Moving Coil Galvanometer

Principle of moving coil galvanometer:

A moving coil galvanometer works on the principle of electromagnetic induction. When a current-carrying coil is placed in a magnetic field, it experiences a torque due to the interaction between the magnetic field and the current in the coil. The direction of the torque is given by Fleming’s left-hand rule.

Construction of moving coil galvanometer:

A moving coil galvanometer consists of a coil of fine wire suspended between the poles of a permanent magnet. The coil is wound on a cylindrical former and is connected to two terminals. A soft iron core is placed inside the coil to increase the magnetic field.

Factors affecting the sensitivity of a moving coil galvanometer:

The sensitivity of a moving coil galvanometer is affected by the following factors:

• Strength of the magnetic field: The stronger the magnetic field, the greater the torque experienced by the coil and the more sensitive the galvanometer.
• Number of turns in the coil: The more turns in the coil, the greater the torque experienced by the coil and the more sensitive the galvanometer.
• Radius of the coil: The smaller the radius of the coil, the greater the torque experienced by the coil and the more sensitive the galvanometer.
• Spring constant of the suspension: The weaker the spring constant, the greater the deflection of the coil for a given current and the more sensitive the galvanometer.

Shunts and multipliers:

Shunts and multipliers are used to extend the range of a moving coil galvanometer. A shunt is a low-resistance resistor connected in parallel with the galvanometer. It reduces the current flowing through the galvanometer, making it less sensitive. A multiplier is a high-resistance resistor connected in series with the galvanometer. It increases the current flowing through the galvanometer, making it more sensitive.

Conversion of galvanometer into ammeter and voltmeter:

A moving coil galvanometer can be converted into an ammeter by connecting a shunt in parallel with it. The shunt is chosen such that only a small fraction of the current flows through the galvanometer, while the majority of the current flows through the shunt. The scale of the ammeter is then calibrated to read the current flowing through it.

A moving coil galvanometer can be converted into a voltmeter by connecting a multiplier in series with it. The multiplier is chosen such that the potential difference across the galvanometer is a small fraction of the potential difference being measured, while the majority of the potential difference is dropped across the multiplier. The scale of the voltmeter is then calibrated to read the potential difference across it.

Reference to NCERT books for 11th and 12th classes:

• NCERT Physics Class 12, Chapter 6: Current Electricity

Potential Energy of a Dipole

Introduction to electric dipole:

An electric dipole is a pair of equal and opposite charges separated by a small distance. The electric dipole moment is defined as the product of the magnitude of one of the charges and the distance between the charges.

Expression for the potential energy of an electric dipole:

The potential energy of an electric dipole in a uniform electric field is given by the following expression:

U = -pEcosθ

where:

• U is the potential energy of the dipole in joules (J)
• p is the dipole moment in coulomb-meters (C-m)
• E is the magnitude of the electric field in volts per meter (V/m)
• θ is the angle between the dipole moment and the electric field in degrees (°)

Factors affecting the potential energy of an electric dipole:

The potential energy of an electric dipole in a uniform electric field depends on the following factors:

• Magnitude of the dipole moment: The greater the dipole moment, the greater the potential energy of the dipole.
• Magnitude of the electric field: The stronger the electric field, the greater the potential energy of the dipole.
• Angle between the dipole moment and the electric field: The closer the angle between the dipole moment and the electric field is to 0°, the greater the potential energy of the dipole.

Potential energy of an electric dipole in a non-uniform electric field:

The potential energy of an electric dipole in a non-uniform electric field is given by the following expression:

U = -p∫E⋅dl

where:

• U is the potential energy of the dipole in joules (J)
• p is the dipole moment in coulomb-meters (C-m)
• E is the electric field vector in volts per meter (V/m)
• dl is a vector of differential length along the path of the dipole

Reference to NCERT books for 11th and 12th classes:

• NCERT Physics Class 12, Chapter 2: Electrostatic Potential