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