# Homework Help: Electron Charge to Mass Ratio

1. Sep 30, 2008

### strawberrysk8

1. The problem statement, all variables and given/known data

Electrons from a heated cathode are accelerated through a potential difference of 25.0 V. Calculate the speed of the electrons escaping through a hole in the anode.

2. Relevant equations

v = (2eV/m)^(1/2)

3. The attempt at a solution

v = (2*25*1.759*10^11)^(1/2) = 2.97*10^6 m/s

but this answer is wrong. why?

2. Sep 30, 2008

### Redbelly98

Staff Emeritus
What are the units for each number you use in the calculation? Perhaps the units are not consistent.

3. Oct 1, 2008

### strawberrysk8

v = (2* 25V * 1.759*10^11 C/kg) ^(1/2) = 2.97*10^6 m/s

(VC/kg)^(1/2) = m/s

or

VC/kg = m^2/s^2

is that true?

4. Oct 1, 2008

### Redbelly98

Staff Emeritus
Okay, I agree with your answer, including the number of significant figures. I wasn't familiar with the value of e/m.

Hmmm ... since the calculated answer is 2.9655... *10^6 m/s, maybe try 2.96*10^6 m/s. They might have done some intermediate rounding off that changed their final answer slightly.

5. Oct 2, 2008

### strawberrysk8

that still turned out to be wrong. maybe i am using the wrong formula. this problem deals with a cathode and anode from a helm's coil. any suggestions?

6. Oct 2, 2008

### Redbelly98

Staff Emeritus
I'm unfamiliar with a helm's coil (or do you mean Helmholtz coil?).

Your formula is correct for electrons with zero, or at least negligible, initial kinetic energy at the cathode.
(Substitute eV = ½ m v2 in the equation to verify it, if you wish.)

Does anything in the problem statement, or in the text book, indicate a nonzero initial speed for the electrons? If not, I am stumped ... does somebody else have any ideas?

7. Oct 4, 2008

### strawberrysk8

thank you so much! i had been typing the answer wrong. the computer did not want the answer in scientific notation. so your advice was correct. thank you

8. Oct 4, 2008

### Redbelly98

Staff Emeritus