Quantrex Quantrex AcademyJEE · NEET · NDA PYQs with solutions Open app
NEETPhysicsCurrent Electricity

A student attempts to measure a very low unknown resistance (around 0.01 ) using a standard Wheatstone bridge. Even though a perfect null point is obtained and the standard formula P Q = R S is applied correctly, the calculated value of the unknown resistance is found to be highly inaccurate. What is the fundamental physical reason for this inaccuracy in this specific regime?

Options

  1. AThe galvanometer cannot detect the extremely small currents that flow in the circuit when measuring low resist
  2. BThe bridge principle mathematically breaks down and becomes invalid for resistance values below one ohm.
  3. CThe resistances of the connecting wires and contact points become comparable to the unknown resistance.
  4. DThe battery gets short-circuited by the low resistance arms, causing its terminal voltage to drop to zero.

Correct answer

C. The resistances of the connecting wires and contact points become comparable to the unknown resistance.

Step-by-step solution

The standard Wheatstone bridge formula P Q = R S assumes that the connecting wires and the contact points at the terminals have zero resistance. When measuring normal or high resistances, these tiny contact and lead resistances (usually a fraction of an ohm) are negligible and do not significantly affect the result. However, when measuring a very low resistance (like 0.01 ), the contact and lead resistances become comparable to or even greater than the unknown resistance itself. This introduces a massive percentage

Practice Current Electricity on Quantrex Academy →

More from Current Electricity

A potentiometer wire of 400 cm length has a resistance of 20 . It is connected in series with a resistance of 15 and a accumulator of EMF 6 V (negligible internal resistance). The 2026In the following figure current I equal to 2026The range of voltmeter is 10 V and its resistance is 50 . To convert it to a voltmeter of range 15 V , how much resistance is to be added? 2026When a galvanometer is shunted with a 2 resistance, the deflection is reduced to one-fourth. If the galvanometer is further shunted with a 1 resistance, the further reduction in de 2026A galvanometer can be converted into a voltmeter of certain range by connecting a resistance of 990 in series with it. When the resistance of 480 is connected in series, the range 2026A resistance is to be measured using a meter bridge; the student chooses the standard resistance R to be 100 . The student finds the null point at l₁=16 cm. To get the null point n 2026Which of the following statements is FALSE? A. In a balanced Wheatstone bridge, if the cell and the galvanometer are exchanged, the null point is disturbed. B. A rheostat can be us 2026A potentiometer wire has length 4 m and resistance 8 . The resistance that must be connected in series with the wire and the driving battery of e.m.f. 2 V, so as to get a potential 2026 Full Current Electricity list All NEET PYQs