# Electrostatics, finding velocity

1. Jun 11, 2013

### physics604

1. A moving proton has 6.4 x 10-16 J of kinetic energy. The proton is accelerated by a potential difference of 5 000 V between parallel plates. The proton emerges from the parallel plates with what speed?

a) 1.3 x 106 m/s
b) 8.8 x 105 m/s
c)1.8 x 106 m/s
d) 9.8 x 105 m/s

2. Relevant equations

$\Delta$Ek + $\Delta$Ep = 0

3. The attempt at a solution

Eki + Ekf = -Ep
6.4×10-16 + 1/2mv2 = -Q$\Delta$v

v= √ [(-1.6x10-19×-5000)-6.4x10-16 ×2 / 1.67x10-27] = 437740.5241...

My answer doesn't match with any of the responses. What did I do wrong?

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2. Jun 11, 2013

### Simon Bridge

Looks like you have misplaced a minus sign or three.
I suspect you have applied the formula without understanding it.

Derive the relation you need using:
Gain in kinetic energy = loss in potential energy

Hint:
what is the initial speed of the proton?
should the final speed be greater than or less than this?
what does this say about the final kinetic energy vs the initial kinetic energy?
how does this relate to the change in potential energy (careful)?

3. Jun 11, 2013

### physics604

Okay, I get it!

$\Delta$Ek = Ekf - Eki

I just used that part of the formula wrong.

4. Jun 11, 2013

### Simon Bridge

Though had you gone to the physics first, you wouldn't have needed to know how to use any particular formula.
You can get a long way just looking for equation to stick the numbers you have into - but that way of thinking will always bite you eventually.

But "no worries" aye?