If a copper wire is stretched to make its radius decrease by 0.1%, then the percentage increase in resistance is approximately -

• A: 0.1%
• B: 0.2%
• C: 0.4%
• D: 0.8%

When a copper wire is stretched, its radius decreases, which affects its resistance. The resistance \( R \) of a wire is given by the formula:

Resistance Formula

R = ρ (L/A)

Where:

• R = resistance
• ρ = resistivity of the material
• L = length of the wire
• A = cross-sectional area

Effect of Stretching

When the radius decreases by 0.1%, the cross-sectional area \( A \) changes. The area of a circle is calculated as:

A = πr²

A decrease in radius leads to a decrease in area, which increases resistance. The area decreases by approximately 0.2% when the radius decreases by 0.1%.

Calculating Percentage Increase in Resistance

Since resistance is inversely proportional to area, if the area decreases by 0.2%, the resistance increases by about 0.2% as well.

Final Answer

The percentage increase in resistance is approximately 0.2%, so the correct option is B: 0.2%.