Final answer:
The claim that two 90 degree bends can be used instead of an expansion joint in situations with extreme temperature changes is false. Expansion joints are specifically designed to accommodate the thermal expansion of materials, which bends cannot adequately replace.
Step-by-step explanation:
It is false that in locations subject to extreme temperature changes, two 90 degree bends may be used instead of an expansion joint in a long run of surface-mounted rigid metal conduit. In engineering practices, especially concerning electrical installations, expansion joints are necessary when dealing with significant temperature fluctuations. These joints accommodate the thermal expansion and contraction of the material, thus preventing damage. While bends in the conduit can provide some minor allowances for movement, they are not a substitute for properly designed expansion joints, which are specifically engineered to handle such changes without stressing the system.
Expansion joints are crucial in many applications beyond electrical systems, such as in bridges like the Auckland Harbour Bridge, where they prevent structural buckling by allowing segments of the bridge to expand and contract with temperature variations. The example of the contact between a steel rod and an aluminum rod in a temperature gradient demonstrates the principle of thermal conductivity and temperature distribution but does not directly address the use of bends versus expansion joints in conduit installations.