(Original post by MrToodles4)
Because the field lines go from the positive plate to the negative plate? And so electrons would be attracted to the positive?
If electron needs to move in the direction of the electric field, then an external agent needs to do work on the electron.
This is usually the confusing part in electric field because students tend to confuse with the movement of positively charged particle in electric field.
That makes sense now.
Coming back to question.
When moving electrons pass through the hole in plate A, the electric field is doing negative work on electrons. This is why electrons need minimum speed (as asked by the question) to reach B. If the initial speeds of the electrons at A is greater than the minimum speed, the electrons would pass B and then it would just go to C naturally
Same reason as above?
If you answer the two why questions based on work done by the electric field, then the two answers would be identical.
However, if you answer using repulsion and attraction, then you need to be careful. Again why?
My first why in post 9, did not mention about any positive plate or negative plate, I just say “when a stationary electron is placed in an uniform electric field”.
As for the second why, it is pertaining to the question that you posted, so your answer is with the positive plate and negative plate is good.
“Because the field lines go from the positive plate to the negative plate. And so electrons would be attracted to the positive.”
This is the usual answer I would get from students when they do not pay attention to the different setup of the question. The difference is subtle but important in understanding the concept of electric field and the work done by the electric field.
Think about the first why based on the work done by the electric field and electric force on the electron before seeing the answer.
When a stationary electron is placed in an uniform electric field, the electric force acts on the electron in the direction opposite to direction of electric field.
This net force would cause the electron to accelerate, the KE of electron would increase. According to conservation of energy, the electric potential energy of the electron-electric field system must decrease.
Can we account for the decrease in electric PE? Yes. If you find the work done by the electric field, it is positive. Since work done by conservative forces increases kinetic energy and decreases potential energy by that amount and electric force is a conservative force, so
W = ΔK = −ΔU
This explanation may sound confusing. It is ok. Compare the movement of positively charged particle and negatively charged particle in a uniform electric field set up by two metal plates that has a p.d. of V
It takes some time to sink in. Your eureka moment will come.