Moving Coil Galvanometer
Principle, sensitivity and conversion to ammeter and voltmeter. (Physics › Magnetic Effects of Current and Magnetism, NEET UG syllabus.)
What is Moving Coil Galvanometer?
A sensitive electromagnetic device used for detecting and measuring small electric currents, based on the principle of torque experienced by a current-carrying coil in a magnetic field.
Key formula / rule: Torque on a current-carrying coil in a radial magnetic field
Key points
- Describe the principle and construction of a moving coil galvanometer.
- Explain the working of an MCG and derive the relationship between current and deflection.
- Define and calculate current sensitivity and voltage sensitivity.
- Explain how to convert an MCG into an ammeter and a voltmeter.
Common exam trap
Confusing current sensitivity with voltage sensitivity.
Definitions
- Term
Moving Coil Galvanometer (MCG)
- Meaning
A sensitive electromagnetic device used for detecting and measuring small electric currents, based on the principle of torque experienced by a current-carrying coil in a magnetic field.
- Term
Current Sensitivity (Is)
- Meaning
The deflection produced in the galvanometer per unit current flowing through it. It is measured in radians/ampere or divisions/ampere.
- Term
Voltage Sensitivity (Vs)
- Meaning
The deflection produced in the galvanometer per unit voltage applied across its terminals. It is measured in radians/volt or divisions/volt.
- Term
Shunt Resistance
- Meaning
A low resistance connected in parallel with a galvanometer to convert it into an ammeter, allowing most of the current to bypass the galvanometer.
- Term
Series Resistance
- Meaning
A high resistance connected in series with a galvanometer to convert it into a voltmeter, limiting the current through the galvanometer and allowing it to measure potential difference.
Learning objectives
Describe the principle and construction of a moving coil galvanometer.
Explain the working of an MCG and derive the relationship between current and deflection.
Define and calculate current sensitivity and voltage sensitivity.
Explain how to convert an MCG into an ammeter and a voltmeter.
Solve numerical problems related to MCG sensitivity and conversions.
Formulae
- Name
Torque on a current-carrying coil in a radial magnetic field
- Note
N = number of turns, I = current, A = area of coil, B = magnetic field strength. In a radial field, sinθ = 1.
- Expression
τ = NIAB
- Name
Restoring torque due to suspension wire
- Note
k = torsional constant of the suspension wire, φ = angular deflection.
- Expression
τr = kφ
- Name
Equilibrium condition for MCG
- Note
Relates current to deflection.
- Expression
NIAB = kφ
- Name
Current Sensitivity of MCG
- Note
Deflection per unit current. Higher value means more sensitive.
- Expression
Is = φ/I = NAB/k
- Name
Voltage Sensitivity of MCG
- Note
Deflection per unit voltage. Rg is the resistance of the galvanometer coil.
- Expression
Vs = φ/V = NAB/(kRg)
- Name
Shunt resistance for Ammeter conversion
- Note
Ig = full scale deflection current of galvanometer, Rg = galvanometer resistance, I = desired range of ammeter.
- Expression
S = (Ig * Rg) / (I - Ig)
- Name
Series resistance for Voltmeter conversion
- Note
Ig = full scale deflection current of galvanometer, Rg = galvanometer resistance, V = desired range of voltmeter.
- Expression
R = (V / Ig) - Rg
Prerequisites
Understanding of magnetic field due to current-carrying conductors.
Concept of Lorentz force and force on a current-carrying conductor in a magnetic field.
Understanding of torque on a current loop in a magnetic field.
Basic knowledge of series and parallel combinations of resistors.
Common mistakes
Confusing current sensitivity with voltage sensitivity.
Incorrectly applying the shunt/series resistance for ammeter/voltmeter conversion.
Forgetting the role of the soft iron core or the radial magnetic field.
Assuming the torque formula always includes sinθ, even in a radial field where θ=90°.
Not understanding that a galvanometer itself has a specific resistance (Rg).
Keywords
Galvanometer
Moving Coil
Magnetic Torque
Current Sensitivity
Voltage Sensitivity
Ammeter
Voltmeter
Shunt Resistance
Series Resistance
Radial Magnetic Field
Practice preview
What is the fundamental principle on which a moving coil galvanometer operates?…
easy
In a moving coil galvanometer, a radial magnetic field is used primarily to ensure that:…
easy
To convert a moving coil galvanometer into an ammeter, a low resistance is connected:…
medium