Help with electric fields PLEASE

The very left plate is point A with 600 V and an electron beam is entered through which passes through point B 0V and goes to point C which is 1200V.

Now the question says "Describe and explain the effect on the current detected at C when the voltage of the grid B is increased negatively."

I have no idea what this is saying - 2824 January 2005 paper for diagram - I don't understand the markscheme either. Please help - thank you.

I can't find a link to that paper tbh... is it a triode valve?

The very left plate is point A with 600 V and an electron beam is entered through which passes through point B 0V and goes to point C which is 1200V.

Now the question says "Describe and explain the effect on the current detected at C when the voltage of the grid B is increased negatively."

I have no idea what this is saying - 2824 January 2005 paper for diagram - I don't understand the markscheme either. Please help - thank you.

I've attached it - 2f

.... I don't understand the markscheme either. Please help - thank you.

It means that when the potential at metal grid plate B becomes more negative, says from -10 V to -100 V, what would happen to the number of electrons hitting the metal plate C per unit time.

It would decrease? As the electrons are more strongly repelled?? So to get to plate c hare are electron even moving through plate B?

It would decrease? …..

… As the electrons are more strongly repelled?? ….

… So to get to plate c hare are electron even moving through plate B?

Yes.



Yes. I would not use “electrons are more strongly repelled”. Use the concept of the work done by the electric field on the electrons.

How could you explain this concept? Would that mean more work is done for the electron to get to plate B? Since the electron is repelled.



Not sure what you are asking here.

Sorry, I didn't make full sense of the diagram - I was asking if to get to plate C the electron has to pass THROUGH plate B first? (which is in the direction of the electric field line from A to B) however, to get to C is has to do work against the repelling from C?

I think you are confused here. It is good that you revise about electric field.

Electron is a negatively charged particle, so when a stationary electron is placed in an uniform electric field, it would move (naturally) in the direction opposite to direction of electric field. (Why? I believe any A level physics students need to know.)

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. (Why?)

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.



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.

Yeah it is quite confusing - So what is the reason for an electric force to be in the opposite direction to the electric field?
And in post 9 when you said 'negative work done' - I haven't really heard this term be used before so Im not entirely sure what you mean - because the pd across plate B has negative values? (do you mean because the plate is attractive - therefore negative work is done?) - Im quite confused with that statement. Many thanks.

I know of electric forces and magnetic forces being (equal?) opposite to each other but Im not sure how electric fields come into this...

I just realized I make a mistake in post 12 and I would correct it later.

I am very surprised that you have not heard about “negative work done”. I believe you have studied work and conservation of energy. Say, for instance, a block of mass is moving on a rough surface horizontally, it would be subjected to frictional force only (ignore normal force and weight). In this scenario, the frictional force is acting in the opposite direction of the mass is moving. This means that the work done by the frictional force is negative and the block would slow down.


When I said “When moving electrons pass through the hole in plate A, the electric field is doing negative work on electrons.”, the italicized word can be rephrased as work done by the electric field or electric force on the electron is negative. When I say the work done by the electric force is negative is based on the concept of work. The electric force acting on the electron (when electron is between plate A and B) is in the direction from B to A and electron is moving from A to B. According to the work done “formula” (Fx cos θ), the work done will be negative because the angle between force and displacement is 180°. This negative work done is similar to the work done by the frictional force which is usually negative.