Page 19 - Physics - XII

Q13. Why is the metre bridge not suitable for measuring very low resistance? (NCERT)
Ans. For measuring low resistance, all the resistances including the resistance of galvanometer should be
low. The end resistance thus introduce error in the result.

Q14. Why is the metre bridge preferred for determining the resistance of a wire over the Ohm’s law
circuit?
Ans. Because the metre bridge works on null defl ection method, so we prefer the metre bridge for
determining the resistance of a wire over the Ohm’s law circuit.
Q15. Why is the metre bridge not suitable for measuring very high resistance? (NCERT)
Ans. The resistance forming bridge should be high and the current in the galvanometer reduces and the
galvanometer will no longer be sensitive.

Q16. Defi ne resistivity.
Ans. It is defi ned as the resistance of the material of the conductor having unit length and unit area of
cross-section.

Q17. Give SI unit of resistivity.
Ans. Its SI unit is Ω-m.

Q18. If the metre bridge wire is not made of material of uniform density, how will it aff ect the
observation? (NCERT)
Ans. For a wire, the density is given by
m m
σ =
V A l
1
∴ σ ∝ [where m, l = constant]
A
1
Also, λ ∝
A
∴ λ ∝ σ
[where, λ is the resistance per unit length]

Thus, the resistance per unit length changes with density of the material. Hence, the resistance ratio
will diff er in our observation.

Q19. What is the electrical conductivity of a material?
Ans. The reciprocal of resistivity is called conductivity.

Q20. What is a rheostat?
Ans. It is a device to change the strength of the current in a circuit by introducing the variable resistance
in the circuit.

EXPERIMENT - 3(a)

Aim
To verify the laws of combination (series) of resistances using a metre bridge.

Apparatus and Materials Required

A metre bridge, two diff erent resistances, a resistance box, a one-way key, a jockey, a sensitive galvanometer,
a cell, sand paper, and connecting wires

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