If a bar of copper is brought near a magnet, the copper bar will be A. unaffected by the magnet. B. made?
First 3 answers are wrong. The act of bringing the magnet near copper will affect the copper, it will induce an electrical current in the copper (an eddy current). The eddy current will create a magnetic field that repells the magnetic field while the eddy current is flowing. This current will stop when the magnetic field stops changing (when the magnet stops moving) but another eddy current will be induced when the magnet is moved away again. The second eddy current will be exactly opposite the first one if the magnet motion is exactly the reverse of it's first motion. The magnitude of this eddy current will depend on the strength of the magnetic field and the speed at which the magnetic field changes. It also depends on the temperature of the copper. If the copper is cooled to near absolute zero, it becomes a superconductor (I'm not sure what the superconducting transition temperature is but most metals become superconductors down near 0K) and that will change the interaction with the magnet (the magnet will be repelled even more strongly). If you want to see a demonstration of the practical effects, get a good strong magnet (NdFeB would be a good choice) and a short length of copper tubing/pipe, say 1 foot long. Drop the magnet thru the pipe and see how long it takes to fall. Drop a marble or a rock or a piece of copper or anything that is not magnetic thru the pipe and see how long that takes to fall. If you neglect the transients and look only at steady state. the first 3 answers are right (well, sort of, right, some of their facts are wrong). Zinc is NOT magnetic. Fe, Ni, Co and (I believe) Gd are magnetic.
A unaffected by the magnet Copper by itself is not magnetic. If you pass electricity through it, you can see a magnetic field, but that is from the movement of electrons, not the copper itself.
The first four answers are all wrong, because copper is diamagnetic, which means the correct answer is C repelled by the magnet. The diamagnetic force is quite weak - but it can be demonstrated as shown in the YouTube video - link 1 below. Diamagnetism is a quantum effect and occurs because the external field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment.
