MAGNETISM AND ELECTROMAGNETSM | Aster Classes

Chapter 5, MAGNETISM AND ELECTROMAGNETSM, 9th std, science, tamilnadu board, 

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


Question 2.

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


Question 3.

Transformer works on

(a) AC only

(b) DC only

(c) both AC and DC

(d) AC nor effectively than DC

Answer:

(a) AC only


Question 4.

The unit of magnetic flux density is

(a) Weber

(b) weber/meter

(c) weber/meter2

(d) weber.meter2

Answer:

weber/meter2


II. Fill in the blanks.

  1. The SI Unit of magnetic field induction is ……………………..
  2. Devices which is used to convert high alternating current to low alternating current is …………………..
  3. An electric motor converts …………………..
  4. A device for producing electric current is ……………………..

Answer:

  1. Tesla
  2. Step down transformer
  3. Electrical energy into mechanical energy
  4. Generator

III. Match the following.

Column – IColumn – II
1. Magnetic material(a) Oersted
2. Non-magnetic material(b) iron
3. Current and magnetism(c) induction
4. Electromagnetic induction(d) wood
5. Electric generator(e) Faraday

Answer:

  1. (b) iron
  2. (d) wood
  3. (a) Oersted
  4. (e) Faraday
  5. (c) induction

IV. True or False.

  1. A generator converts mechanical energy into electrical energy – True

  1. Magnetic field lines always repel each other and do not intersect – True
  2. Fleming’s Left hand rule is also known as Dynamo rule – True
  3. The speed of rotation of an electric motor can be increased by decreasing the area of the coil – False
    Correct Statement: The speed of rotation of coil can be increased by increasing the area of the coil.
  4. A transformer can step up direct current – False
    Correct Statement: A transformer can step up alternating current.
  5. In a step down transformer the number of turns in primary coil is greater than that of the number of turns in the secondary coil – True

V. Answer in brief.

Question 1.

State Fleming’s Left Hand Rule.

Answer:

It states that while stretching the three fingers of left hand in perpendicular manner with each other, if the direction of the current is denoted by middle finger of the left hand and the second finger is for direction of the magnetic field then the thumb of the left hand denotes the direction of the force or movement of the conductor.


Question 2.

Define magnetic flux density.

Answer:

The number of magnetic field lines crossing unit area kept normal to the direction of field lines is called magnetic flux density.

Its unit is Wb/m2


Question 3.

List the main parts of an electric motor.

Answer:

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

Question 4.

Draw and label the diagram of an AC generator.

Answer:


Question 5.

State the advantages of AC over DC.

Answer:

Advantages of AC over DC:

  • The Voltage of AC can be varied easily using a device called transformer. The AC can be carried over long distances using step up transformers. The loss of energy while distributing current in the form of AC is negligible.
  • Direct current cannot be transmitted as such. The AC can be easily converted into DC. Generating AC is easier than DC. The AC can produce electromagnetic induction which is useful in several ways.

Question 6.

Differentiate step up and step down transformer.

Answer:

Step up transformerStep down transformer
The transformer used to change a low alternative voltage to a high alternating voltage is called a step up transformer, i.e. (Vs>Vp).The transformer used to change a high alternating voltage to a low alternating voltage is called a step down transformer (Vs <Vp).
In a step up transformer, the number of turns in the secondary coil is more than the number of turns in the primary coil (Ns > Np)In a step down transformer, the number of turns in the secondary coils are less than the number of turns in the primary coil (Ns < Np)

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?

Answer:

A step-down transformer is used in a portable radio in order to reduce the voltage.


Question 8.

State Faraday’s laws of electromagnetic induction.

Answer:

Whenever there is a change in the magnetic flux linked with a closed-circuit an emf is produced and the amount of emf induced varies directly as the rate at which the flux changes.

This emf is known as induced emf and the phenomenon of producing an induced emf due to change in the magnetic flux linked with a closed circuit is known as electromagnetic induction.


VI. Answer in detail.

Question 1.

Explain the principle, construction and working of a DC motor.

Answer:

A motor is an electrical machine which converts electrical energy into mechanical energy. The principle of working of a DC motor according to Faraday’s laws of electromagnetic induction is that “whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force”.

The various parts of a DC motor are; Permanent magnets on both sides of a coil which consists of carbon brush and commutator as shown in

Working of electric motor is primarily dependent upon the interaction between magnetic field and current.

The direction of this force is given by Fleming’s left hand rule and it’s magnitude is given by F = BIL. Where, B = magnetic flux density, I = current and L = length of the conductor within the magnetic field.


Question 2.

Explain two types of transformer.

Answer:

Transformer is a device used for converting low voltage into high voltage and high voltage into low voltage.

It works on the principle of electromagnetic induction.

It consists of primary and secondary coil insulated from each other.

Depending upon the number of turns in the primary and secondary coils, the two types of transformers are; step-up or step-down transformers.

Step up transformer:

The transformer used to change a low alternative voltage to a high alternating voltage is called a step up transformer, ie (Vs > Vp) .

In a step up transformer, the number of turns in the secondary coil is more than the number of turns in the primary coil (Ns > Np).

Step down transformer:

The transformer used to change a high alternating voltage to a low alternating voltage is called a step down transformer (Vs < Vp).

In a step down transformer, the number of turns in the secondary coils are less than the number of turns in the primary.

coil (Ns < Np).

The formulae pertaining to the transformers are given in the following equations.

  • The number of primary turns Np / The number of secondary turns Ns = The primary voltage Vp/ The secondary voltage Vs
  • The number of secondary turns Ns / The number of primary turns Np – The primary current Ip/ The secondary current Is

Question 3.

Draw a neat diagram of an AC generator and explain its working.

Answer:

An alternating current (AC) generator, consists of a rotating rectangular coil ABCD called armature placed between the two poles of a permanent magnet.

The two ends of this coil are connected to the two slip rings S1 and S2.

The inner sides of these rings are insulated.

Two conducting stationary brushes B1 and B2 are kept separately on the rings S1 and S2 respectively.

The two rings S1 and S2 are internally attached to an axle.

The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.

Outer ends of the two brushes are connected to the external circuit.

When the coil is rotated, the magnetic flux linked with the coil changes.

This change in magnetic flux will lead to generation of induced current.

The direction of the induced current, as given by Fleming’s Right Fland Rule, is along ABCD in the coil and in the outer circuit it flows from B2 to B1. During the second half of rotation, the direction of current is along DCBA in the coil and in the outer circuit it flows from B1 to B7.

As the rotation of the coil continues, the induced current in the external circuit is changing its direction for every half a rotation of the coil.


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