# Thomsons e/m experiment

I have to do a question and im stuck on it, can anyone help me with it?

You are trying to do Thomsons e/m experiment and you have a 0 to 250V variable high voltage supply and a wide gap electromagnet with field of 0.5 T in the gap

a) draw a diagram of the apparatus showing the placcement of the internatl components and any needed dimensions, which will be given to a glass blowing vacuum technology section for constrution of the unit

b) You discover that your variable supply can only go to 200 and not 250.
Your supervisor said he will buy a cheap fixed version rather than a new variable. You realise that using both supples will allow you to solve the problem. Explain how using the two supplies can get over the problem

c) Calculate the output voltage required from this supply for your purpose

If anyone can help with any parts i wood be greatful

I have tried to draw the apparatus but i cant find any thing on how to find out the dimensions of an electromagnet of 250V and 0.5T. If you give me the formula used to figure this out i wood be greatful

For part B i would think if you put the two power supplies in series you would get the needed output, but i dont no if that would work in reality or only im my world

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berkeman
Mentor
I have to do a question and im stuck on it, can anyone help me with it?

You are trying to do Thomsons e/m experiment and you have a 0 to 250V variable high voltage supply and a wide gap electromagnet with field of 0.5 T in the gap

a) draw a diagram of the apparatus showing the placcement of the internatl components and any needed dimensions, which will be given to a glass blowing vacuum technology section for constrution of the unit

b) You discover that your variable supply can only go to 200 and not 250.
Your supervisor said he will buy a cheap fixed version rather than a new variable. You realise that using both supples will allow you to solve the problem. Explain how using the two supplies can get over the problem

c) Calculate the output voltage required from this supply for your purpose

If anyone can help with any parts i wood be greatful

I have tried to draw the apparatus but i cant find any thing on how to find out the dimensions of an electromagnet of 250V and 0.5T. If you give me the formula used to figure this out i wood be greatful

For part B i would think if you put the two power supplies in series you would get the needed output, but i dont no if that would work in reality or only im my world
I'm assuming that you're using the Electric field to accelerate electrons, and the perpendicular B field of the magnet to deflect the electrons (and measure the deflection based on the bias voltage). Is this correct? Can you post a sketch of how you would position the plates and the magnet? What are you supposed to use for your cathode (electron source)?

And you are correct to series-connect the fixed and variable power supplies. As long as at least one of them is floating (the outputs are isolated from Earth ground), you can series connect them. So if you nave a 200V variable supply, and you want to be able to get up to 250V, what value of fixed supply should you specify?

it wood look something like this
http://www.calstatela.edu/faculty/kaniol/f2000_lect_nuclphys/lect1/jj_aparatus.jpg [Broken]

The electrons from the cathode are accelerated and forms a beam by the PD (of 250V) between the anodes. This beam then passes through the wide gap electromagnet with field of 0.5T in the gap. Then the beam hits the screen at the end.

I wood use a 50V fixed power supply in series with the 200V power supply.

If that is right then all i need help with is the dimensions of the elctromagnet. is there a formula to relate voltage, tesla and the area?

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berkeman
Mentor
it wood look something like this
http://www.calstatela.edu/faculty/kaniol/f2000_lect_nuclphys/lect1/jj_aparatus.jpg [Broken]

The electrons from the cathode are accelerated and forms a beam by the PD (of 250V) between the anodes. This beam then passes through the wide gap electromagnet with field of 0.5T in the gap. Then the beam hits the screen at the end.

I wood use a 50V fixed power supply in series with the 200V power supply.

If that is right then all i need help with is the dimensions of the elctromagnet. is there a formula to relate voltage, tesla and the area?
Sounds like you are on the right track. Here is an introduction to electromagnets:

http://en.wikipedia.org/wiki/Electromagnet

Quiz Question -- what are the advantages and disadvantages of using a Helmholtz coil versus an iron core electromagnet for this application?

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thanks but is there any equation for connecting voltage to tesla?

the advantage is that the Helmholtz coil can produce an almost uniform magnetic field while the electromagnet wiht an iron core could cause more of an non uniform magnet field as the iron core could have flaws in it

berkeman
Mentor
thanks but is there any equation for connecting voltage to tesla?
Yes, see the wikipedia link I posted.

the advantage is that the Helmholtz coil can produce an almost uniform magnetic field while the electromagnet wiht an iron core could cause more of an non uniform magnet field as the iron core could have flaws in it
Correct about the Helmholtz coil. The iron core electromagnet has a less uniform field, but not because of flaws. It's more because fringing -- see the figures in the wikipedia link.