Consider the bar in worked Example 5.5 to be made of (a) concrete and (b) aluminum. The length of each bar is given as L = 0.1 m. The thermal diffusivity of concrete and aluminum are \alpha _{c} = 5.19 ×10^{-7} m^{2}/s and \alpha _{a} = 9.71 ×10^{-5} m^{2}/s , respectively. Determine the time beyond which the one-term approximation is valid, in these two materials.