✔ 最佳答案
The explanation given in your physics textbook is quite misleading (but unfortunately, most local physics textbooks use such explanation).
Induced emf on the moving rod is produced when the rod cuts through magnetic field lines. The reason behind is that the charge carriers (which are electrons in the case of a metal rod) inside the rod interact with the magnetic field. Such interaction gives rise to a force (known as Lorentz force) that propels the electrons to move along the rod in a certain direction. This is how induced emf (or induced current) is produced.
Faraday discovered that the magnitude of the induced emf is proportional to the number of field lines cut by the rod in a unit time, which (for reason given below) he described it as the "rate of change of magnetic flux" . This is Faraday's Law of electromagentic induction.
Because the number of field lines cannot be physically counted, the term "magntic flux" is used instead in physics. Therefore, the concept of "rate of change of magnetic flux" can be interpreted as "the number of magnetic field lines being cut ( by the moving rod) in a unit time".
When the rod moves faster, either to the left or right, the number of field lines cut in a unit time would increase. This indicates a higher induced emf produced on the rod.
In the actual fact, in classical electromagnetism, electric or magnetic fields are based on the concept of "field lines".
2013-07-21 16:46:31 補充:
Q:通過metal rod既magnetic field lines既總數唔係應該不變咩?
A: I think you still cannot grasp the concept of electromagnetic induction.
Induced emf occurs is not because of the field lines that pass through the rod, but due to the MOTION of the rod that CUTS through few lines.
2013-07-21 16:56:02 補充:
As word limit is reached, continue on [意見] section.
2013-07-21 16:57:19 補充:
Q:...本書就話冇induced emf,點解個結果會同metal rod果個唔同既?
A: Your book is wrong. There is indeed emf produced, but no induced current.
Because of the motion of the rectangle, the two sides of the rectangle,that are perpendicular to the field lines,are SEPARATELY CUTTING through magnetic field lines...
2013-07-21 16:58:14 補充:
(cont'd)..Hence,emf are induced on each of the 2 sides of the rectangle. Using Fleming's Right hand Rule, you would find that these two induced emf are in the same direction, thus tends to drive current opposing each other. Resulting in no current at all...
2013-07-21 16:59:14 補充:
(cont'd)...This situation is similar to two cells connected in parallel to form a closed circuit. There are emfs but no current could be delivered.
2013-07-21 17:01:32 補充:
typo correction, last sentence "... CUTS through field lines"