Explanation

- Resistivity (ρ) is a measure of how strongly a material opposes the flow of electric current.

- The reciprocal of resistivity is called conductivity (σ).

Mathematically:  

σ = 1 / ρ

- Conductivity indicates how easily a material allows the flow of electric current.

- The other options:  

  - Impedance relates to resistance and reactance in AC circuits.  

  - Admittance is the reciprocal of impedance.  

  - Resistance is the opposition to current flow but depends on dimensions, unlike resistivity.

Explanation

Percentage elongation measures how much a material stretches before breaking under tensile load.

Higher elongation indicates greater ability to deform plastically without fracture.

Percentage elongation is a direct measure of ductility.

Explanation

Steel is more elastic than rubber because the ratio of stress to strain is more for steel.

This indicates that steel deforms less under the same amount of stress compared to rubber, making it more elastic.

Explanation

Permissible error defines the tolerance allowed in a measurement.

Ensures accuracy and reliability in engineering measurements.

Permissible error = allowed deviation from true value.

Explanation

Thermal diffusivity indicates how quickly heat spreads through a material.

It depends on thermal conductivity, density, and specific heat of the material.

Thermal diffusivity is an intrinsic physical property of a material.

Explanation

Gas turbine blades require very high dimensional accuracy and smooth surface finish.

Investment moulding (lost wax process) is ideal for producing complex and precise shapes.

Gas turbine blades are cast by investment moulding due to high precision requirements.

Explanation

Cast iron has excellent fluidity, allowing it to fill moulds easily.

It is economical and widely used for machine parts, pipes, and engine blocks.

Cast iron is the most commonly used metal for casting.

Explanation

In belt drives, slip and creep cause a reduction in velocity ratio between driver and driven pulleys.

For efficient power transmission, their combined effect is usually limited to about 4%.

Slip + creep together should not exceed 4% in belt drives.

Explanation

Rivets are specified mainly by the diameter of their shank, which determines strength and load-carrying capacity.

Length is selected separately based on plate thickness and head formation.

Rivet size is identified by shank diameter.

Explanation

Induction hardening uses high-frequency alternating current to heat the steel surface rapidly.

Immediate quenching produces a hard surface layer in a very short time.

Induction hardening is the fastest surface-hardening process.