Question .1. Describe the construction and working of a simple step-up transformer.
Or,
What is the function of a step-up transformer?
Solution:
Construction : It consists of two coils, primary and secondary coils. The number of turns in the secondary coil is more than that of primary coil. These coils are wounded over a laminated core made of thin rectangular sheets of soft iron or silicon steel dipped in insulating paint or varnish. The insulated sheets are stacked together to form a solid looking rectangular frame, called laminated core.
Working : The ac source connected to the primary coil causes a varying current to flow through this coil and produces a varying magnetic field whose lines of force pass through the secondary coil. These varying magnetic field produces a changing flux in the secondary coil. This produces an induced emf in the secondary coil. The emf induced is directly proportional to the number of turns in the secondary coil. Thus
VS / VP = NS / NP
The step up transformer is used to increases the alternating (not D. C.) voltage.
Question .2. Name the principle on which the functioning of a transformer depends?
Solution:
functioning of a transformer depends upon the principle of mutual inductance.
Question .3. Can a transformer work when it is connected to a dc source?
Solution:
No, a transformer will not work when it is connected to a dc source.
Question .4. (i) Draw a neat labelled diagram of an ac generator.
(ii) What is the magnitude of emf induced in the coil when its plane becomes parallel to the magnetic field.
(iii) State two ways by which the emf in an ac generator can be increased.
Solution:
(i) Consult a Text Book for diagram.
(ii) In this case the magnitude of emf induced is maximum.
(iii) The emf of an ac generator is increased by increasing the speed of armature.
Question .5. State two ways of increasing the speed of a dc motor.
Solution:
(i) By increasing the current through the motor and
(ii) By increasing the number of turns of the motor.
Question .6. State the energy transformations taking place in the following
(i) electric cell and
(ii) electric generator.
Solution:
(i) From chemical energy to electrical energy.
(ii) From mechanical energy to electrical energy.
Question .7. (i) Draw a labelled diagram of the device you would use to transform 200 V ac to 15 V ac.
(ii) What is the name of this device?
Or,
Draw a labelled diagram to show the various components of a step down transformer .
Solution:
(i) For diagram consult a Text Book.
(ii) The name of the device is step down transformer.
Question .8. What is an electromagnet? State two ways in which, the strength of an electromagnet can be increased.
Solution:
(i) An electromagnet is a coil wounded over a soft iron core. By passing the current through the coil the core is magnetized.
(ii) The strength of an electromagnet is increased by :
(a) increasing the number of turns and
(b) increasing the current through the coil.
Question .9. State two characteristics of a primary coil of a step-up transformer when compared to the secondary coil.
Solution:
(i) The number of turns of the primary coil are less than the number of turns of the secondary coil.
(ii) The primary coil consists of thick copper wire.
Question .10. With reference to the dc motor state
(i) the energy change that takes place and
(ii) the principle on which it operates.
Solution:
(i) In dc motors electrical energy is converted into mechanical energy.
(ii) The principle is that a current carrying conductor when placed in a magnetic field experiences a force.
Question .11. (a) Draw a representative diagram of a dc motor . Label the following in your diagram
(i) the field magnet
(ii) the armature
(iii) commutator and
(iv) brushes. What is the energy changes involved in this case?
(b) State the function of the split ring.
Solution:
(a) For diagram consult a Text Book.
(b) A split ring ensure the flow of current in the same direction. This makes the armature always rotate in the same direction.
Question .12. Draw a sketch of an electric bell with electrical connections and label the main parts. Why the armature is made of soft iron and not steel?
Solution:
For diagram consult a T. B.
Armature is made of soft iron and not steel because steel once magnetized does not lose its magnetism.
Question .13. State the main difference between a step-up and step-down transformer.
Solution:
| Step – up | Step – down |
1. Number of turns in the secondary coil is more than the primary. | 1. Number of turns in the primary coil is less than the secondary. |
2. Output voltage is greater than the input voltage. | 2. Output voltage is less than the input voltage. |
3. Primary coil is of thick copper wire. | 3. Primary coil is of thin copper wire. |
4. Secondary coil is of thin copper wire. | 4. Secondary coil is of thick copper wire. |
Question .14. Explain briefly how a magnet can be demagnetized using an alternating current.
Solution:
A copper wire is wounded around the magnet and alternating current is passed through it. This will change the polarities of the magnet rapidly and the magnetic property of the magnet is lost.
Question .15. State the energy change which takes place when a magnet is moved inside a coil having a galvanometer at its ends. Name this phenomenon.
Solution:
Mechanical energy is converted into electrical energy. The name of this phenomenon is electromagnetic induction.
Question .16. Mention two reasons why a soft iron core is used within the coil of a moving coil galvanometer.
Solution:
(i) It concentrates the lines of magnetic field through the galvanometer coil.
(ii) It makes the field radial.
Question .17. State two advantages of an electromagnet over a permanent magnet.
Solution:
| Electromagnet | Permanent magnet |
1. The strength of electromagnet can be changed by changing the strength of the current. | 1. The strength of a permanent magnet can not be changed. |
2. The polarity of an electromagnet can be changed by reversing the directing of current in it. | 2. Polarity of a permanent magnet can not be changed. |
Question .18. (i) What will happen to a compass needle when the compass is placed below a wire and a current is made to flow the wire? Give a reason to justify your answer.
(ii) What energy conversion takes place during the working of a d.c. motor?
Solution:
(i) The compass needle will deflect.
Reason : When current flows in a wire, a magnetic field is produced around it.
(ii) Electrical energy is converted into mechanical energy.
Q.19. (i) State two factors on which the strength of an induced current depends.
(ii) When a solenoid that is carrying current is freely suspended, it comes to rest along a particular direction. Why does this happen?
(iii) What will be the effect on the working of an electric bell if instead of a direct current, an alternating current is used?
Solution:
(i) The strength of an induced current depends upon (a) induced emf which in turn depends upon the rate of change of flux (b) resistance of the circuit (I = V/R).
(ii) A solenoid carrying current behaves like a bar magnet and when suspended freely it rest in north – south direction.
(iii) The electric bell will not ring as the make and break arrangement will not be able to respond to the high frequency AC.
