A bar of silicon is 4 cm long with a circular cross section. If the resistance of the bar is 240 Ω at room temperature, what is the cross-sectional radius of the bar? 室温下,截面为圆形的硅棒电阻为240Ω,那么硅棒的横截面积是? 提示:需要查硅的电阻率,并根据电阻的定义公式计算。