Answer check

I agree with your answer, so most probably a typo.

Reply 2

OP

Original post by Pangol

I agree with your answer, so most probably a typo.

Some more answers that the book may have got wrong.

1. Electrons moving at a speed of 1.35x10⁷ms^-1 are directed into a uniform magnetic field of flux density 1.54x10^-3T in such a direction as to form a cirlce of diameter 50mm. Calculate the specific charge, e/m, of the electron.

I got 3.51x10¹¹Ckg^-1 whilst the book gets 1.80x10¹¹Ckg^-1. Now obviously the specific charge of the electron is way off what I got but given the information in the question, is my answer correct (book may have used diameter rather than radius, as it did in a worked example which makes me feel a bit suspicious I'm going wrong).

2. Electrons emitted from the heated filament are accelerated througha pd of 550V to form a beam which is then directed into a uniform magnetic field of magnetic flux density 2.8mT. The magnetic field deflects the beam into a circle of radius 28mm. Use these data to calculate the specific charge of the electron.

I got 1.77x10¹¹Ckg^-1 whilst the book gets 1.79x10¹¹Ckg^-1. Not a big difference but can't see why there is one.

Thankyou.

(edited 11 years ago)

Reply 3

First question - they obviously meant to say radius when they said diameter (although I make it 1.75 x 10^11 with that data).

Second question - I agree with them, to three significant figures. Mis-typed digit on your calculator?

Reply 4

OP

Original post by Pangol

First question - they obviously meant to say radius when they said diameter (although I make it 1.75 x 10^11 with that data).

Second question - I agree with them, to three significant figures. Mis-typed digit on your calculator?

Well I calculated the velocity from 0.5mv²=eV
and then used e/m=v/Br to get my answer.

I've found that if I used the equation mentioned in the book
e/m=2V/(B²r²)
I get the correct answer.

What is wrong with my method :s

Reply 5

Can't tell without seeing your working, but it is almost certainly a rounding error introduced by using a value of the velocity to a certain number of significant figures part way through. Not the biggest deal, the main thing is that you know exactly what you are doing.

Reply 6

OP

Original post by Pangol

Can't tell without seeing your working, but it is almost certainly a rounding error introduced by using a value of the velocity to a certain number of significant figures part way through. Not the biggest deal, the main thing is that you know exactly what you are doing.

Actually I've realised that my method may not be allowed...

Because if I find the velocity from KE=eV then I am making an assumption about the charge on the electron aren't I? Given that the question wants e/m, then me taking e as 1.6x10^-19 to find the velocity would alter the answer a bit. I should substitute the expression for the speed into r=mv/Be and the make e/m the subject before solving. Hence why the book gets a slight different answer. Right?

Reply 7

Ooh, extremely good point. Yes, I think you are right in saying that you cannot assume the value of e. If you could, why not also assume the value of m and just calculate e/m directly?! Obviously outside the spirit of the question, so I am sure that you are right to say that you have to do it without using a value for either of these quantities.

The algebra comes out quite nicely, anyway.

Reply 8

OP