Moving charges and Magnetism 3

Question 1

what is a moving coil galvanometer?

Answer 1

It is a device used to detect the presence of current in a circuit.

Question 2

principle of mcg

Answer 2

A current carrying coil placed in a magnetic field, experiences a torque which tries to rotate the coil and produces angular deflection.

Question 3

costruction of mcg

Answer 3

• It consists of a fine insulated copper wire that is wound on a non magnetic support frame
  -It is placed symmetrically between 2 strong cylindrical pole pieces of magnet/.
  -The motion of the coil is controlled by 2 springs that arre placed above and below the coil
• An aluminium pointer is attached to the coil and is allowed to move freely on a graduated scale.

Question 4

working of an mcg

Answer 4

When a current is passed through the coil, it experiences a torque due the magnetic field and gets rotated, as a result of which the spring gets twisted.

a restoring torque is developed in the spring.

The coil rotates till the deflecting torque on it due to the mag field is balanced by the restoring torque produced in the spring.

Question 5

what is torsional constant

Answer 5

It is the restoring torque per unit twist. it is called torsional constant of a spring. It depends on the material of the spring.

tau= k x alpha

theta= angle of twist

Question 6

why is there a linear relationship between I and theta

Answer 6

There is alinear relationship between i and alpha because the field is radial and theta=90

The radial magnetic field is obtained while using the cylindrical concave shaped pieces of magnet. The soft iron core also helps in the production of radial field and it also increases the strength of the field.

Question 7

current sensitivity

Answer 7

The amt of deflection produces in the galvanometer when unit current passes through unit current is passed through it is called current sensitiviy.

It is defined as deflection per unit current.
I=alpha/I
=NBA/k

current sensitivity, by increasing the no of turns, increasing the mag field, increasing the area and decreasing galvanometer constant/figure of merit

Question 8

voltage sensitivity

Answer 8

The amt of eflection produced in the galvanometer when unit potential difference is applied.
V= alpha/V= alpha/IR= BNA/kR

increasing no of turns, mag field, area and decreasing resistance,

Question 9

does increasing the current sensitivity always increase the voltage sensitivity

Answer 9

No because increasing the current sensitivity by increasing the no of coils but result in the decrease in the voltage sensitivity.
we use B,A for the manipulation of factors for sensitivity,

increasing the current sensitivity
may not necessarily increase the voltage sensitivity. Let us take Eq. (4.27)
which provides a measure of current sensitivity. If N → 2N, i.e., we double
the number of turns, then
2
I I
φ φ →
Thus, the current sensitivity doubles. However, the resistance of the
galvanometer is also likely to double, since it is proportional to the length
of the wire. In Eq. (4.28), N →2N, and R →2R, thus the voltage sensitivity,
V V
φ φ →
remains unchanged. So in general, the modification needed for conversion
of a galvanometer to an ammeter will be different from what is needed for
converting it into a voltmeter.

Question 10

why is galvanometer not used for measuring current

Answer 10

(i) Galvanometer is a very sensitive device, it gives a fullscale deflection for a current of the order of µA.

(ii) For measuring currents,
the galvanometer has to be connected in series, and as it has a large
resistance, this will change the value of the current in the circuit.