Question

A proton passes through a potential difference of 350 V. Find it's kinetic energy and velocity. What's wrong with my work?

Here's my work:

`DeltaU_e=qDeltaV=-W_e=-DeltaK=-(K_f-cancel(K_i))=-K_f`

`K_f=-qDeltaV=-(1.6*10^{-19})(350)=-5.6*10^{-19}`

So `K_f = -5.6*10^{-19} J`

This doesn't make sense though as I don't think kinetic energy can be negative. And if it is negative then what would the velocity be imaginary which definitely doesn't make sense

What am I doing wrong?

Answer 1

`Delta V` is negative for protons as protons go from high potential to low potential.

Explanation:

I realized what I did wrong. All my math was correct, but at the final step where I plugged my values into `K_f = -q DeltaV`, I didn't realize that `Delta V` was negative.

This is because protons go from high potentials to low potentials. `Delta V = V_f-V_i = V_{low}-V_{high} = 0 - 350 = -350V`

So when you plugin the values, you get:

`K_f = -q DeltaV = -(1.6*10^{-19})(-350) = 5.6*10^{-19}J`

Thanks for trying to help :P