Particle physics Q

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Jaydude
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It says initial energy at 60mm, therefore I used graph and at 60mm there are 2000 ionisations per mm.

Then I used this and fact that one ionisation equals 30eV. Using basic maths I calculated the initial energy to be 60000 eV.

But this is wrong, They say the AREA needs to be calculated, but I don't understand why...it says initial energy at 60mm so why won't my method work?

Thanks!


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Jaydude
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Bump! (Sorry)


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Doctor_Einstein
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(Original post by Jaydude)
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It says initial energy at 60mm, therefore I used graph and at 60mm there are 2000 ionisations per mm.

Then I used this and fact that one ionisation equals 30eV. Using basic maths I calculated the initial energy to be 60000 eV.

But this is wrong, They say the AREA needs to be calculated, but I don't understand why...it says initial energy at 60mm so why won't my method work?

Thanks!


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We want to first compute the number of ionisations the alpha particle produced. The x-axis shows how many ionisations alpha particle produces for every mm it travels.

Therefore it is not correct to go 2000*30, because at 60mm, the ionisation rate is 2000 ionisations per mm, not 2000 isonisations altogether.

It is also not correct to multiply 2000*distance in mm = 2000*60, and then multiply by 30. This is wrong because the rate of ionisation varied along the alpha particle's journey.

The number of ionisations made from 0 to 60mm is calculated by the area under the curve. This effectively calculates the rate of ionisations per mm (x-axis) multiplied by distance traveled in mm (y-axis) to give total number of ionisations. Energy is therefore Area under curve*30eV.
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Jaydude
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(Original post by Doctor_Einstein)
We want to first compute the number of ionisations the alpha particle produced. The x-axis shows how many ionisations alpha particle produces for every mm it travels.

Therefore it is not correct to go 2000*30, because at 60mm, the ionisation rate is 2000 ionisations per mm, not 2000 isonisations altogether.

It is also not correct to multiply 2000*distance in mm = 2000*60, and then multiply by 30. This is wrong because the rate of ionisation varied along the alpha particle's journey.

The number of ionisations made from 0 to 60mm is calculated by the area under the curve. This effectively calculates the rate of ionisations per mm (x-axis) multiplied by distance traveled in mm (y-axis) to give total number of ionisations. Energy is therefore Area under curve*30eV.
Oh I think I understand, at 60mm at that INSTANT there are 2000ionisations, but it has actually lost energy from travelling THROUGH 60mm of air, so it has initial energy at 0, then losses it gradually.
Thus the area gives the total number of ionisations through travelling 60mm of air and so if one ionisation is 30eV, like you said Area times 30eVgives total energy.
The phrase "initial " in the question confused me, as I thought it wanted rate of energy at a specific instant, but what you stated makes sense, thanks again! (I'm I'm wrong above please correct me!)


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Doctor_Einstein
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(Original post by Jaydude)
Oh I think I understand, at 60mm at that INSTANT there are 2000ionisations, but it has actually lost energy from travelling THROUGH 60mm of air, so it has initial energy at 0, then losses it gradually.
Thus the area gives the total number of ionisations through travelling 60mm of air and so if one ionisation is 30eV, like you said Area times 30eVgives total energy.
The phrase "initial " in the question confused me, as I thought it wanted rate of energy at a specific instant, but what you stated makes sense, thanks again! (I'm I'm wrong above please correct me!)


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Yes I think you are understanding, just note that at 60mm there are 2000 ionisations per mm, not 2000 ionisations.

The estimated energy of the alpha particle at the beginning will be equal to the energy it can give off through ionisations throughout its journey, given by area under curve*30.
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