# Help with physics magnetic field question

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#1
http://pmt.physicsandmathstutor.com/...%20A-level.pdf

for question 3d... it says to describe and explain the deflection so I wrote that it would be deflected less as the mass is larger - but the answer says 'downwards'??

So Im a bit confused - how would I know that what they wanted from me? it didn't ask for resultant force :/

http://pmt.physicsandmathstutor.com/...%20A-level.pdf
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3 years ago
#2
(Original post by MrToodles4)
for question 3d... it says to describe and explain the deflection so I wrote that it would be deflected less as the mass is larger - but the answer says 'downwards'??
You are having to compare what happens in this new situation to what had just happened in part (c). But in that part, there is no deflection of the ions at all, as the magnetic and electric fields have been set so that the forces due to the magnetic field and the electric field are of equal magnitude and opposite directions.

So, for part (d), you would be expected to descibe how these two forces change (if at all), and what the overall effect of this is.

You can't say that they would be deflected less, as in (c) they are not deflected at all!
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#3
(Original post by Pangol)
You are having to compare what happens in this new situation to what had just happened in part (c). But in that part, there is no deflection of the ions at all, as the magnetic and electric fields have been set so that the forces due to the magnetic field and the electric field are of equal magnitude and opposite directions.

So, for part (d), you would be expected to descibe how these two forces change (if at all), and what the overall effect of this is.

You can't say that they would be deflected less, as in (c) they are not deflected at all!
Ohhh, that makes a lot of sense. thank you so much You know for question c you use F=EQ and F=BQv

I first used F=W*d. I thought W = eV? (kinetic energy) - and the horizontal distance is 16mm... So then I plugged that force into F=BQv to find B but that method was wrong - why is that?

Again, many thanks.
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3 years ago
#4
(Original post by MrToodles4)
I first used F=W*d. I thought W = eV? (kinetic energy) - and the horizontal distance is 16mm... So then I plugged that force into F=BQv to find B but that method was wrong - why is that?

Are you sure that F = Wd is correct? But the key to this is the first thing you say;

(Original post by MrToodles4)
Ohhh, that makes a lot of sense. thank you so much You know for question c you use F=EQ and F=BQv
You can see the the electric force is going to be unchanged, because E and Q are both unchanged. As for the magnetic force; B and Q are both unchanged, but you need to think about the effect of the increased mass on v.
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#5
(Original post by Pangol)
Are you sure that F = Wd is correct? But the key to this is the first thing you say;

You can see the the electric force is going to be unchanged, because E and Q are both unchanged. As for the magnetic force; B and Q are both unchanged, but you need to think about the effect of the increased mass on v.
OMG. Okay I see where i went wrong now. And yes v would be less and so force would be less on the magnetic field - and magnetic field is going upwards? and therefore since that upward force is now less - particle will deflect downwards!

Will they ever give a question with magnetic field and electric field ins are direction? Or does that just simply not happen?
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3 years ago
#6
(Original post by MrToodles4)
OMG. Okay I see where i went wrong now. And yes v would be less and so force would be less on the magnetic field - and magnetic field is going upwards? and therefore since that upward force is now less - particle will deflect downwards!
Ta-da!

(Original post by MrToodles4)
Will they ever give a question with magnetic field and electric field ins are direction? Or does that just simply not happen?
Do you mean in the same direction? (And if so, I'm assuming that you mean the forces felt by the particles are in the same direction, not the fields themselves.) Well, they could do, but it would be a less interesting question.
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#7
(Original post by Pangol)
Ta-da!

Do you mean in the same direction? (And if so, I'm assuming that you mean the forces felt by the particles are in the same direction, not the fields themselves.) Well, they could do, but it would be a less interesting question.
Yeah thats what i meant - ok

Could you help me with 4civ here:

http://pmt.physicsandmathstutor.com/...%20A-level.pdf

How would you find the emf? I know its gradient so would I need to draw a tangent at 0.05?
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3 years ago
#8
(Original post by MrToodles4)
How would you find the emf? I know its gradient so would I need to draw a tangent at 0.05?
That would give you an idea of the rate of change of flux when t = 0.005 (which I am sure is what you mean, not 0.05). But you have to find the average emf from 0 to 0.005. Just calculate the change in flux from t = 0 to t = 0.005, and then use this to find the rate of change of flux linkage between these times. (And be careful to use flux linkage when you need to - is this what you can get directly from the graph, or is there something else to do?)
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#9
(Original post by Pangol)
That would give you an idea of the rate of change of flux when t = 0.005 (which I am sure is what you mean, not 0.05). But you have to find the average emf from 0 to 0.005. Just calculate the change in flux from t = 0 to t = 0.005, and then use this to find the rate of change of flux linkage between these times. (And be careful to use flux linkage when you need to - is this what you can get directly from the graph, or is there something else to do?)
Oh so since this is average we use values from the graph?

Many thanks Yes I read off 8.8*20^-5 and multiples by 500 before dividing by 0.005 to get 8.8V
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