# Electron launched at an angle in a parallel plate field.

1. Jan 12, 2014

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

http://puu.sh/6i6HY.png [Broken]

2. Relevant equations

Fe = q*Ee

3. The attempt at a solution

a)
dx = vx*t
0.0682 = 4.92*106 * cos(68) * t
t = 0.0682 / [4.92*106 * cos(68)]
= 3.70036*10-8 seconds

dy = dyi + vy*t + at2
0 = 0 + 4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) + a * [0.0682 / [4.92*106 * cos(68)]]2
a = -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2
= -1.23278 * 1014 m/s2

mass of electron = 9.10938291 * 10-31 kg
charge of electron = 1.60217657 * 10-19 C

Fe = ma
= 9.10938291 * 10-31 * [ -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2 ]

Ee * q = Fe
Ee * 1.60217657 * 10-19 = 9.10938291 * 10-31 * [ -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2 ]
Ee = 9.10938291 * 10-31 * [ -4.92*106 * [0.0682 / [4.92*106 * cos(68)]] * sin(68) / [0.0682 / [4.92*106 * cos(68)]]2 ] / (1.60217657 * 10-19)

I got part b correct, so I must have the correct acceleration - now I can't figure out where I'm going wrong.

Last edited by a moderator: May 6, 2017
2. Jan 12, 2014

### Simon Bridge

If you have the acceleration right, then E=a(m/q)
If that gives the wrong E then (a) you have used a bad value for q/m for an electron - maybe a rounding error? or (b) you didn;t get the right acceleration and the correctness of the answer to part b is a fluke.

Best practice is to do all the algebra before putting the numbers in.

Last edited: Jan 12, 2014
3. Jan 12, 2014

### ehild

The time of flight is correct if you meant in in seconds.

Is it the same d as before, when you wrote that d=vt?

The electric field is normal to the capacitor plates. What is the direction of the acceleration? Does the electron accelerate horizontally?

ehild

Last edited by a moderator: May 6, 2017
4. Jan 12, 2014

No I used horizontal velocity and distance to find time, and then used vertical velocity, distance and time to find acceleration. I'll add subscripts to the OP to make that more clear

I took the electron values to many more decimals than the homework system checks (3).

I changed all my calculations to not use any previously calculated values and reformatted the exponents. Do you see any specific incorrect logic or calculation now?

5. Jan 12, 2014

### ehild

The equation

dy = dyi + vy*t + at2

is wrong. Check.

ehild

6. Jan 12, 2014

Oh man, messed up that equation after using it hundreds of times. Thank you very much, that fixed it.

(and I forget the 1/2 both times and the mistakes cancelled out which is why I got the right answer for part b )