanical energy
(b) mechanical energy into heat energy
(c) electrical energy into electrical energy
(d) mechanical energy into electrical energy
Answer: (d) mechanical energy into electrical energy
In simple words: An electric generator takes energy from movement, like spinning a turbine, and turns it into electrical energy. It is the opposite of an electric motor.

🎯 Exam Tip: Remember that a generator produces electricity, so it converts another form of energy (mechanical) into electrical energy.

II. Fill In The Blanks:

 

Question 1. The magnetic field inside a ___________ is uniform.
Answer: magnet
In simple words: Inside a bar magnet, or more specifically, inside a long solenoid, the magnetic field is generally the same strength everywhere. This is considered a uniform field.

🎯 Exam Tip: Understand that a uniform magnetic field implies constant strength and direction within a specific region, which is ideal in solenoids.

 

Question 2. An AC generator is provided with ___________ slip rings which rotate with the coil.
Answer: two
In simple words: An AC generator uses two slip rings that turn with the coil to collect the electricity it makes. These rings ensure the current can be continuously collected from the rotating coil.

🎯 Exam Tip: AC generators use two slip rings to maintain a continuous connection to the external circuit as the coil rotates, transferring alternating current.

 

Question 3. The moving part of an electric motor is called ____________.
Answer: shaft
In simple words: In an electric motor, the part that spins and does the work is called the shaft. This spinning shaft is where the mechanical energy comes out.

🎯 Exam Tip: The armature, along with the shaft, forms the rotor (rotating part) of the motor, responsible for converting electrical energy into mechanical motion.

 

Question 4. A magnetic field is a ____________ quantity.
Answer: Vector
In simple words: A magnetic field is a vector because it has both a strength (how strong it is) and a direction (where it points). You need both pieces of information to describe it fully.

🎯 Exam Tip: Remember that vector quantities have both magnitude and direction, differentiating them from scalar quantities which only have magnitude.

 

Question 5. The SI unit of magnetic field strength is ____________.
Answer: Tesla
In simple words: The standard way to measure the strength of a magnetic field is using a unit called Tesla. A higher Tesla value means a stronger magnetic field.

🎯 Exam Tip: The Tesla (T) is a very large unit, so smaller units like Gauss are often used for weaker magnetic fields.

 

Question 6. The laws of induction were given by ____________.
Answer: Faraday
In simple words: Michael Faraday was the scientist who discovered how changing magnetic fields can create electric currents. His work led to the laws of induction, which explain this process.

🎯 Exam Tip: Faraday's Law of Induction is a fundamental principle in electromagnetism, describing how a varying magnetic field induces an electromotive force.

 

Question 7. The relation between weber and Tesla is ____________.
Answer: \( 1 \text{Wb} = 1\text{T} \times 1\text{m}^2 \)
In simple words: One Weber is the unit for magnetic flux, which tells you how much magnetic field passes through an area. One Tesla is the unit for magnetic field strength. This formula shows that if you have a magnetic field of 1 Tesla across an area of 1 square meter, the total magnetic flux is 1 Weber.

🎯 Exam Tip: Magnetic flux (Weber) is the product of magnetic field strength (Tesla) and the area it passes through (square meters).

 

Question 8. Unlike magnetic poles ___________ whereas like poles __________.
Answer: attract, repel
In simple words: Magnets behave like people: opposite poles (north and south) pull each other together, but poles that are the same (north and north, or south and south) push each other away. This is a basic rule of magnetism.

🎯 Exam Tip: Always remember the fundamental rule: "Opposites attract, likes repel" when dealing with magnetic poles.

 

Question 9. Magnetic lines of force never ___________ each other.
Answer: intersect
In simple words: Magnetic field lines are like invisible paths; they never cross each other. If they did, it would mean the magnetic field has two directions at one point, which is not possible.

🎯 Exam Tip: The property of magnetic field lines never intersecting is crucial for the uniqueness of the magnetic field direction at any given point.

 

Question 10. Sailors use ___________ to find direction in order to navigate on the sea.
Answer: Magnetic compass
In simple words: Sailors use a magnetic compass, which is a tool with a needle that always points north. This helps them know their direction on the sea and find their way.

🎯 Exam Tip: The Earth's magnetic field allows compasses to function, making them vital navigation tools for centuries.

 

Question 11. The strongest natural magnet is ____________.
Answer: lodestone magnetite
In simple words: Lodestone, also known as magnetite, is a type of rock that is naturally magnetic and is considered the strongest natural magnet. It was one of the first magnetic materials discovered and used by humans.

🎯 Exam Tip: Lodestone is a naturally occurring mineral (iron oxide) that exhibits permanent magnetism.

 

Question 12. The ___________ produces its own magnetic field, which shields the earth's ozone layer from the ____________ and is important in navigation.
Answer: Earth, solar wind
In simple words: Our planet Earth has its own magnetic field, which acts like a protective shield. This field helps protect the ozone layer from harmful solar wind particles and is why compasses work for navigation.

🎯 Exam Tip: The Earth's magnetic field (magnetosphere) is crucial for protecting the atmosphere from charged particles from the sun, which could otherwise strip away the ozone layer.

 

Question 13. Sea turtles return to their birth beach many decades after they were born due to ____________.
Answer: geomagnetic imprinting
In simple words: Sea turtles can find their way back to the exact beach where they were born, even after many years, by using the Earth's magnetic field as a map. This ability is called geomagnetic imprinting.

🎯 Exam Tip: Geomagnetic imprinting is a fascinating example of how animals use subtle environmental cues, like magnetic fields, for long-distance navigation.

 

Question 14. Magnetic field at a point is ___________ to the magnetic field lines.
Answer: tangential
In simple words: At any point, the direction of the magnetic field is shown by drawing a line that just touches the magnetic field line at that point. This line is called a tangent.

🎯 Exam Tip: The tangent to a magnetic field line at any point indicates the direction of the magnetic force a north pole would experience at that specific location.

 

Question 15. ___________ is the number of magnetic field lines passing through a given area.
Answer: Magnetic flux
In simple words: Magnetic flux is a measurement of how many magnetic field lines pass through a certain space. It tells us the total amount of magnetic field in that area.

🎯 Exam Tip: Magnetic flux is a key concept in Faraday's law of induction, as its change over time leads to an induced electromotive force.

 

Question 16. The direction of the magnetic lines around a current-carrying conductor can be easily understood using the ___________ rule.
Answer: right-hand thumb rule
In simple words: When you hold a current-carrying wire with your right hand, your thumb points in the direction of the current. Your curled fingers then show the direction of the magnetic field lines around the wire.

🎯 Exam Tip: The right-hand thumb rule is essential for determining the direction of the magnetic field produced by a straight current-carrying conductor.

 

Question 17. The magnetic field lines are stronger near the ___________ and it diminishes as you go away from it.
Answer: current-carrying wire
In simple words: The magnetic field is strongest very close to the wire where the current is flowing. As you move further away from the wire, the magnetic field becomes weaker.

🎯 Exam Tip: The strength of the magnetic field generated by a current-carrying wire is inversely proportional to the distance from the wire.

 

Question 18. A charge moving in a magnetic field, in a direction other than the direction of the magnetic field, experiences a force. It is called the ___________ force.
Answer: magnetic Lorentz
In simple words: When a charged particle moves through a magnetic field, it feels a push or pull if its movement is not parallel to the field. This specific force is known as the magnetic Lorentz force.

🎯 Exam Tip: The Lorentz force is a combined effect of electric and magnetic fields on a charged particle, where the magnetic component is crucial for understanding its behavior in a magnetic field.

 

Question 19. When the conductor is perpendicular to the magnetic field, the force will be the ___________ When it is parallel to the magnetic field, the force will be ___________.
Answer: maximum, zero
In simple words: A conductor feels the strongest magnetic force when it is at a 90-degree angle to the magnetic field. However, if the conductor is perfectly aligned with the magnetic field (parallel), it feels no force at all.

🎯 Exam Tip: The force on a current-carrying conductor in a magnetic field depends on the sine of the angle between the current and the field, explaining maximum force at 90° and zero force at 0° or 180°.

 

Question 20. An electric bell contains an ___________, consisting of coils of insulated wire wound around iron rods.
Answer: electromagnet
In simple words: Inside an electric bell, there is an electromagnet. This is made of wire coiled around an iron piece, which becomes magnetic when electricity flows through it, causing the bell to ring.

🎯 Exam Tip: Electromagnets are used in many devices, like electric bells, because their magnetic properties can be turned on and off with electricity.

 

Question 21. No force acts in a current-carrying conductor when it is to the magnetic field.
Answer: parallel
In simple words: If a wire carrying electricity is placed exactly in the same direction as a magnetic field, it will not feel any push or pull from that field. The force only happens when the wire cuts across the magnetic field lines.

🎯 Exam Tip: This reinforces the concept that the magnetic force on a current-carrying conductor is zero when the conductor is parallel to the magnetic field.

III. Match The Following:

 

Question 1.

Answer: 1-a, 2-b, 3-d, 4-c
In simple words: An electric generator works using electromagnetic induction. A dynamo is linked to Fleming's left-hand rule. Magnetic field lines typically go from north to south. Electric motors use slip rings.

🎯 Exam Tip: Matching questions often test fundamental concepts; ensure you clearly understand the core function or property of each item.

 

Question 2.

Answer: 1-b, 2-c, 3-d, 4-a
In simple words: Hans Christian Oersted discovered the link between electricity and magnetism. The force on a conductor is calculated using F=BIL. An electric bell uses an electromagnet. When the number of turns in the secondary coil is less than the primary coil, it describes a step-down transformer.

🎯 Exam Tip: Understand the key scientists associated with fundamental discoveries, formulas, applications, and principles of transformers for accurate matching.

IV. State Whether True Or False. If False, Correct The Statement:

 

Question 1. Fleming's left-hand rule is also known as the dynamo rule.
Answer: False.
Correct statement: Fleming's right-hand rule is also known as the dynamo rule.
In simple words: Fleming's left-hand rule helps figure out the direction of force on a current-carrying wire in a magnetic field, which is used in motors. The right-hand rule is for generators, helping to find the direction of the induced current.

🎯 Exam Tip: Clearly differentiate between Fleming's Left-Hand Rule (for motors, direction of force) and Right-Hand Rule (for generators, direction of induced current).

 

Question 2. Magnetic field lines always repel each other and do not intersect.
Answer: True.
In simple words: Magnetic field lines are like invisible paths that never cross each other. They push away from each other, which shows that the magnetic field has a clear direction at every point.

🎯 Exam Tip: The non-intersecting nature of magnetic field lines ensures a unique direction for the magnetic field at any point.

 

Question 3. A current flowing in a wire gives rise to a magnetic field.
Answer: True.
In simple words: When electricity flows through a wire, it creates an invisible magnetic field around that wire. This discovery showed the strong connection between electricity and magnetism.

🎯 Exam Tip: This statement describes Oersted's discovery, a foundational concept in electromagnetism.

 

Question 4. The magnetic field lines due to a straight wire carrying current are parallel.
Answer: False.
Correct statement: The magnetic field lines due to a straight wire carrying current are concentric circles.
In simple words: Around a straight wire carrying current, the magnetic field lines are not parallel. Instead, they form perfect circles around the wire, like rings around a target, with the wire in the middle.

🎯 Exam Tip: Use the right-hand thumb rule to visualize and remember the circular nature of magnetic field lines around a straight current-carrying conductor.

 

Question 5. A generator converts mechanical energy into electrical energy.
Answer: True.
In simple words: A generator is a machine that takes movement energy (mechanical energy) and changes it into electricity (electrical energy). It's how power plants make the electricity we use.

🎯 Exam Tip: Remember that generators are key devices in energy conversion, making them essential for electricity production.

 

Question 6. The magnetic field is a quantity that has magnitude only.
Answer: False.
Correct statement: The magnetic field is a quantity that has magnitude and direction.
In simple words: A magnetic field has both a strength (magnitude) and a direction, so it's a vector quantity. You need to know both how strong it is and where it points to describe it fully.

🎯 Exam Tip: Always recall that magnetic fields, like electric fields, are vector quantities and must be described by both their strength and direction.

 

Question 7. Magnetic fields do not interact with electric charges in motion.
Answer: False.
Correct statement: Magnetic fields do not interact with electric charges at rest.
In simple words: Magnetic fields do not affect electric charges that are sitting still. However, if an electric charge starts to move, then the magnetic field will interact with it and exert a force.

🎯 Exam Tip: Understand that the force exerted by a magnetic field on a charged particle is velocity-dependent, meaning a stationary charge experiences no magnetic force.

 

Question 8. The force on a charged particle moving in a magnetic field is maximum when the angle between the direction of motion and field is 90°.
Answer: True.
In simple words: When a charged particle moves exactly across the magnetic field lines (at a 90-degree angle), it feels the strongest possible force. This angle causes the biggest interaction.

🎯 Exam Tip: The magnetic force on a charged particle is proportional to the sine of the angle between its velocity and the magnetic field, thus maximum at 90°.

 

Question 9. An electric motor converts mechanical energy into electrical energy.
Answer: False.
Correct statement: An electric motor converts electrical energy into mechanical energy.
In simple words: An electric motor takes in electricity and uses it to create movement (mechanical energy). It's the opposite of a generator, which makes electricity from movement.

🎯 Exam Tip: Distinguish clearly between motors (electrical to mechanical) and generators (mechanical to electrical) based on their energy conversion functions.

 

Question 10. The field at the centre of the long circular coil carrying current will be parallel straight lines.
Answer: False.
Correct statement: The field at the centre of the long circular coil carrying current will be perpendicular straight lines.
In simple words: Inside a long coil, especially at its center, the magnetic field lines are straight and run perpendicular to the plane of the coil. They are not parallel to the plane of the coil itself.

🎯 Exam Tip: For a circular coil, the magnetic field lines at the center are uniform and perpendicular to the coil's plane.

 

Question 11. A wire with green insulation is usually the live wire of an electric supply.
Answer: False.
Correct statement: A wire with green insulation is usually the earth wire of an electric supply.
In simple words: In electrical wiring, a green insulated wire is typically used for the earth connection, which provides safety. The live wire, which carries the main electricity, is usually red or brown.

🎯 Exam Tip: Knowing the standard color codes for electrical wires (live, neutral, earth) is crucial for safety and correctly identifying components.

 

Question 12. An electric generator works on the principle of electromagnetic induction.
Answer: True.
In simple words: Electric generators create electricity by using the idea of electromagnetic induction. This means they make current flow by moving a conductor through a magnetic field, or by changing the magnetic field around a conductor.

🎯 Exam Tip: Electromagnetic induction is the fundamental principle behind how generators produce electricity.

 

Question 13. Permanent magnets are made of hard magnetic materials like alnico.
Answer: True.
In simple words: Permanent magnets are made from materials like alnico (an alloy of aluminum, nickel, and cobalt). These hard materials keep their magnetism for a very long time, making them suitable for permanent magnets.

🎯 Exam Tip: Alnico is a common alloy used for permanent magnets due to its high coercivity and strong magnetic properties.

 

Question 14. The pole of a freely suspended magnet which points towards the north is called the south pole and what points towards south is celled north pole.
Answer: False.
Correct statement: The pole of a freely suspended magnet that points towards the north is called the north pole and that points towards the south are called a south pole.
In simple words: When a magnet is hung freely, the end that points to the Earth's geographic north is called the magnet's North pole. The end that points to the Earth's geographic south is called the magnet's South pole.

🎯 Exam Tip: Remember that a magnet's north pole is attracted to the Earth's geographic north (which is actually a magnetic south pole).

 

Question 15. The magnetic poles always exist in pairs.
Answer: True.
In simple words: You can never have a single North pole or a single South pole by itself. If you break a magnet, each new piece will still have both a North and a South pole. They always come in pairs.

🎯 Exam Tip: The non-existence of magnetic monopoles is a fundamental concept in magnetism, meaning poles always appear in pairs.

 

Question 16. A galvanometer is an instrument that can detect the presence of a current in a circuit.
Answer: True.
In simple words: A galvanometer is a sensitive tool used to find out if there's an electric current flowing in a circuit. It can also tell you how strong the current is.

🎯 Exam Tip: Galvanometers are crucial for detecting small currents, often serving as the basis for ammeters and voltmeters.

V. Assertion And Reason Type Questions:

Mark the correct choice as:
(a) If both assertion and reason are true and the reason is the correct explanation of assertion.
(b) If both assertion and reason are true but the reason is not the correct explanation of assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true