Electrical Measurements Assessment Questions and Answers on “Measurement Errors in a Wheatstone Bridge”.
1. The Wheatstone bridge is used for the measurement of _________
a) 1ῼ to a few megaohms
b) 1kῼ to a few megaohms
c) 1Mῼ to a few gigaohms
d) 1mῼ to a few ohms
Clarification: A Wheatstone bridge is used for the measurement of medium resistance in the range of 1ῼ to a few megaohms. Kelvin bridge is used for the measurement of low resistance.
2. The source of error in a Wheatstone bridge is due to _________
a) range of galvanometer used
b) type of the source of emf used
c) limiting errors of the three known resistances
d) balance condition
Clarification: The main source of error in a Wheatstone bridge is because of the limiting errors of the three known resistances. As a result very precise resistances are required with 1 % tolerance.
3. What is the main source of error in a null detector?
a) lower current detection ability
b) insufficient sensitivity of the null detector
c) wiring resistance
d) power dissipation
Clarification: Resistance of contact wires and power dissipation are some of the causes for error in a null detector. But insufficient sensitivity is the major source of error in a null detector such as a galvanometer.
4. When current passes through the resistances in the Wheatstone bridge what happens?
a) voltage drop occurs
b) null detector burns
c) back emf is induced
d) temperature increases due to power dissipation
Clarification: Current flow through the resistances causes power dissipation i.e. I2R leading to an increase in the temperature. As the bridge consists of pure resistances, there is no back e.m.f induced.
5. What is the effect of the increase in temperature on the resistance elements?
a) directly proportional
c) inversely proportional
d) changes by a factor of 10
Clarification: Temperature affects resistance of the circuit. Resistance has a positive temperature coefficient and increases linearly with rise in temperature.
6. Heating effect in the Wheatstone bridge can be avoided by _________
a) using an expensive emf source
b) limiting current to a safe value
c) using heating sinks
d) immersing the setup in ice
Clarification: Heating effect can be eliminated by limiting the current through the circuit to a safe value and by calculating the power dissipation in the arms. Heat sinks are typically used in power electronic applications.
7. The main error when measuring low resistances is _________
a) power dissipation of the arms
b) voltage drop across the emf source
c) lead and contact resistance
d) current flowing through the circuit
Clarification: Voltage drop across the circuit along with power dissipation lead to errors in the measurement of low resistances. The resistance of leads and contacts of the external circuit adds to the values in the circuit. This is the main source of error while measuring low resistances.
8. How is thermal emf generated?
a) when current flows
b) when voltage is applied
c) when the null detector is connected
d) when dissimilar metals come in contact
Clarification: When dissimilar metals come into contact with each other in a galvanometer circuit, thermal emf is generated. Null detector is used for detecting the balance condition.
9. Thermal emf can be prevented by _________
a) using more sensitive galvanometers
b) using similar metals
c) by separating the dissimilar metals
d) reducing the supply emf
Clarification: Thermal emf can be eliminated by making use of sensitive galvanometers consisting of copper coils and copper suspension systems. Dissimilar metals can’t be physically separated.
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