I think i did this electric potential problem correctly but

[punktux]

Homework Statement

a helium nucleus (charge =2e, mass = 6.63E-27 kg) traveling at a speed of 6.20E5 m/s enters an electric field, traveling from point A, at a potential of 1.5E3 V, to point B, at 4.0E3 V. what is the speed at point B?

Homework Equations

charge = 2e = 2 * 1.6E-19 C
delta V is potential difference.
i used equation delta V = - (delta KE)/q and derived the relationship:
delta V = - mass/2q (vf^2 - vi^2)

The Attempt at a Solution

since the charge is 2e and e for electron is -1.6E-19 C, so the right hand side is positive. the left side is 4.0E3-1.5E3...finally i solved vf and got 7.91E5 m/s but the answer is 3.78E5 m/s

i don't get it...where is the mistake?

 

[Chrisas]

I think you are making a sign error. Electric fields point from higher potential to lower potential. The helium nucleus is +2e (not a negative charge). This positive charge is going from A to B, or lower potential to higher potential. This means it is going against the E field. This means that the nucleus will slow down, not speed up. Notice that the answer you want is smaller than the initial velocity.

Note: I haven't done the math to check your equations. I'm just looking at what the answer, in general, should look like.

 

[nlightner]

Based on the information provided, your approach and equations seem correct. However, it is always important to double check your calculations and make sure that you are using the correct values and units. In this case, it is possible that you may have made a mistake in your calculations which led to the incorrect answer. I would recommend going through your steps again and carefully checking each calculation to see if you can identify where the mistake may have occurred. Additionally, it is always helpful to have a second set of eyes look over your work, so you may want to consider asking a classmate or your instructor for assistance. Keep up the good work and don't be discouraged, science is all about trial and error and learning from mistakes!