# Homework Help: Electron in electric field

1. Jul 30, 2010

### Neesha

1. The problem statement, all variables and given/known data
a cathode ray tube contains parallel plates that are 30mm long and have an electric field strenght between them of 2.5x10^4. an electron travelling at 2.9x10^7m/s enters the field at right angles. explain why the electron experiences a change in velocity.

2. Relevant equations

3. The attempt at a solution
im thinking that only the direction of the electron changes but the speed stays the same. so the velocity changes in terms of direction. is that right?

Last edited: Jul 30, 2010
2. Jul 30, 2010

### rock.freak667

Do you have any idea of what principles you might need to apply?

You need to show some attempt before anyone at PF can help you.

3. Jul 30, 2010

### Neesha

okay i did make an attempt but im not really sure about the principles i need to apply. i think only the direction changes but not the speed. but im not sure why or if its true.

4. Jul 30, 2010

### rock.freak667

Well firstly what will cause a change in velocity?

5. Jul 30, 2010

### Neesha

probably the +ve plate attracting the electron

6. Jul 31, 2010

### rock.freak667

Which causes what type of force to be exerted on the electric charge?

7. Jul 31, 2010

### Neesha

electric force.i think thats what its called?

8. Jul 31, 2010

### sagardip

Listen no doubt the direction of velocity changes, but the electron also gathers a certain velocity in the upward direction towards the positive plate.And this velocity increases because of the constant acceleration the electron experiences.So now the net velocity of the electron is the resultant of both the initial horizontal component as well as the perpendicular component both in magnitude and direction.Got it??

9. Jul 31, 2010

### Neesha

yeah i get your last sentence but i thought the acceleration increased and that it wasn't constant.

10. Aug 1, 2010

### rock.freak667

Before the electron enters the electric field, it is in a gravitational field. So the effects of gravity are quite small. But as it enters the electric field, it causes an acceleration which affects the vertical component of its velocity.

F=ma=EQ.

So its horizontal velocity will stay the same, but the vertical velocity changes with time.

11. Aug 4, 2010

### Neesha

oh ok yh thanks :)