First, you should know that if
is large enough (i.e. high photon energy), electron has enough energy to free itself from work potential, i.e.
. This amount of energy,
allows electrons to travel. Hence, the purpose of
is to stop electrons, i.e. currents.
Hence, (A) and (C) are only possible answers.
Now, it is time to think a bit more. The electrons in the tube travels from the photocathode to the collector. Hence, the current travels from the collector to the photocathode inside the tube when
. In order to oppose the current, the gradient of potential, i.e., electric field has to be in opposite direction. That is, the electric field generated by additional potential must point from the photocathode to the collector. Since the potential at the photocathode is zero (ground),
at the collector must be negative.
The correct answer is (A). 34 of 100 people answered it correctly.
ฟิสิกส์โอลิมปิก วิทยาศาสตร์โอลิมปิก ข้อสอบแข่งขัน ข้อสอบชิงทุน
