If the length of a copper wire is increased by twice, then its resistivity will be
- A.Doubled
- B.Halved
- C.Same
- D.One-fourth
Correct answer
Explanation
The correct answer is C, Same. Resistivity is a property of the material itself, so changing the length of a copper wire does not change it.
The resistance of a wire is given by R = ρL/A, where ρ is the resistivity, L the length and A the area of cross-section. When the length is increased, the resistance R rises in proportion, but ρ stays fixed because it depends only on the nature of the material and its temperature. Copper has a very low resistivity, about 1.7 × 10⁻⁸ ohm-metre at room temperature, which is why it is used in household wiring. Resistivity is measured in ohm-metre (Ω m).
A is wrong: it is the resistance, not the resistivity, that changes with length. B is wrong: neither quantity is halved by making the wire longer. D is wrong: a quarter change would apply to resistance only if the length were quartered at the same cross-section.
Exam tip: resistance depends on length, area and material; resistivity depends only on the material and temperature.
Practice Questions
View allA negative work is done when an applied force F and the corresponding displacement S are
- A.perpendicular to each other.
- B.parallel to each other.
- C.anti-parallel to each other.
- D.equal in magnitude.
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Work is negative when the force acts opposite to the displacement, because the angle between them is then 180°. Work done is W = F S cos θ, where θ is the angle between the force and the displacement.
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- C.Satellite requires energy from solar panels and solid fuels for orbiting.
- D.Satellite does not require any energy for orbiting.
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- A.Frictional force
- B.Electric force
- C.Gravitational force
- D.Spring force
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- A.electrical signals to sound waves
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- C.microwaves to sound waves
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- B.bending away from the normal
- C.propagating in straight line
- D.reflected towards the opposite direction
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