Magnetic effects of electric current

fiftysixpowersElectronics - Devices

Oct 18, 2013 (3 years and 9 months ago)

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Chapter 13

Magnetic effects of electric current


1. Why does needle compass deflects when it is brought near a current carrying wire?

A. Current carrying wire produces a magnetic effect. So the compass needle deflects.


2. Who showed that electricity a
nd magnetism are related phenomena?

A. Oersted.


3. What is the unit of magnetic field strength?

A. Oersted.


4. What is magnetic field?

A. The region surrounding a magnet, in which the force of the magnet can be
detected, is called magnetic field.


5
. Which is magnetic field


scalar or vector quantity?

A. Magnetic field is vector quantity.


6. What is the direction of magnetic field lines inside and outside the magnet?

A. Inside the magnet magnetic field lines move from South Pole to North Pole;
but
outside they move from North Pole to South Pole.


7. How is the relative strength of magnetic field shown?

A. The relative strength of the magnetic field is shown by the degree of closeness of
the field lines.


8. Can two field lines cross over eac
h other? Justify.

A. No, because at the point of intersection there will be two directions simultaneously
which is impossible.


9. On what factors do the magnetic field of a current carrying wire depend?

A. The magnetic field produced by a current carr
ying wire depends directly on the
current passing through it.


10. Describe right hand thumb rule.

A. Imagine that you are holding a current
-
carrying straight conductor in your right
hand such that the thumb points towards the direction of the current.

Then your
fingers will wrap around the conductor in the direction of the field lines of the
magnetic field. This is known as right hand thumb rule.


11. What is solenoid?

A. A coil of many circular turns of insulated copper wire wrapped closely in the
shape of a cylinder is called a solenoid.


12. Field lines inside solenoid are in the form of parallel lines. What does this
indicate?

A. This indicates that the magnetic field is same at all points inside the solenoid.
That is, the field is uniform
inside the solenoid.


13. How can we make an electromagnet by using solenoid?





OR


Describe a way by which we can magnetise a magnetic material.

A. A strong magnetic field produced inside a solenoid can be used to magnetise a
piece of magn
etic material like soft iron, when placed inside the coil. The magnet so
formed is called an electromagnet.


14. Name a material in which magnetic field is uniform.

A. Solenoid.


15. Who suggested that both magnet and current
-
carrying conductor exer
t equal and
opposite force?

A. Andre Marie Ampere.


16. On what factors do the direction of the force of conductor depend?

A.
Factors on which direction of the force of conductor depend.

1.

Direction of current.

2.

Direction of magnetic field.


17. State F
leming’s left hand rule.

A. According to Fleming’s left hand rule, stretch the thumb, forefinger and middle
finger of your left hand such that they are mutually perpendicular. Then the
forefinger is in the direction of magnetic field, middle finger in t
he direction of
current and thumb in the direction of motion.


18. Name some devices, which use current
-
carrying conductors and magnetic fields.

A. Electric motor, electric generator, loudspeakers, microphones and measuring
instruments.


19. Name the o
rgans in human body, which produces magnetic field.

A. Heart and Brain.


20. What is MRI?

A. The magnetic field inside the body formed the basis of obtaining the images of
different parts of the body part. This is done using a technique called Magnet
ic
Resonance Imaging.


21. What is an electric motor?

A. An electric motor is a rotating device that converts electrical energy to
mechanical energy.


22. Name some devices, which has electric motor as important component.

A. Electric fans, refrigera
tors, mixers, washing machines, computers, MP3 players
etc.


23. What is commutator? In electric motor what acts as commutator?

A. A device that reverses the direction of flow of current through a current is called a
commutator. In electric motor, the

split ring acts as a commutator.





24. How are different components arranged in electric motor?

A. An electric motor consists of a rectangular coil ABCD of insulated copper wire.
The coil is placed between the two poles of the magnetic field such th
at the arm AB
and CD perpendicular to the direction of the magnetic field. The ends of the coil are
connected to the two halves P and Q of a split ring. The inner sides of these halves
are insulated and attached to an axle. The external conducting edges

of P and Q touch
two conducting stationary brushes X and Y.


25. Describe the working of electric motor.

A. Current in the coil ABCD enters from the source battery through conducting
brush X and flows back to the battery through brush Y. The force act
ing of arm AB
pushes it downwards while the force acting on arm CD pushes it upwards. Thus the
coil and the axle O mounted free to turn about an axis, rotate anti
-
clockwise. At half
rotation, Q makes contact with the brush X and P with brush Y. Therefor
e the current
in the coil in the coil gets reversed and flows along the path DCBA. The reversal of
current also reverses the direction of force acting on the two arms AB and CD. Now
AB is pushed up and CD is pushed down. Therefore the coil and the axle
rotate half a
turn more in the same direction. The reversing of the current is repeated at each half
rotation giving rise to continuous rotation of the coil and to the axle.


26. What enhances the power of the motor?

A. Armature.


27. What is armatu
re?

A. The soft iron core, on which the coil is wound, plus the coils, is called an
armature.


28. How are simple electric motors different from commercial motors?

A. Commercial motors use

a.

An electromagnet in place of permanent magnet.

b.

Large number of

turns of the conducting wire in the current
-
carrying coil.

c.

A soft iron core on which the coil is wound.


29. Who discovered that moving magnet can be used to generate electric currents?

A. English physicist Michael Faraday in 1831.


30. What is a gal
vanometer?

A. A galvanometer is an instrument that can detect the presence of current in a
circuit.


31. What is electromagnetic induction?

A. The process by which a changing magnetic field in a conductor induces a current
in another conductor is call
ed electromagnetic induction.


32. How can we induce current in a coil?

A. We can induce current in a coil by either moving it in magnetic field or by
changing the magnetic field around it. It is convenient in most situations to move the
coil in a magn
etic field.


33. State Fleming’s right hand rule.

A. Stretch the thumb, forefinger and middle finger of right hand so that they are
perpendicular to each other. Forefinger indicates the direction of magnetic field and
thumb shows the direction of motio
n of conductor, then the middle finger will show
the direction of induced current.


34. Describe the components of electric generator.

A. An electric generator consists of a rotating rectangular coil ABCD placed
between the two poles of a permanent magn
et. The two ends of this coil are
connected to the two rings R
1

and R
2
. The inner side of these rings are made
insulated. The two conducting stationary brushes B
1

and B
2

are kept pressed
separately on the rings R1 and R
2
. The two rings R
1

and R
2

are in
ternally 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 galvanometer
to show the flow of current in the given external circuit.


35.

Describe the working of electric generator.

A. When the axle attached to the two rings is rotated such that the arm AB moves up
and CD moves down in the magnetic field produced by a permanent magnet. An
induced current flows in the direction ABCD. If
there are larger number of turns in
the coil, the current generated in each turn adds up to give a large current through the
coil. This means that the current in the external circuit flows from B
1

to B
2
. After
half a rotation arm CD starts moving up and
AB moving down. As a result, the
directions of the induced currents in both the arms change, giving rise to the net
induced current in the external circuit now flows from B
1

to B
2
. Thus after half
rotation the polarity of the current in the respective ar
ms change.


36. What is Alternating Current?

A. A current, which changes direction after equal intervals of time, is called an
Alternating Current (AC).


37. After how much time AC changes its direction?

A. 1/100 second.


38. What is the frequen
cy of AC in India?

A. 50Hz.


39. What is the potential difference between live wire and neutral wire?

A. 220V.


40. What is the colour of live wire?

A. Red.


41. What is the colour of neutral wire?

A. Black.


42. What is the colour of earth wir
e?

A. Green.



43. What is the use of green or earth wire?

A. Earth wire is used as a safety measure, especially for that appliance that has a
metallic body. The metallic body is connected to the earth wire, which provides a low
resistance
-
conducti
ng path for the current. Thus, it ensures that any leakage of
current to the appliance keeps its potential to that of the earth, and the user may not
get a severe electric shock.


44. When does Overloading occur?

A. Overloading occurs when the live wi
re and the neutral wire come into direct
contact (This occurs when the insulation of wires is damaged or there is a fault in a
the appliance) or connecting too many appliances to a single socket.


45. What is short
-
circuiting?

A. During overloading, the

current in the circuit abruptly increases. This is called
short
-
circuiting.


46. What is the use of fuse?

A. The use of an electric fuse prevents the electric circuit and the appliance from a
possible damage by stopping the flow of unduly high electr
ic current.


47. Can our nerve impulse produce magnetic field?

A. Yes.


48. Compare the magnetic field produced by earth and nerve impulse.

A. The magnetic field produced by nerve impulse is about one billionth of the
earth’s magnetic field.


49. Wr
ite an important advantage of AC over DC?

A. An important advantage of AC over DC is that electric power can be transmitted
over long distances without much loss of energy.


50. Why are domestic circuits connected in parallel?

A. In order that each ap
pliance has equal potential difference, circuits are connected
in parallel.


51. How fuse prevents electric circuit during short
-
circuiting?

A. During short
-
circuiting, Joule heating takes place in the fuse, melts it to break the
electric circuit.


52.

Is magnetic field produced by solenoid and bar magnet similar?

A. Yes.