# Electrons moving in electric field

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1. Sep 2, 2016

### ooohffff

1. The problem statement, all variables and given/known data
Suppose electrons enter the electric field midway between two plates at an angle θ0 to the horizontal, as shown in the figure, where L = 5.1 cm and H = 1.1 cm. The path is symmetrical, so they leave at the same angle θ0 and just barely miss the top plate. What is θ0? Ignore fringing of the field.

2. Relevant equations
F = qE

3. The attempt at a solution
Not sure where to start but I'm assuming I have to break things into components and that there's trig involved...

2. Sep 2, 2016

### Simon Bridge

Very good - that is an excellent start.
You have actually done problems like this before.
Hint: the electron is a projectile fired at an angle to the horizontal in a uniform vertical force field

3. Sep 2, 2016

### feynmann93

Indeed you can solve it by using projectile motion. By getting the ratio of maximum heights (H) and the range (L), you can determine the angle.

4. Sep 3, 2016

5. Sep 3, 2016

### Staff: Mentor

Can you demonstrate that mathematically? Somehow I doubt that it is so

6. Sep 5, 2016

### Simon Bridge

For the benefit of someone else with the same issue, please post what you did.

7. Sep 5, 2016

### feynmann93

The motion of electrons can be calculated by using the equations of projectile motion.
The electrons reach the maximum height at
H= v2*(sin(θ))2/2*g
and leave the plate at the range of
R=v2sin(2θ)/g
(Note: H is the distance between two plates, R is the length of plate, v is the initial velocity of electron, g is the acceleration acts on the electron )
by getting the ratio, we can eliminate the v and g by division thus leads to
H/R= sin2(θ)/2sin(2θ)
from the identity of trigonometry we know that 2sin(2θ)=2sinθcosθ
Therefore,
tanθ=2H/R. Put in the values of H and R we get
θ=tan-1(2H/R)

8. Sep 5, 2016

### Staff: Mentor

Nice. I didn't see that as what you were implying from your hint.