# HELP Electromagtism Physics Please

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Its part d thats confusing me; I'll have it attached as an image. I have working out the acceleration correctly to be 1.76*10^15 but Im not sure how to do the next part using this?? Any help is greatly appreciated thanks.

Edit: Not letting me post? :/

But the question basically says, "Electrons are accelerated upwards by the electric force on them. Calculate their acceleration, and use your answer to deduce the upward component of the electron's velocity as it emerges from the space between the plates."

So I've managed to work out the acceleration correctly but how would I use this for the second part its asking for?? Im so confused

Edit: Not letting me post? :/

But the question basically says, "Electrons are accelerated upwards by the electric force on them. Calculate their acceleration, and use your answer to deduce the upward component of the electron's velocity as it emerges from the space between the plates."

So I've managed to work out the acceleration correctly but how would I use this for the second part its asking for?? Im so confused

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#2

Equations of motion?

Without seeing the Q I'm fumbling in the dark, but I'm imagining the electrons enter some kind of electric field which is at right angles to their direction of motion?

If that's the case, the you will need to use the horizontal speed to work out how long they will be between the plates, then use this time, plus the acceleration, to work out what their upwards/vertical velocity will increase to in this time.

Remember it's like projectile motion so horizontal and vertical motion are completely independent which means that, assuming they enter the plates horizontally, their vertical velocity at that time is zero.

Of course, your Q might be nothing like that, but it's my best guess until I see it....

Without seeing the Q I'm fumbling in the dark, but I'm imagining the electrons enter some kind of electric field which is at right angles to their direction of motion?

If that's the case, the you will need to use the horizontal speed to work out how long they will be between the plates, then use this time, plus the acceleration, to work out what their upwards/vertical velocity will increase to in this time.

Remember it's like projectile motion so horizontal and vertical motion are completely independent which means that, assuming they enter the plates horizontally, their vertical velocity at that time is zero.

Of course, your Q might be nothing like that, but it's my best guess until I see it....

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(Original post by

Equations of motion?

Without seeing the Q I'm fumbling in the dark, but I'm imagining the electrons enter some kind of electric field which is at right angles to their direction of motion?

If that's the case, the you will need to use the horizontal speed to work out how long they will be between the plates, then use this time, plus the acceleration, to work out what their upwards/vertical velocity will increase to in this time.

Remember it's like projectile motion so horizontal and vertical motion are completely independent which means that, assuming they enter the plates horizontally, their vertical velocity at that time is zero.

Of course, your Q might be nothing like that, but it's my best guess until I see it....

**phys981**)Equations of motion?

Without seeing the Q I'm fumbling in the dark, but I'm imagining the electrons enter some kind of electric field which is at right angles to their direction of motion?

If that's the case, the you will need to use the horizontal speed to work out how long they will be between the plates, then use this time, plus the acceleration, to work out what their upwards/vertical velocity will increase to in this time.

Remember it's like projectile motion so horizontal and vertical motion are completely independent which means that, assuming they enter the plates horizontally, their vertical velocity at that time is zero.

Of course, your Q might be nothing like that, but it's my best guess until I see it....

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#4

Yes, as I imagined, so try doing as suggested in my first reply.

How long will it take the electron to travel the 10cm horizontally and how much will its vertical speed increase in this time.

(A variation on this question is to ask about its vertical displacement in the time taken, in order to work out whether it emerges from the plates or hits one of them first.)

How long will it take the electron to travel the 10cm horizontally and how much will its vertical speed increase in this time.

(A variation on this question is to ask about its vertical displacement in the time taken, in order to work out whether it emerges from the plates or hits one of them first.)

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(Original post by

Yes, as I imagined, so try doing as suggested in my first reply.

How long will it take the electron to travel the 10cm horizontally and how much will its vertical speed increase in this time.

(A variation on this question is to ask about its vertical displacement in the time taken, in order to work out whether it emerges from the plates or hits one of them first.)

**phys981**)Yes, as I imagined, so try doing as suggested in my first reply.

How long will it take the electron to travel the 10cm horizontally and how much will its vertical speed increase in this time.

(A variation on this question is to ask about its vertical displacement in the time taken, in order to work out whether it emerges from the plates or hits one of them first.)

The answer says the magnetic field will be much greater than the electric field and so will curve the ion upwards - but I don't understand why this is so - why'd the magnetic field greater?

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**phys981**)

Equations of motion?

Without seeing the Q I'm fumbling in the dark, but I'm imagining the electrons enter some kind of electric field which is at right angles to their direction of motion?

If that's the case, the you will need to use the horizontal speed to work out how long they will be between the plates, then use this time, plus the acceleration, to work out what their upwards/vertical velocity will increase to in this time.

Remember it's like projectile motion so horizontal and vertical motion are completely independent which means that, assuming they enter the plates horizontally, their vertical velocity at that time is zero.

Of course, your Q might be nothing like that, but it's my best guess until I see it....

EDIT: I see that I get the wrong answer using 9.81 and the right answer using the acceleration I worked out - why is this the case? that acceleration I worked out - is it just for the vertical direction since it says going UPWARDS

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#7

There are now two forces acting on those charges, in opposite directions

E-field will exert force Eq in one direction, B-field will exert force Bqv in the other.

For the charges to pass through the slit, they must be undeflected which means the resultant force on them must be zero.

Hopefully that's enough for you to answer it.

E-field will exert force Eq in one direction, B-field will exert force Bqv in the other.

For the charges to pass through the slit, they must be undeflected which means the resultant force on them must be zero.

Hopefully that's enough for you to answer it.

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#8

(Original post by

The acceleration I'd use here would be 9.81 right? as it vertical motion? So I don't use the acceleration I just worked out? or do i...

**MrToodles4**)The acceleration I'd use here would be 9.81 right? as it vertical motion? So I don't use the acceleration I just worked out? or do i...

NOOOOOOO!!!!!! Use the one you've worked out.

Gravitational pull on an electron will be much smaller than the force due the the E field so ignore it.

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(Original post by

NOOOOOOO!!!!!! Use the one you've worked out.

Gravitational pull on an electron will be much smaller than the force due the the E field so ignore it.

**phys981**)NOOOOOOO!!!!!! Use the one you've worked out.

Gravitational pull on an electron will be much smaller than the force due the the E field so ignore it.

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(Original post by

There are now two forces acting on those charges, in opposite directions

E-field will exert force Eq in one direction, B-field will exert force Bqv in the other.

For the charges to pass through the slit, they must be undeflected which means the resultant force on them must be zero.

Hopefully that's enough for you to answer it.

**phys981**)There are now two forces acting on those charges, in opposite directions

E-field will exert force Eq in one direction, B-field will exert force Bqv in the other.

For the charges to pass through the slit, they must be undeflected which means the resultant force on them must be zero.

Hopefully that's enough for you to answer it.

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#11

(Original post by

I understand that it won't be equal anymore - so magnetic force is greater as BQV and speed is greater.

**MrToodles4**)I understand that it won't be equal anymore - so magnetic force is greater as BQV and speed is greater.

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(Original post by

yes, so it will accelerate (vertically) under what is now an unbalanced force, which means it will be deflected and will miss the slit.

**phys981**)yes, so it will accelerate (vertically) under what is now an unbalanced force, which means it will be deflected and will miss the slit.

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#13

The Q tells us that they are + charges so we know they will be deflected downwards by the electric field, ie towards the negative plate.

I would (lazily) assume that the magnetic field is in the opposite direction - we know this has to be true if some pass through undeflected. However, if you want to check, it's the Left hand rule for magnetic force, yes.

I would (lazily) assume that the magnetic field is in the opposite direction - we know this has to be true if some pass through undeflected. However, if you want to check, it's the Left hand rule for magnetic force, yes.

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(Original post by

The Q tells us that they are + charges so we know they will be deflected downwards by the electric field, ie towards the negative plate.

I would (lazily) assume that the magnetic field is in the opposite direction - we know this has to be true if some pass through undeflected. However, if you want to check, it's the Left hand rule for magnetic force, yes.

**phys981**)The Q tells us that they are + charges so we know they will be deflected downwards by the electric field, ie towards the negative plate.

I would (lazily) assume that the magnetic field is in the opposite direction - we know this has to be true if some pass through undeflected. However, if you want to check, it's the Left hand rule for magnetic force, yes.

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#15

Yes, they're positive so like conventional current.

And yes, with negative charges you have to watch what you do with the current finger in the L H rule.

And yes, with negative charges you have to watch what you do with the current finger in the L H rule.

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(Original post by

Yes, they're positive so like conventional current.

And yes, with negative charges you have to watch what you do with the current finger in the L H rule.

**phys981**)Yes, they're positive so like conventional current.

And yes, with negative charges you have to watch what you do with the current finger in the L H rule.

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#17

This is just trig. You know the horizontal and vertical components of the velocity - draw them as vectors and find the angle between.

something like this

You probably did some stuff like this when you did the forces and motion topic, but that will seem like a long time ago now.

something like this

You probably did some stuff like this when you did the forces and motion topic, but that will seem like a long time ago now.

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(Original post by

This is just trig. You know the horizontal and vertical components of the velocity - draw them as vectors and find the angle between.

something like this

You probably did some stuff like this when you did the forces and motion topic, but that will seem like a long time ago now.

**phys981**)This is just trig. You know the horizontal and vertical components of the velocity - draw them as vectors and find the angle between.

something like this

You probably did some stuff like this when you did the forces and motion topic, but that will seem like a long time ago now.

for part f I would use SUVAT right? I tried doing it with the information I already worked but I keep getting 2.5cm whereas the correct answer is 25cm. So I think I'm meant to look at it horizontally again - find the time, then use SUVAT again with the vertical motion to find the displacement? So horizontally would I use the distance from just outside the plates to the screen? or the whole distance of the entire diagram

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(Original post by

I will try it to check, 25cm is quite big for one of those screens...

**phys981**)I will try it to check, 25cm is quite big for one of those screens...

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