Sensitivity of Wheatstone Bridge Questions & Answers – Electrical Measurements

1. In a Wheatstone bridge what is the relation between the sensitivity and deflection for a galvanometer?

Directly proportional

Inversely proportional

Independent of each other

Depends on the type of galvanometer used

Answer.1. Directly Proportional

Explanation:-

When the bridge is balanced, the current through the galvanometer is zero. but when the bridge is in an unbalanced condition, current flows through the galvanometer, causing a deflection of its pointer.

The amount of deflection is a function of the sensitivity of the galvanometer. Sensitivity can be thought of as deflection per unit current.

Sensitivity (S) = Deflection (D)/Current (I)

From the above equation, it is clear that the sensitivity of the Wheatstone bridge is directly proportional to the Deflection. A more sensitive galvanometer deflects by a greater amount for the same current.

2. Maximum sensitivity in a Wheatstone bridge for small unbalanced occurs when?

R_{2} ⁄ R_{x} = 1

R_{1} ⁄ R_{x} = 1

R_{3} ⁄ Rx = 1

R_{3} ⁄ R_{x} = 1

Answer.3. R_{3} ⁄ Rx = 1

Explanation:-

Different galvanometers have different current/voltage sensitivities. Hence, in order to determine whether the galvanometer has the required sensitivity to detect an unbalance condition the bridge circuit can be solved for a small unbalance by converting the Wheatstone bridge into its equivalent Thevenin circuit.

The bridge sensitivity for a small unbalanced load is given by

Where

S_{B} = Bridge sensitivity

S_{V} = Current sensitivity of galvanometer

E = EMF

R_{3} = Standard Arm

R_{x} = Unknown Resistance

Hence the Maximum sensitivity in a Wheatstone bridge for small unbalanced occurs when R_{X} = R_{3}

Wheatstone Bridge is said to be more sensitive when all four resistances are nearly equal.

3. When the bridge is balanced, what is the current flowing through the galvanometer?

0

Depends on the ratio arms R_{1} and R_{2}

Varies by a factor of 2

Depends on the type of null detector used

Answer.1. 0

Explanation:-

When there is no current through the meter, the galvanometer pointer rests at 0, i.e. mid-scale. To have zero current through the galvanometer, the points b and d must be at the same potential. Thus potential across arm ab must be the same as the potential across arm ad.

Current in one direction causes the pointer to deflect on one side and current in the opposite direction to the other side.

4. The Sensitivity of the Wheatstone bridge is expressed in _____

cm/A

m/mA

mm/µA

inch/nA

Answer.3. mm/µA

Explanation:-

The amount of deflection is a function of the sensitivity of the galvanometer. Sensitivity can be thought of as deflection per unit current.

Sensitivity (S) = Deflection (D)/Current (I)

Deflection may be expressed in linear or angular units of measure i.e mm, Degree, or radian, and the current is expressed in µA

Therefore the sensitivity can be expressed in units of S = mm/µA or degree/µA or radians/µA.

5. Amount of deflection of the galvanometer depends on _______

Resistance of the ratio arms

Sensitivity

EMF across the circuit

None of the above

Answer.2. Sensitivity

Explanation:-

When the bridge is balanced, the current through the galvanometer is zero. but when the bridge is in an unbalanced condition, current flows through the galvanometer, causing a deflection of its pointer.

The amount of deflection is a function of the sensitivity of the galvanometer. Sensitivity can be thought of as deflection per unit current.

Sensitivity (S) = Deflection (D)/Current (I)

or

Deflection (D) = Sensitivity (S) × Current (I)

From the above equation, it is clear that the sensitivity of the Wheatstone bridge is directly proportional to the Deflection. A more sensitive galvanometer deflects by a greater amount for the same current.

6. The voltage sensitivity of a galvanometer is given by ______

S_{v} = e/θ

S_{v} = θ/e

S_{v} = 1/e

S_{v} = 1/θ

Answer.2. S_{v} = θ/e

Explanation:-

Voltage sensitivity: It is defined as the amount of deflection per until voltage across the galvanometer.

S_{V} = θ/e

θ = Voltage across galvanometer

e = Deflection of galvanometer

It is measured in degrees per volts or radian per volt.

7. Unit of voltage sensitivity is ________

Volts per degrees

Amps per ohms

Degrees per volts

Watts per amps

Answer.3. Degrees per Volt

Explanation:-

Voltage sensitivity: It is defined as the amount of deflection per until voltage across the galvanometer.

S_{V} = θ/e

θ = Voltage across galvanometer

e = Deflection of galvanometer

It is measured in degrees per volts or radian per volt.

8. If we increase the current sensitivity then what will be the effect on the voltage sensitivity of the instrument.

Voltage sensitivity will definitely increase

Voltage sensitivity will definitely decrease

No change in the voltage sensitivity

The change in voltage sensitivity is not absolute

Answer.4. The change in voltage sensitivity is not absolute

Explanation:-

Voltage sensitivity: lt is defined as the deflection of coil per unit potential difference across its ends.

S_{V} = θ/V = N.A.B/R_{g}.C ——- (1)

Where

R_{g} = Galvanometer resistance

N = Number of turns

A = Area

B = Magnetic field strength

C = torsional Rigidity

Clearly, for greater sensitivity number of turns N, area A, and magnetic field strength B should be large, and torsional rigidity C of suspension should be small.

Also, current sensitivity is

S_{I} = θ/I = NAB/C ——- (2)

S_{V}/S_{I} = 1/G

S_{V} = S_{I}/G

Clearly the voltage sensitivity depends on the current sensitivity and the resistance of the galvanometer. If we increase current sensitivity then it is not certain that voltage sensitivity will be increased. Thus, the increase of current sensitivity does not imply an increase in voltage sensitivity.