Class 10 Chapter 2 Magnetic Effect of Electric Current Physics Solution S.Chand. Part 5

 Part 5 Page 102

 Very Short Answer Type Questions

1. Name the device which converts mechanical energy into electric energy.

1. The device that converts mechanical energy into electric energy is a generator.

2. Out of an A.C. generator and a D.C. generator :

(a) which one uses a commutator (split rings) ?

(b) which one uses slip rings ?

1. (a) A D.C. generator uses a commutator (split rings). (b) An A.C. generator uses slip rings.

3. Name the phenomenon which is made use of in an electric generator.

1. The phenomenon made use of in an electric generator is electromagnetic induction.

4. Name the rule which gives the direction of induced current.

1. The rule that gives the direction of induced current is Fleming’s right-hand rule.

5. What condition is necessary for the production of current by electromagnetic induction ?

1. The condition necessary for the production of current by electromagnetic induction is the relative motion between a conductor and a magnetic field.

6. What type of generator is used at Power Stations ?

1. Power stations typically use AC generators (alternators).

7. What change should be made in an a.c. generator so that it may become a d.c. generator ?

1. To convert an AC generator into a DC generator, the commutator would need to be replaced with a split ring commutator.

8. State whether the following statements are true or false :

(a) A generator works on the principle of electromagnetic induction.

(b) A motor works on the principle of electromagnetic induction.

1. (a) True. A generator works on the principle of electromagnetic induction. (b) False. A motor works on the principle of electromagnetic interaction, not induction.

9. What is the function of brushes in an electric generator ?

1. The brushes in an electric generator maintain electrical contact between the rotating armature (or rotor) and the stationary components (such as the terminals or slip rings), allowing the flow of current.

10. When a wire is moved up and down in a magnetic field, a current is induced in the wire. What is this

phenomenon known as ?

1. When a wire is moved up and down in a magnetic field, a current is induced in the wire. This phenomenon is known as electromagnetic induction.

11. When current is ‘switched on’ and ‘switched off’ in a coil, a current is induced in another coil kept near it.

What is this phenomenon known as ?

1. When current is 'switched on' and 'switched off' in a coil, a current is induced in another coil kept near it. This phenomenon is known as mutual induction.

12. What is the major difference between the simple alternator and most practical alternators ?

1. The major difference between a simple alternator and most practical alternators is that practical alternators typically have a rotating field magnet and a stationary armature, whereas a simple alternator may have a rotating armature and a stationary field magnet.

13. Why are Thermal Power Stations usually located near a river ?

1. Thermal Power Stations are usually located near a river to utilize water for cooling purposes in the generation of electricity. The water from the river is used as a coolant in the condenser, which helps in dissipating the heat produced during the generation of electricity.

14. List three sources of magnetic fields.

1. Three sources of magnetic fields are:

   • Permanent magnets
   • Electromagnets
   • Current-carrying conductors

15. Complete the following sentence :

A generator with commutator produces...........current.

direct

Short Answer Type Questions

16. Two circular coils A and B are placed close to each other. If the current in coil A is changed, will some

current be induced in the coil B ? Give reason for your answer.

Yes, some current will be induced in coil B if the current in coil A is changed. This is due to the principle of electromagnetic induction, which states that a changing magnetic field induces an electromotive force (EMF) in a nearby conductor. Therefore, the changing current in coil A will create a changing magnetic field, which in turn induces a current in coil B.

17. (a) Explain the principle of an electric generator.

(b) State two ways in which the current induced in the coil of a generator could be increased.

(a) The principle of an electric generator is based on electromagnetic induction. It involves rotating a coil of wire within a magnetic field. As the coil rotates, the magnetic flux through it changes, which induces an electromotive force (EMF) or voltage across the ends of the coil, resulting in the generation of electricity. (b) Two ways to increase the current induced in the coil of a generator are: - Increasing the speed of rotation of the coil within the magnetic field. - Increasing the strength of the magnetic field.

18. (a) What is the difference between alternating current and direct current ?

(b) What type of current is given by (i) a dry cell, and (ii) a Power House generator ?

(a) The main difference between alternating current (AC) and direct current (DC) is the direction of the flow of electric charge. In AC, the direction of the electric current reverses periodically, while in DC, the flow of electric charge remains in one direction. (b) (i) A dry cell typically provides direct current (DC). (ii) A power house generator typically provides alternating current (AC).

19. State and explain Fleming’s right hand rule.

Fleming’s right-hand rule is used to determine the direction of the force experienced by a current-carrying conductor placed in a magnetic field. When the thumb, forefinger, and middle finger of the right hand are extended such that they are mutually perpendicular, if the forefinger points in the direction of the magnetic field, and the middle finger in the direction of the current, then the thumb indicates the direction of the force experienced by the conductor.

20. Name and state the rule to find the direction of :

(a) current induced in a coil due to its rotation in a magnetic field.

(b) force experienced by a current-carrying straight conductor placed in a magnetic field which is

perpendicular to it.

(a) The rule to find the direction of current induced in a coil due to its rotation in a magnetic field is Fleming’s right-hand rule. (b) The rule to find the direction of force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it is the right-hand palm rule.

21. (a) In what respect does the construction of an A.C. generator differ from that of a D.C. generator ?

(b) What normally drives the alternators in a Thermal Power Station ? What fuels can be used to heat water

in the boiler ?

(a) The construction of an AC generator differs from that of a DC generator mainly in the arrangement of the commutator. In an AC generator, slip rings are used to collect the induced current, while in a DC generator, a split-ring commutator is used to convert the induced alternating current into direct current. (b) The alternators in a Thermal Power Station are typically driven by steam turbines. The water in the boiler is heated using fuels such as coal, natural gas, or oil.

Long Answer Type Questions

22. Draw the labelled diagram of an A.C. generator. With the help of this diagram, explain the construction and

working of an A.C. generator.

23. (a) What do you understand by the term “electromagnetic induction”? Explain with the help of a diagram.

(b) Name one device which works on the phenomenon of electromagnetic induction.

(c) Describe different ways to induce current in a coil of wire.

24. (a) What do you understand by the terms ‘direct current’ and ‘alternating current’ ?

(b) Name some sources of direct current and some of alternating current.

(c) State an important advantage of alternating current over direct current.

(d) What is the frequency of A.C. supply in India ?

(a) Understanding Direct Current and Alternating Current:

• Direct Current (DC): Direct current is an electric current that flows in one direction only, maintaining a constant polarity. In other words, the flow of electric charge remains constant in magnitude and direction over time.
• Alternating Current (AC): Alternating current is an electric current that periodically reverses direction, changing polarity over time. It flows first in one direction and then reverses to flow in the opposite direction, continuously oscillating back and forth.

(b) Sources of Direct Current and Alternating Current:

• Sources of Direct Current (DC):

  • Batteries (e.g., dry cells, car batteries)
  • Photovoltaic (solar) cells
  • DC power supplies

• Sources of Alternating Current (AC):

  • Power grid (electricity supplied by utility companies)
  • Generators (produce AC electricity)
  • Inverters (convert DC power to AC power)
  • Portable generators (some produce AC power)

(c) Advantage of Alternating Current over Direct Current: One important advantage of alternating current over direct current is that it can be easily transformed to different voltage levels using transformers. This makes it more suitable for long-distance transmission of electricity over power lines. AC allows for efficient voltage transformation, reducing power losses during transmission.

(d) Frequency of AC Supply in India: The frequency of AC supply in India is typically 50 Hz (Hertz).

Multiple Choice Questions (MCQs)

Questions Based on High Order Thinking Skills (HOTS)

37. A coil is connected to a galvanometer. When the N-pole of a magnet is pushed into the coil, the galvanometer

deflected to the right. What deflection, if any, is observed when :

(a) the N-pole is removed ?

(b) the S-pole is inserted ?

(c) the magnet is at rest in the coil ?

State three ways of increasing the deflection on the galvanometer.

(a) When the N-pole of a magnet is removed from the coil, the galvanometer will deflect to the left. This is because the magnetic field produced by the N-pole of the magnet induces a current in the coil in one direction (according to Lenz's law). When the magnet is removed, the magnetic field collapses, inducing a current in the opposite direction, resulting in a deflection in the opposite direction on the galvanometer.

(b) When the S-pole of a magnet is inserted into the coil, the galvanometer will deflect to the left. This is because the magnetic field produced by the S-pole of the magnet induces a current in the coil in the opposite direction compared to when the N-pole was inserted.

(c) When the magnet is at rest in the coil (not moving), no deflection will be observed on the galvanometer. This is because there is no change in magnetic flux through the coil, so no current is induced according to Faraday's law of electromagnetic induction.

Three ways of increasing the deflection on the galvanometer are:

1. Increasing the strength of the magnet: Using a stronger magnet will increase the magnetic field strength, resulting in a larger induced current and hence a greater deflection on the galvanometer.
2. Increasing the number of turns in the coil: More turns in the coil will increase the amount of magnetic flux passing through the coil, leading to a larger induced current and greater deflection on the galvanometer.
3. Increasing the speed of motion: Moving the magnet into or out of the coil more rapidly will change the magnetic flux through the coil more quickly, resulting in a larger induced current and greater deflection on the galvanometer.

38. When the magnet shown in the diagram below is moving towards the coil, the galvanometer gives a reading

to the right.

(i) What is the name of the effect being produced by the moving magnet ?

(ii) State what happens to the reading shown on the galvanometer when the magnet is moving away from

the coil.

(iii) The original experiment is repeated. This time the magnet is moved towards the coil at a great speed.

State two changes you would notice in the reading on the galvanometer.

39. If you hold a coil of wire next to a magnet, no current will flow in the coil. What else is needed to induce a

current ?

To induce a current in a coil of wire next to a magnet, a change in magnetic flux through the coil is needed. Simply holding the coil next to a magnet without any motion or change in magnetic field strength will not induce a current.

Therefore, to induce a current in the coil, one of the following actions is needed:

1. Move the magnet towards or away from the coil.
2. Move the coil towards or away from the magnet.
3. Change the magnetic field strength by varying the strength of the magnet or by altering the distance between the magnet and the coil.

Any of these actions will result in a change in magnetic flux through the coil, which according to Faraday's law of electromagnetic induction, induces an electromotive force (EMF) or voltage across the ends of the coil, leading to the flow of current if the circuit is closed.

40. The wire in Figure below is being moved downwards through the magnetic field so as to produce induced current.

What would be the effect of :

(a) moving the wire at a higher speed ?

(b) moving the wire upwards rather than downwards ?

(c) using a stronger magnet ?

(d) holding the wire still in the magnetic field ?

(e) moving the wire parallel to the magnetic field lines ?

41. Two coils A and B of insulated wire are kept close to each other. Coil A is connected to a galvanometer

while coil B is connected to a battery through a key. What would happen if :

(i) a current is passed through coil B by plugging the key ?

(ii) the current is stopped by removing the plug from the key ?

Explain your answer mentioning the name of the phenomenon involved.

(i) When a current is passed through coil B by plugging the key, it creates a magnetic field around coil B. This changing magnetic field induces an electromotive force (EMF) or voltage in coil A, according to Faraday's law of electromagnetic induction. This induced EMF causes a current to flow in coil A, which is detected by the galvanometer. Therefore, the galvanometer will show a deflection indicating the presence of current in coil A.

(ii) When the current is stopped by removing the plug from the key, the magnetic field around coil B collapses. This change in magnetic flux induces an EMF in coil A, but this time the induced EMF opposes the change that caused it (Lenz's law). Therefore, the direction of the induced current in coil A will be opposite to the direction of the original current in coil B. As a result, the galvanometer will show a deflection in the opposite direction compared to when the current was passing through coil B.

The phenomenon involved in both cases is electromagnetic induction, where a changing magnetic field induces an EMF in a nearby conductor.

42. 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 ?

The built-in transformer in the portable radio that allows it to work from the mains instead of batteries is most likely a step-down transformer.

Here's why:

1. Purpose: The main purpose of using a transformer in this scenario is to reduce the high voltage from the mains electricity to a lower voltage suitable for the operation of the radio's internal circuits.

2. Voltage Requirement: Portable radios typically operate on lower voltages, often in the range of a few volts to tens of volts. The mains electricity typically has a much higher voltage, such as 110V or 220V, depending on the region. Therefore, to power the radio, the voltage needs to be reduced.

3. Step-Down Transformation: A step-down transformer is designed to reduce the input voltage to a lower output voltage. It achieves this by having fewer turns in the secondary coil compared to the primary coil. This configuration allows for stepping down the voltage while increasing the current proportionally (according to the transformer equation Vp/Vs = Np/Ns).

Therefore, the transformer in the portable radio is most likely a step-down transformer, converting the high voltage from the mains electricity to a lower voltage suitable for powering the radio's internal components.