IEMDaily - Video Lecture Notes

Definition

Working principle of the moving coil galvanometer

When a current flows through the coil, a torque acts on it. The magnetic torque tends to rotate the coil. A spring provides a counter torque that balances the magnetic torque resulting in a steady angular deflection

Example

Galvanometer

Galvanometer is an instrument used to check the presence of electric current and find its direction in a circuit. It is connected in series to the device through which current is to be checked. Ideal galvanometer has zero resistance.

Example

Deflection obtained in moving coil galvanometer

A current of

10^{- 5}

A produces a deflection of

10^{0}

in a moving coil galvanometer. A current of

10^{- 6}

amp in the same galvanometer produces a deflection of:Given that,

θ_{1} = 10^{\circ}

I_{1} = 10^{- 5} A

I_{2} = 10^{- 6} A

The current

I

, through the moving coil galvanometer is given by:

I = G θ

\Rightarrow I \propto θ

Hence, we can write

\frac{I_{1}}{I_{2}} = \frac{θ_{1}}{θ_{2}}

θ_{2} = \frac{θ_{1} I_{2}}{I_{1}}

θ_{2} = \frac{10^{\circ} 10^{- 6}}{10^{- 5}} = 1^{\circ}

Definition

Shunt resistance

Since galvanometer is a very sensitive instrument therefore it can't measure heavy currents. In order to convert a galvanometer into an ammeter, a very low resistance known as "shunt" resistance is connected in parallel to galvanometer. Value of shunt is so adjusted that most of the current passes through the shunt.

Formula

Current sensitivity

The current sensitivity of the galvanometer is the deflection per unit current.

\frac{ϕ}{I} = \frac{N A B}{k}

Formula

Voltage sensitivity

voltage sensitivity is the deflection per unit voltage.

\frac{ϕ}{V} = \frac{N A B I}{k V} = \frac{N A B}{k R}

Definition

Describe the sensitivity of a moving coil galvanometer and how can it be increased

The sensitivity of a galvanometer is defined as the scale deflection per microampere.
So, for high sensitivity we must have:
(1) a coil of large area;
(2) a large number of turns in the coil;
(3) a special alloy permanent magnet which gives high magnetic flux;
(4) weak hair springs to give a small control couple.

Example

Electric Bell

Electric bell is based on the principle of electromagnets. When switch is pressed, electromagnet is activated and the hammer moves and hits the gong. As the hammer moves, circuit breaks and the electromagnet is disabled. This causes the hammer to go back to initial position due to the spring and the circuit completes again. This process is continuously repeated giving the ringing sound of the bell.

Definition

Electromagnet

An electromagnet is a temporary magnet constructed by winding insulated wires around a soft iron piece (called core) and passing current through the wire.
Generally two types of electromagnets are used: Bar electromagnet and U-shaped electromagnet.
Electromagnet loses its properties soon after the current is turned off. Field lines of a bar electromagnet are shown in the diagram.

Definition

Moving charges or current produce magnetic field

When a compass is placed near a current carrying straight wire, then deflection occurs in the needle of the compass.
The alignment of the needle is tangential to an imaginary circle which has the straight wire as its centre and has its plane perpendicular to the wire.
It is noticeable when the current is large and the needle sufficiently close to the wire so that the earths magnetic field may be ignored. Reversing the direction of the current reverses the orientation of the needle.
The deflection increases on increasing the current or bringing the needle closer to the wire.
Thus moving charges or currents produced a magnetic field in the surrounding space.