# To find the HORIZONTAL speed of an electron entering an electric field at an angle?

## Homework Statement

So if an electron enters an electric field (between two charged parallel plates) at an angle (i.e not parallel to the x axis) and knowing that the speed is, say, 5 x 10^6 m/s, how would you find the HORIZONTAL velocity of the electron?

you know that horizontaly it must be (5 x 10^6)cos(theta) and that vertically it must therefore be (5 x 10^6)sin(theta) but how would you find the horizontal velocity?

## The Attempt at a Solution

I thought it might help knowing that sin(theta)/cos(theta) = tan(theta) ?
But no idea how to find the horizontal speed...

tiny-tim
Homework Helper
Hi Amahia11 ! (have a theta: θ and try using the X2 icon just above the Reply box )

I don't understand … what are the unkowns, and what is given? If 5 x 106 and θ are both given, then it's just 5 x 106 cosθ. Hi Amahia11 ! (have a theta: θ and try using the X2 icon just above the Reply box )

I don't understand … what are the unkowns, and what is given? If 5 x 106 and θ are both given, then it's just 5 x 106 cosθ. hi tiny-tim! thanks for the tips! :D

Ok, basically my physics exam on particle dynamics in electric fields is next week and my teacher told us to remember that sinθ/cosθ=tanθ (something which we btw never used in class)

now i understand (you're right) that the horizontal component will be 5 x 196cosθ but my question is how could you use the equation "sinθ/cosθ=tanθ"? I'm just trying to figure out how that could be useful in anyway... any ideas?

(sorry my initial post was quite unclear and vague!)

tiny-tim
Homework Helper
Hi Amahia11! Ok, basically my physics exam on particle dynamics in electric fields is next week and my teacher told us to remember that sinθ/cosθ=tanθ (something which we btw never used in class)

…but my question is how could you use the equation "sinθ/cosθ=tanθ"? I'm just trying to figure out how that could be useful in anyway... any ideas?

ahhh …

yes, you will find tan = sin/cos useful, not when you're given the angle, but when you want to find it …

for example, if you know that the horizontal component of a force is 13, and the vertical component is 10, then the magnitude will be √(100 + 169), and the angle from the horizontal will have tan = 10/13 btw, you should at some stage be learning the basic trigonometric formulas:

(that's adjacent, opposite, hypotneuse )​