# Physics Questions and Answers – Wheatstone Bridge

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Wheatstone Bridge”.

1. How many resistances are used in a Wheatstone bridge?
a) 3
b) 4
c) 5
d) 6

Explanation: Wheatstone bridge is an arrangement of four resistors P, Q, R, and S, such that if we know the value of the resistances of any three of them, we can obtain the value of fourth unknown resistance. Therefore, there are 4 resistances in a Wheatstone bridge.

2. The values of resistances P, Q, R, and S of a Wheatstone bridge are 20, 15, 30, and 35 ohms respectively. Calculate the current passing through the battery of negligible resistance.

a) 0.36A
b) 22.22A
c) 0.25A
d) 0.45A

Explanation: The Wheatstone bridge is balanced, i.e. there is no current passing through the galvanometer. So, resistances P and R are in series → R1 = 20 + 30 = 50 ohms.
Also, Q and S are in series → R2 = 15 + 35 = 40 ohms.
R1 and R2 are parallel to each other, so $$\frac {1}{R} = \frac {1}{50} + \frac {1}{40} = \frac {(50 + 40)}{50 \times 40} = \frac {90}{200}$$
Thus, R = $$\frac {200}{90}$$ = 22.22.
Now, current (I) = $$\frac {V}{R} = \frac {10}{22.22}$$ = 0.45A
Therefore, the current passing through the battery is 0.45A.

3. Which instrument is used as the null detector in the Wheatstone bridge?
a) Voltmeter
b) Ammeter
c) Galvanometer
d) Multimeter

Explanation: A galvanometer is used as the null detector in a Wheatstone bridge. The null point means the situation in which no current flows through the circuit. The galvanometer is used for measuring the current and also to determine the voltage between any two points of the circuit. Galvanometer is used due to its sensitivity and therefore, even small currents can be measured.
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4. The equation of a balanced Wheatstone bridge is PR = QS.
a) True
b) False

Explanation: No. A Wheatstone bridge is an electrical device used to measure unknown resistance by placing it in one of the branches and balancing the two legs of the bridge. If R is the unknown resistance, S is varied until the galvanometer shows null deflection, whereas P and Q are fixed. This is the balanced condition. So the equation is $$\frac {P}{Q} = \frac {R}{S}$$ → PS = QR. Also, in the balanced condition, no current passes through the galvanometer.

5. Identify the definition of sensitivity of a galvanometer from the following.
a) Ratio of unit change in unknown resistance to the deflection in the galvanometer
b) Ratio of unit change in unknown resistance to twice the deflection in the galvanometer
c) Ratio of deflection in the galvanometer to the unit change in unknown resistance
d) Ratio of deflection in the galvanometer to half the unit change in unknown resistance

Explanation: Sensitivity of a galvanometer is defined as ratio of deflection in the galvanometer to the unit change in unknown resistance. Sensitivity of a galvanometer can be increased by increasing the number of turns, increasing the area of the coil, or using a strong magnet.
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6. In the Wheatstone bridge given below, what should be values of R1 and R2 so that the bridge is balanced?

a) R1 = 6 ohms; R2 = any finite value
b) R1 = 6 ohms; R2 = 3 ohms
c) R1 = 29.4; R2 = 2 ohms
d) R1 = any finite value; R2 = 3 ohms

Explanation: The given Wheatstone bridge is balanced, so the equation to be used is: $$\frac {P}{Q} = \frac {R}{S}$$
$$\frac {21}{R_1} = \frac {35}{10}$$ → R1 = 21 × $$\frac {10}{35}$$
= 3 × 2
= 6 ohms
Since, the bridge is balanced, no curent will pass through the arm BD
Hence, R2 can take any finite value.
Therefore, R1 = 6 ohms; R2 = any finite value.

7. Find the false statement.
a) Wheatstone bridge is analogous to simple level system
b) A galvanometer with low resistance in series is an ammeter
c) Wheatstone bridge cannot be used forprecision measurement because of the error introduced in contact resistance
d) Wheatstone bridge is susceptible to high dc current

Explanation: The false statement is Wheatstone bridge is susceptible to high dc current. It is actually not susceptible to high dc current and can give inaccurate readings if not balanced. The Wheatstone bridge measures resistance from few ohms tomega ohms.All the other statements are valid.

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