Question

In a resistance welding of a lap joint for two mild steel plates of 15 mm thickness, a current 10000 A is passed for a period of 0.1 seconds. The effective resistance of the joint is $120\mu \Omega$. Density of steel is $0.0076{\text{g/mm}}^3$ and heat required to melt steel is 1381 J/g. The joint can be considered as a cylinder of 5 mm diameter and 4.5 mm in height. The percentage of heat distributed to surroundings is the ________%.

• A. 22.7%
• B. 33.6%
• C. 20%
• D. 45.2%

Answer 1

The correct option is A 22.7%

$m_n=0.0076\times \frac{\pi }{4}\times 5^2\times 4.5=0.671\text{gm}$

$H_m=0.671\times 1381=927.36J$

$H_s=I^2\text{Rt}$

$={10000}^2\times 120\times {10}^{-6}\times 0.1=1200J$

${\eta }_m=\frac{927.36}{1200}\times 100=77.28\%$

Percentage of heat lost = 100-77.28

$=22.7\%$