Homework Help: Speed of the Proton

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1. Feb 17, 2017

Vanessa Avila

1. The problem statement, all variables and given/known data
In the figure, a proton, mass 1.67×10^−27 kg, is projected horizontally midway between two parallel plates that are separated by 0.20cm, with an electrical field with magnitude 7.8×10^5N/C between the plates. If the plates are 5.40cm long, find the minimum speed of the proton that just misses the lower plate as it emerges from the field.

2. Relevant equations
q(E) = ma
x = vt
y = 1/2a(t^2)

3. The attempt at a solution

This is what I did:
1.6*10^-19(7.8*10^5) = 1.67*10^-27 (a)
solving for a I get: 7.47*10^13 m/s^2

then i solve for t
2*10^-3 = (1/2)(7.47*10^13)(t^2)
t = sqrt(7.47*10^13/((2)(2*10^-3))
I get 7.32*10^-9 s

then i plug that in the x = vt eqn
0.054/7.23*10^-9 = v
v = 7.4*10^6 m/s

But that is wrong

2. Feb 17, 2017

Xiaochao Zheng

It says the proton comes in midway, which means y is 0.1cm, not 0.2cm

3. Feb 17, 2017

Vanessa Avila

Oh I see.Thank you!