Class 10 Chapter 2 Magnetic Effect of Electric Current Physics Solution S.Chand. Part 4

 Part 4 Page 91

Very Short Answer Type Questions

1. What happens when a current-carrying conductor is placed in a magnetic field ?

1. When a current-carrying conductor is placed in a magnetic field, it experiences a force perpendicular to both the direction of the current and the magnetic field lines. This phenomenon is known as the magnetic force or Lorentz force.

2. When is the force experienced by a current-carrying conductor placed in a magnetic field largest ?

The force experienced by a current-carrying conductor placed in a magnetic field is largest when the direction of the current is perpendicular to the direction of the magnetic field.

3. In a statement of Fleming’s left-hand rule, what do the following represent ?

(a) direction of centre finger.

(b) direction of forefinger.

(c) direction of thumb.

In Fleming’s left-hand rule:

(a) The direction of the centre finger represents the direction of the magnetic field.

(b) The direction of the forefinger represents the direction of the current. (c) The direction of the thumb represents the direction of the force acting on the conductor.

4. Name one device which works on the magnetic effect of current.

An example of a device that works on the magnetic effect of current is an electric motor.

5. Name the device which converts electrical energy into mechanical energy.

The device that converts electrical energy into mechanical energy is called an electric motor.

6. A motor converts one form of energy into another. Name the two forms.

A motor converts electrical energy into mechanical energy.

7. State whether the following statement is true or false :

An electric motor converts mechanical energy into electrical energy.

False. An electric motor converts electrical energy into mechanical energy.

8. For Fleming’s left-hand rule, write down the three things that are 90° to each other, and next to each one

write down the finger or thumb that represents it.

For Fleming’s left-hand rule:

• Magnetic field direction is represented by the thumb.
• Current direction is represented by the forefinger.
• Force direction is represented by the middle finger.

9. Name the device which is used to reverse the direction of current in the coil of a motor.

The device used to reverse the direction of current in the coil of a motor is called a commutator.

10. What is the other name of the split ring used in an electric motor ?

The other name of the split ring used in an electric motor is a commutator.

11. What is the function of a commutator in an electric motor ?

The function of a commutator in an electric motor is to reverse the direction of the current in the coil, ensuring that the coil continues to rotate in the same direction.

12. Of what substance are the brushes of an electric motor made ?

The brushes of an electric motor are usually made of carbon or graphite.

13. Of what substance is the core of the coil of an electric motor made ?

The core of the coil of an electric motor is typically made of soft iron or other ferromagnetic materials.

14. In an electric motor, which of the following remains fixed and which rotates with the coil ?

Commutator ; Brush

In an electric motor, the commutator rotates with the coil, while the brushes remain fixed.

15. What is the role of the split ring in an electric motor ?

The role of the split ring (commutator) in an electric motor is to reverse the direction of the current in the coil as it rotates, ensuring continuous rotation in the same direction.

16. Fill in the following blanks with suitable words : (a) Fleming’s Rule for the motor effect uses the left hand. (b) A motor contains a kind of switch called a commutator which reverses the current every half rotation.

Short Answer Type Questions

17. (a) A current-carrying conductor is placed perpendicularly in a magnetic field. Name the rule which can be

used to find the direction of force acting on the conductor.

(b) State two ways to increase the force on a current-carrying conductor in a magnetic field.

(c) Name one device whose working depends on the force exerted on a current-carrying coil placed in a

magnetic field

(a) The rule used to find the direction of force acting on a current-carrying conductor placed perpendicularly in a magnetic field is the Right-Hand Rule.

17. (b) Two ways to increase the force on a current-carrying conductor in a magnetic field are increasing the current flowing through the conductor and increasing the strength of the magnetic field. (c) An example of a device whose working depends on the force exerted on a current-carrying coil placed in a magnetic field is an electric motor.

18. State Fleming’s left-hand rule. Explain it with the help of labelled diagrams.

Fleming’s left-hand rule states that if the forefinger, middle finger, and thumb of the left hand are stretched mutually perpendicular to each other, and if the forefinger represents the direction of the magnetic field, the thumb represents the direction of motion (force), then the direction in which the middle finger points represents the direction of current. It's a mnemonic for determining the direction of force experienced by a current-carrying conductor in a magnetic field.

19. What is the principle of an electric motor ? Name some of the devices in which electric motors are used.

The principle of an electric motor is based on the interaction between a current-carrying conductor and a magnetic field, resulting in the generation of mechanical motion. Electric motors are used in various devices such as fans, washing machines, elevators, electric vehicles, and industrial machinery.

20. (a) In a d.c. motor, why must the current to the coil be reversed twice during each rotation ?

(b) What device reverses the current ?

(a) In a d.c. motor, the current to the coil must be reversed twice during each rotation to ensure continuous rotation in the same direction. (b) The device that reverses the current is called a commutator.

21. (a) State what would happen to the direction of rotation of a motor if : (i) the current were reversed (ii) the magnetic field were reversed (iii) both current and magnetic field were reversed simultaneously. (b) In what ways can a motor be made more powerful ?

(a) (i) Reversing the current in a motor would reverse the direction of its rotation. (ii) Reversing the magnetic field in a motor would also reverse the direction of its rotation. (iii) If both the current and the magnetic field in a motor were reversed simultaneously, the direction of rotation would remain the same as the original direction. (b) A motor can be made more powerful by increasing the current flowing through it, increasing the number of turns in the coil, or using stronger magnets to create a stronger magnetic field.

 Long Answer Type Questions

22. (a) What is an electric motor ? With the help of a labelled diagram, describe the working of a simple electric

motor.

(b) What are the special features of commercial electric motors ?

(a) An electric motor is a device that converts electrical energy into mechanical energy by utilizing the interaction between magnetic fields and electric currents.

• Working of a simple electric motor: In a simple electric motor, a rectangular coil is placed between two magnetic poles. The coil is connected to a DC power source through a split-ring commutator and brushes. When current flows through the coil, a magnetic field is produced, interacting with the external magnetic field created by the permanent magnets. This interaction causes a torque on the coil, causing it to rotate. As the coil rotates, the split-ring commutator ensures that the direction of the current in the coil reverses every half rotation, allowing continuous rotation in the same direction. This continuous rotation can be used to perform mechanical work.

(b) Special features of commercial electric motors include:

• Efficiency: Commercial electric motors are designed to operate efficiently, converting a high percentage of electrical energy into mechanical energy.
• Durability: They are built with durable materials and components to withstand prolonged use and environmental conditions.
• Variable Speed: Many commercial motors have variable speed controls to adapt to different operating conditions and requirements.
• Size and Power Range: Commercial motors are available in a wide range of sizes and power ratings to suit various applications, from small household appliances to industrial machinery.
• Safety Features: They may include safety features such as overload protection and thermal protection to prevent damage from excessive heat or current overload.
• Ease of Maintenance: Commercial motors are designed for easy maintenance and repair, with accessible components and standardized parts.

Multiple Choice Questions (MCQs)

23. In an electric motor, the direction of current in the coil changes once in each :

(a) two rotations (b) one rotation (c) half rotation (d) one-fourth rotation

24. An electron beam enters a magnetic field at right angles to it as shown in the Figure.

The direction of force acting on the electron beam will be :

(a) to the left (b) to the right (c) into the page (d) out of the page

25. The force experienced by a current-carrying conductor placed in a magnetic field is the largest when the

angle between the conductor and the magnetic field is :

(a) 45° (b) 60° (c) 90° (d) 180°

26. The force exerted on a current-carrying wire placed in a magnetic field is zero when the angle between the

wire and the direction of magnetic field is :

(a) 45° (b) 60° (c) 90° (d) 180°

27. A current flows in a wire running between the S and N poles of a magnet lying horizontally as shown in

Figure below :

The force on the wire due to the magnet is directed :

(a) from N to S (b) from S to N (c) vertically downwards (d) vertically upwards

28. An electric motor is a device which transforms :

(a) mechanical energy to electrical energy

(b) heat energy to electrical energy

(c) electrical energy to heat energy only

(d) electrical energy to mechanical energy

29. A magnetic field exerts no force on :

(a) an electric charge moving perpendicular to its direction (b) an unmagnetised iron bar

(c) a stationary electric charge (d) a magnet

30. A horizontal wire carries a current as shown in Figure below between magnetic poles N and S :

Is the direction of the force on the wire due to the magnet :

(a) in the direction of the current (b) vertically downwards

(c) opposite to the current direction (d) vertically upwards

 Questions Based on High Order Thinking Skills (HOTS)

31. In the simple electric motor of figure given below, the coil rotates anticlockwise as seen by the eye from the

position X when current flows in the coil.

Is the current flowing clockwise or anticlockwise around the coil when viewed from above ?

Clockwise

32. Which way does the wire in the diagram below tend to move ?

Upward (out of the page)

33. If the current in a wire is flowing in the vertically downward direction and a magnetic field is applied from

west to east, what is the direction of force on the wire ?

The direction of force on the wire can be determined using the right-hand rule for the magnetic force on a current-carrying conductor:

• Point the thumb of your right hand in the direction of the current (vertically downward).
• Point your fingers in the direction of the magnetic field (from west to east).

The direction in which your palm faces indicates the direction of the force on the wire. In this case, if the current is flowing vertically downward and the magnetic field is applied from west to east, the force on the wire would be directed towards the south.

34. Which way does the wire in the diagram below tend to move ?

Downward (into the page)

35. What is the force on a current-carrying wire that is parallel to a magnetic field ? Give reason for your answer.

The force on a current-carrying wire that is parallel to a magnetic field is zero.

This is because the force experienced by a current-carrying wire in a magnetic field is given by the formula:

$𝐹=𝐼\cdot 𝐿\cdot 𝐵\cdot \sin (𝜃)$

Where:

• $𝐹$ is the force on the wire,
• $𝐼$ is the current flowing through the wire,
• $𝐿$ is the length of the wire in the magnetic field,
• $𝐵$ is the magnetic field strength, and
• $𝜃$ is the angle between the direction of the current and the direction of the magnetic field.

When the wire is parallel to the magnetic field, the angle $𝜃$ between the current and the magnetic field is $0^{\circ }$, and the sine of $0^{\circ }$ is $0$. Therefore, the force $𝐹$ on the wire becomes zero.

36. A charged particle enters at right angles into a uniform magnetic field as shown :

What should be the nature of charge on the particle if it begins to move in a direction pointing vertically out

of the page due to its interaction with the magnetic field ?

The charged particle in the image needs to be positively charged to move in a direction pointing vertically out of the page when entering a uniform magnetic field at a right angle.

We can determine the charge of the particle using Fleming's left-hand rule. This rule states that if you extend your thumb, index finger, and middle finger of your left hand perpendicular to each other, the thumb points in the direction of the motion of the positive charge (conventional current), the index finger points in the direction of the magnetic field, and the middle finger points in the direction of the force exerted on the charged particle.

In the image, the magnetic field is entering the page (represented by a dot), and the force is exerted vertically out of the page. Using Fleming's left-hand rule, the conventional current must be moving downwards. Since conventional current is the flow of positive charge, the particle must be positively charged.

Alternatively, the right-hand rule can be used to determine the direction of the force exerted on a moving charged particle in a magnetic field. This rule states that if you curl your right hand fingers in the direction of the current (positive charge flow) and point your thumb in the direction of the magnetic field, your fingers will point in the direction of the force acting on the particle.

In conclusion, based on Fleming's left-hand rule and the right-hand rule, the charged particle must be positively charged to experience a force in the direction vertically out of the page.