Final answer:
The statement regarding materials that are round or rust-covered needing less preheat flame is false. High-voltage wires are held by insulating connectors but are not wrapped in insulating materials, so that's false. The ice cube threaded with salt experiment is true, and dropping a bar magnet through a copper tube inducing an electric current is also true.
Step-by-step explanation:
True or False: Materials that are round or covered in rust require less preheat flame. This statement is False. Preheat flame may be necessary based on the material thickness, its thermal conductivity, and surface conditions. Round or rust-covered materials might actually require more preheating to ensure a uniform temperature and proper welding quality.
High-voltage Insulation: The high-voltage wires you see are indeed held aloft by insulating connectors, but they are typically not wrapped in an insulating material because air acts as a natural insulator at high altitudes, where these wires are placed. So the answer here is False.
Ice Cube and Salt Experiment: When you sprinkle salt on an ice cube, it causes the ice to melt at the points of contact due to the lowering of the freezing point. When the water refreezes, it captures the thread, resulting in it sticking to the cube. You can lift the ice cube by picking up the thread afterward, therefore this statement is True.
Bar Magnet and Copper Tube: Dropping a bar magnet through a copper tube does indeed induce an electric current in the tube due to the changing magnetic field creating an electromotive force according to Faraday's law of induction. This is an example of Lenz's Law, which states that the induced current will flow in such a way as to create a magnetic field opposing the motion of the magnet. So this statement is True.