Magnetic

Question 1

Who discovered magnetic effect of current ? What is it?

Answer 1

H C Oersted. When a current flows through a conductor it produces a magnetic field around it. He observed that a tiny magnetic needle placed above or below a straight conductor deflected when a current was passed through the conductor.

Question 2

Stare amperes swimming rule

Answer 2

If a swimmer is imagined to swim along the conductor in the direction of current , facing the needle , then the north pole of the needle is deflected to his left.

Question 3

State maxwell’s right hand cork screw rule.

Answer 3

If a right handed cork screw is turned such that it’s tip advances in the direction of the current , then the direction in which the head of the screw is rotated gives the direction of the magnetic field round the conductor.

Question 4

Right hand clasp rule

Answer 4

If the conductor carrying current is clasped with the right hand such that the thumb gives the direction of the current , then the direction in which the other fingers encircle the conductor gives the direction of the magnetic field.

Question 5

If the current is clock wise in a circular conductor
If it is anti clockwise 
It acts as a 
Represented by
Direction of magnetic field

Answer 5

Clockwise - represented by a dot. Magnetic field along the axis is directed towards the observer facing the coil . Acts as a north pole

Anticlockwise - represented by a cross. Magnetic field along the axis directed away from observer facing the coil . Acts as a south pole.

Question 6

State Biot - Savart ( Laplace )law.

Answer 6

Laplace’s law states that the magnetic field dB at P is dierectly proportional to the
Strength of the current I
Length of the current element dl
Sine of the angle between the element and the line joining the point to the element sinθ
Inversely proportional to the square of the distance between the point and the current element r^2

Question 7

Direction of magnetic field dueto current element

Answer 7

The direction of field at a point due to a current element is perpendicular to the plane containing the current element and the point

Question 8

Factors on which flux density of magnetic field at the centre of a circular coil depend upon ?
How does it vary on the axis as the distance from the centre increases ?

Answer 8

Current
Radius
Number of turns in the coil.

Flux density decreases as distance from the centre increases.

Question 9

Rule used to find the direction of dipole moment of a circular current loop

Answer 9

If the current on the circular loop is directed along the four curved fingers of the right hand then the direction of the dipole magnetic moment is along the extended thumb.

Question 10

Solenoid

Answer 10

A solenoid is a cylindrical coil made of several turns of wire
The wire is coated with an insulating material so that even if they touch each other they are electrically insulated.

Question 11

Ideal solenoid

Answer 11

If the turns are closely wound and the length is much greater than the radius then the solenoid is treated as an ideal solenoid.
For such a solenoid the field outside the solenoid is weaker than that inside the solenoid and the field inside the solenoid is uniform over a considerable volume and is along the axis of the solenoid.
A solenoid can be considered ideal of its length is greater than five times its radius.

Question 12

Earths principal magnetic elements

Answer 12

Are quantities which completely determine the earths magnetic field at any place.
Declination δ
Inclination or magnetic dip θ
Horizontal component of the earths field BH

Question 13

Declination

Answer 13

Declination at a place is the angle between the geographic meridian and the magnetic meridian.
The geographic meridian at a place is the vertical plane passing through the geographic north and south poles at the given place.
The magnetic meridian at a place is the vertical plane passing through the place and containing the magnetic axis of a freely suspended magnetic needle at that place.

Question 14

Inclination or magnetic dip

How does it vary from equator to poles

Answer 14

Dip at a place is the angle between the earths total magnetic field and the horizontal drawn in the magnetic meridian.
Increases from 0 to 90 when we move from equator to poles.

Question 15

Horizontal component of the earths magnetic field
How does it vary when be move from equator to poles
Vertical component

Answer 15

The component of the earths magnetic field B at a place along the horizontal drawn in the magnetic meridian is called the horizontal component BH of the earths field at that place.
Is maximum at equator and 0 at poles.
The component of B along the vertical is called the vertical component of the earths magnetic field.

Question 16

Classification of magnetic variations

Answer 16

Daily or diurnal change 
Lunar variations 
Annual changes
Secular changes
Magnetic storms

Question 17

Magnetic variations daily or diurnal changes.

Answer 17

These are small changes in the magnetic elements over a period of 24 hours. The variation differs from day to day. The variations are greater in summer than in winter. Measured using magneto graphs.

Question 18

Magnetic variation
Lunar variations
Annual changes

Answer 18

Lunar variation - these variations occur twice a day and at the same interval as that of the tides. Hence they are considered to be due to the moon.
Annual changes. These are periodic variations in the magnetic elements between a maximum and minimum value extending over a period of one year. These variations are small and occurring opposite directions in the north and south hemispheres.

Question 19

Magnetic variations secular changes.

Answer 19

These are gradual changes in all the magnetic elements extending over a very long period (960 years ). These changes are due to the slow rotation of the magnetic axis of the earth around its geographic axis.

Question 20

Magnetic variations

Magnetic storms

Answer 20

These are sudden and large changes in the magnetic elements produced simultaneously all over the world. They occur during volcanic eruptions, earth quakes and appearance of sun spots. Magnetic storms also accompany aurora borealis a spectacular glow in the sky at the polar regions. The changes are non periodic and often interfere with telegraph and radio communication.

Question 21

Probable factors that give rise to earths magnetic field

Answer 21

Magnetic deposits in the earths crust
Electric circuits circulating within the earth
Convection currents set up in the ionosphere around the earth.

Question 22

Isogonals

Agonic lines

Answer 22

Lines passing through places of equal declination are called isogonals.
The isogonals passing through places having 0 declination are called Agonic lines.

Question 23

Isoclinals

Aclinic line or magnetic equator

Answer 23

Lines passing through places of equal dip are called isoclinals
Isoclinals passing through places of zero dip are called aclinic lines or magnetic equator.

Question 24

Isodynamic lines

Answer 24

Lines passing through places of the same value of horizontal component of earths magnetic field are called Isodynamic lines.

Question 25

Tangent law

Answer 25

For a given restoring field the deflecting field is directly proportional to the tangent of the angle of deflection.

Question 26

Tangent galvanometer

Answer 26

A tangent galvanometer is an instrument used to measure current which works on the basis of tangent law.

Question 27

Reduction factor of TG

Answer 27

The reduction factor of a TG is numerically equal to the current in ampere through the TG which produces a deflection of 45°, when the plane of the coil lies in the magnetic meridian.

Question 28

Why is the plane mirror mounter below the magnetic needle of a compass used in a TG

Answer 28

To avoid error due to parallax.

Question 29

Why are both ends of the pointer read while measuring deflection in a TG ?

Answer 29

The point of pivot of the magnetic needle may not coincide with. The centre of the circular scale. Hence both ends of the pointer are read

Question 30

Why is the current through a TG reversed and 2 more readings noted? What is the use if commutator?

Answer 30

The plane of the coil may not lie along the magnetic meridian A commutator is used to reverse the direction of flow of current in a given part of a circuit.

Question 31

While determining the reduction factor of TG the rheostat is placed at a large distance from it . Why?

Answer 31

Because the current through the rheostat produces a magnetic field which may affect the deflection in the TG

Question 32

For what value of deflection in a TG is accuracy of measurement of current maximum. What is the range of deflection in TG experiments in order that error may not be large

Answer 32

45° 30° - 60°

Question 33

Characteristics of magnetic field lines

Answer 33

They are continuous closed loops The tangent to the magnetic field lines at a point give the direction of magnetic field at that point. Two magnetic field lines never intersect.

Question 34

Magnetic flux

Answer 34

The number of field lines normal to a given surface is called the flux of magnetic field or magnetic flux Weber Wb

Question 35

Magnetic flux density

Answer 35

The flux per unit area is the strength of the magnetic field or simply magnetic field or magnetic flux density Wb/m^2