# Question Pertaining to Voltage

Shinster007

## Homework Statement

If an electron travels 0.350 m from an electron gun to a TV screen in 46.0 ns, what voltage was used to accelerate it? (Note that the voltage you obtain here is lower than actually used in TVs to avoid the necessity of relativistic corrections.)

## Homework Equations

E=k(q/r^2) ΔV=|Ed| ΔKE + ΔPE = 0

## The Attempt at a Solution

I have no clue how to answer this question. I don't know if it's because I am trying to use the wrong equations, or because I am just not seeing the connections

## Answers and Replies

Homework Helper
If the electron travels 0.35 m in 46 ns, what velocity does it travel at? Hence what KE does it posses at this velocity? Also how does voltage and charge relate to energy?

Shinster007
If the electron travels 0.35 m in 46 ns, what velocity does it travel at? Hence what KE does it posses at this velocity? Also how does voltage and charge relate to energy?

Well the velocity is going to be at about 7.6x10^6 m/s. I know that K.E.= 1/2mv^2, so can I calculate the kinetic energy of that and use it as the energy to calculate voltage? I am a little lost

Mentor
Keep in mind that the electron will be accelerating throughout its journey; It doesn't cover the interval at a constant speed.

What's the formula for the energy gained by a charged particle falling through a given potential difference?

Shinster007
E = |qΔV|? is that the equation you are talking about?

Mentor
E = |qΔV|? is that the equation you are talking about?

Yes.

Shinster007
Well the only thing that I can think of using is the Δ Kinetic Energy + Δ Electric Potential=0 and then trying to solve for V (voltage).

Mentor
Well the only thing that I can think of using is the Δ Kinetic Energy + Δ Electric Potential=0 and then trying to solve for V (voltage).

Sounds like a good plan Shinster007
Ha ha okay, I actually tried doing that earlier but I got the wrong answer. Which of the Velocities and voltages are zero? And for the mass of the electron do I use 9.1X10^-31?

Mentor
Ha ha okay, I actually tried doing that earlier but I got the wrong answer. Which of the Velocities and voltages are zero? And for the mass of the electron do I use 9.1X10^-31?

Show your work so we can see where something might have gone wrong.

The electron starts out with zero velocity and accelerates to the screen thanks to the potential difference.

Shinster007
1.)1/2m(Vf^2-Vi^2) + q(Vf-Vi)=0, cancel out initial velocity and final voltage (I think) since they are zero.
2.)1/2mVf^2-qV=0
3.) 1/2mVf^2=qV
4.)1/2mVf^2/q=V

That's what I tried, and got the wrong answer so I guess that I am setting up the equation wrong.

Shinster007
m=mass of electron, 9.1x10^-31 kg
Vf= 7.61x10^6
q= 1.6x10^-19

Those are the numbers I am using

Mentor
m=mass of electron, 9.1x10^-31 kg
Vf= 7.61x10^6
q= 1.6x10^-19

Those are the numbers I am using

How did you determine Vf? Remember that the electron is undergoing accelerated motion, not constant velocity...

Shinster007
Hmmm that is true. I am kinda stumped then

hms.tech
How did you get this Velocity ?

I think you have erred in calculating the Final Velocity at x=0.35m

Mentor
Hmmm that is true. I am kinda stumped then

It's just the usual accelerated motion of a projectile. Use the information given (distance and time) to find the final velocity assuming that a constant acceleration is applied. Hint: you can begin by determining the acceleration.

Shinster007
Okay, I will try to figure it out. In the meantime, can you help me with another question?

The problem is as follows:

In nuclear fission, a nucleus splits roughly in half.

(a) What is the potential 3.50 10-14 m from a fragment that has 50 protons in it?
I calculated this to be 2.05x10^6, which is correct.

(b) What is the potential energy in MeV of a similarly charged fragment at this distance?

I have no idea what this part is asking for; our professor did not go over those units at all and the textbook we are using is very vague on the subject.

Mentor
Okay, I will try to figure it out. In the meantime, can you help me with another question?

The problem is as follows:

In nuclear fission, a nucleus splits roughly in half.

(a) What is the potential 3.50 10-14 m from a fragment that has 50 protons in it?
I calculated this to be 2.05x10^6, which is correct.

(b) What is the potential energy in MeV of a similarly charged fragment at this distance?

I have no idea what this part is asking for; our professor did not go over those units at all and the textbook we are using is very vague on the subject.

eV stands for 'electron volt'. It's a unit of energy equivalent to the energy gained by a unit electric charge (the magnitude of the charge on the electron) falling through a potential difference of 1 volt. Hence electron volt.

MeV is Mega eV, that is, a million eV.

Shinster007
Okay, so how can I go from the volts to ev?

Mentor
Okay, so how can I go from the volts to ev?

First you'll have to determine the potential energy of the second fragment at the stated location. You know its charge and the potential at the given distance. Convert the resulting energy value (Joules) to eV.