# V required so ions passing through the region don't devitate

• moenste
In summary, the conversation discusses the movement of hydrogen ions in the presence of electric and magnetic fields. The first question asks for the value of the potential difference needed for the ions to pass through the fields without being deviated, and the answer is 0.1V. The second question asks for the kinetic energy of the ions as they leave the fields, and the answer is 8.4 * 10-24 J. Different attempts at solving the problem are also mentioned, including using the formula F = B e v to calculate the force needed to overcome the magnetic field. The conversation also touches on the purpose of the parallel plates and the importance of considering only electric and magnetic forces at this stage.
cnh1995 said:
First term is incorrect. And the charge should be q instead of e since the ion is not an electron.
gneill said:
e is commonly used to refer to the unit of elementary charge though, so it actually works in this case. I'd be more picky about this if the ions involved had a different charge than a single elementary charge.
Maybe it's e E = B e v?

moenste said:
Maybe it's e E = B e v?
Right. So how would you proceed then?

moenste
moenste said:
Maybe it's e E = B e v?
Yes, that is a valid expression of the condition that the magnitude of the electric force balances the magnitude of the magnetic force on the moving charge. The result of course is no net force, so no deviation.

moenste
cnh1995 said:
Right. So how would you proceed then?
E = B v = 0.1 * 100 = 10 V, not 0.1 V.

moenste said:
E = B v = 0.1 * 100 = 10 V, not 0.1 V.
That is the expression for electric field, not voltage. How is electric field related to voltage?

moenste
moenste said:
E = B v = 0.1 * 100 = 10 V, not 0.1 V.

You've found the magnitude of E, but check your units: The units of E are not volts! What's the expression for the electric field established between parallel plates?

moenste
cnh1995 said:
That is the expression for electric field, not voltage. How is electric field related to voltage?
E = V / d?

moenste said:
E = V / d?
Yes!

It's easy to remember because the units of E are volts/m.

moenste
gneill said:
Yes!

It's easy to remember because the units of E are volts/m.
Why do we need it? We need to find V and we don't know E.

moenste said:
Why do we need it? We need to find V and we don't know E.
You found E in post #39.

moenste
gneill said:
You found E in post #39.
So, E = V / d, V = B v d = 0.1 * 100 * (10 / 10 / 100) = 0.1 V.

How do we find the kinetic energy per ion?
KE = 0.5 m v2 = 0.5 * 1.67 * 10-27 * 1002 = 8.35 * 10-24 J?

Last edited:
moenste said:
So, E = V / d, V = B v d = 0.1 * 100 * (10 / 10 / 100) = 0.1 V.

How do we find the kinetic energy per ion?
KE = 0.5 m v2 = 0.5 * 1.67 * 10-27 * 1002 = 8.35 * 10-24 J?
Yes and yes.

moenste

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