PetitePanda
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Could anyone help me with this question? I have no idea how to start no matter how much I look at it. It's Q2 of this link: https://pmt.physicsandmathstutor.com...0Atom%20QP.pdf
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Stonebridge
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First the direction, up or down.

If the ion is positive then the electric field direction needs to oppose and balance the downwards force on the ion due to gravity.This gives the direction of the field.

The magnitude of the field needs to balance the weight of the ion.
Its weight is mg downwards.
You know that if the field is of strength E then the force on a charge q is = Eq
Finally they give you a value for q/m for the ion as 2.4 x 107
This info, rearranged a bit and equated, will give you a value for E.
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PetitePanda
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(Original post by Stonebridge)
First the direction, up or down.

If the ion is positive then the electric field direction needs to oppose and balance the downwards force on the ion due to gravity.This gives the direction of the field.

The magnitude of the field needs to balance the weight of the ion.
Its weight is mg downwards.
You know that if the field is of strength E then the force on a charge q is = Eq
Finally they give you a value for q/m for the ion as 2.4 x 107
This info, rearranged a bit and equated, will give you a value for E.
Thank you so much. I'm so sorry to ask but could you explain the direction a bit more because I do understand why it would be upwards with your explanation but why does the electric field direction need to oppose if the ion is positive? I can follow the instruction of the equation but I dont understand why it is tbh or how I could even think of that method. Is there any topics I could revise on so I can understand the method to do so better?
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Stonebridge
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The direction of an electric field is, by definition, the direction of the force on a positive charge.
So to balance the gravitational force downwards on the positive ion, the electric force needs to be upwards.
So the electric field needs to be directed upwards.
This is in order to hold the ion stationary. The two forces are balanced. Gravity downwards and the electric field upwards.

In the calculation, you are equating these 2 forces, mg downwards and the electric field force upwards

so Eq = mg where q is the charge on the ion and m is its mass
so
E=mg/q
We are given the charge to mass ratio of this ion which means we have a value of q/m
This eliminates q and m.
We also have a value for g, so we can find E.

When doing a question like this, asking you to find E, the first thing I would do is write down any equation I know with E in it and which involves charge and force. These are the things in the question.
So I think of F=Eq for the ion.
As it is being suspended I think of the ion's weight, mg, being balanced and equal to that force Eq.

Just find and try more similar questions. After a while, the pattern and method will be familiar.
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PetitePanda
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(Original post by Stonebridge)
The direction of an electric field is, by definition, the direction of the force on a positive charge.
So to balance the gravitational force downwards on the positive ion, the electric force needs to be upwards.
So the electric field needs to be directed upwards.
This is in order to hold the ion stationary. The two forces are balanced. Gravity downwards and the electric field upwards.

In the calculation, you are equating these 2 forces, mg downwards and the electric field force upwards

so Eq = mg where q is the charge on the ion and m is its mass
so
E=mg/q
We are given the charge to mass ratio of this ion which means we have a value of q/m
This eliminates q and m.
We also have a value for g, so we can find E.

When doing a question like this, asking you to find E, the first thing I would do is write down any equation I know with E in it and which involves charge and force. These are the things in the question.
So I think of F=Eq for the ion.
As it is being suspended I think of the ion's weight, mg, being balanced and equal to that force Eq.

Just find and try more similar questions. After a while, the pattern and method will be familiar.
PRSOM Ohh that makes more sense. I understand what you're getting at and everything but how do I find g? I've tried to arrange it but I got 2.4 x 10^7 e = g/q - am I doing this part wrong? Do you know what topic this is in so I can find similar questions?
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Stonebridge
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You don't need to find g. It is always given as a value either in the question itself, or in the data list for the examination.
It's usually either 9.8 or 9.81 m/s2 Sometimes even 10m/s2
Whatever value is given.

So when you get
Eq = mg and rearrange to get
E=mg/q
I imagine you are given the value of g to be 9.81 m/s2 as the answers for A to D are given to 3 significant figures.
You are given the charge to mass ratio q/m for the ion as 2.40 x 107
so in the formula you have there m/q is 1 / 2.40 x 107

This question touches on 2 topic areas in physics.
1) Electric fields
2) Atomic physics - in particular, a) Millikan's experiment to measure charge by suspending a charged particle in an electric field, and b) measurement of the mass of an ion using a mass spectrometer.
Take a look in those topic areas to find possible similar questions.
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PetitePanda
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(Original post by Stonebridge)
You don't need to find g. It is always given as a value either in the question itself, or in the data list for the examination.
It's usually either 9.8 or 9.81 m/s2 Sometimes even 10m/s2
Whatever value is given.

So when you get
Eq = mg and rearrange to get
E=mg/q
I imagine you are given the value of g to be 9.81 m/s2 as the answers for A to D are given to 3 significant figures.
You are given the charge to mass ratio q/m for the ion as 2.40 x 107
so in the formula you have there m/q is 1 / 2.40 x 107

This question touches on 2 topic areas in physics.
1) Electric fields
2) Atomic physics - in particular, a) Millikan's experiment to measure charge by suspending a charged particle in an electric field, and b) measurement of the mass of an ion using a mass spectrometer.
Take a look in those topic areas to find possible similar questions.
OMG why did I overcomplicate everything? Thank you so much I'm glad you helped me once again. Thank you so much I'm going to learn that and practice it further
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