Why is a metal piece heated up when cathode rays strike it?
When cathode rays strike a metal plate, their kinetic energy is transformed into heat energy, which increases the temperature of the plate.
1. Can we perform Millikan's experiment with drop of any size?
No, it is not advised to perform Millikan's oil drop experiment with drops of any size due to limitations of experimental accuracy.
When we use a drop of larger size (as compared to microscopic sizes), they will have comparatively higher velocity which is difficult to detect.
Similarly, a large electric field should be applied to move the drop towards the positive terminal against gravity.