# Homework Help: Path of positivley charged particle in electric field

1. Jun 4, 2010

### ghostbuster25

A positivley charged particle is travelling in a straight line with velocity v at a small angle from the horizontal.
At a certain instant a constant uniform electrical field is applied in the downward vertical.
what is the direction of the charged particle?

I figured that the equation for a particle in a electric field is Fel=is qE(r) with E(r) equal to the electric force at distance r.

knowing this i believed the particle would not continue on its path but be pushed downwards in the vertical position. I used the right hand grip rule to figure it wouldnt just spiral in acircular clockwise path but in a helical anticlockwise path.

this the right idea?

sorry if ive written this in a confusing way lol

2. Jun 4, 2010

### housemartin

You need right hand grip rule for magnetic field i guess. This one is uniform electric field. So the path will be something like a parabola, cause direction of electric field is vertical, it will change only vertical component of velocity, horizontal component will stay same.

3. Jun 4, 2010

### ghostbuster25

sorry i didnt mean the grip rule just the right hand rule. i checked it again and figured that if the particle was travelling along the +x axis and the electric field was in the -y direction...the force on the particle would be giong into the page(so to speak) this would make it travel in an anticlockwise direction. like you said the x direction wouldnt have changed so it would still be travelling with the same V so a helical path would be the right one.....i hope :)

4. Jun 4, 2010

### housemartin

there will be no helical path, for this one you need certain combination of electric and magnetic fields. just google for helical path, you will see what it looks like. This problem is more like throwing balls - no helical - just a parabolic path.

5. Jun 4, 2010

### ghostbuster25

the choices i have are
a) the particle stays on the same path,
b)the particle follows a circular path in an anticlockwise direction
c)the particle follows a circular path in a clockwise direction
d)the particle the particle follows a helical path in an anticlockwise direction
e)the particle follows a helical path in a clockwise direction

i cant see how it could be a because the electric field must have some sort of effect, even though the v is not stated.
i cant see b or c either because if it follows a circular path there must be a force pushing it back up against the electric field.
which is why i chose helical.

Im guessing this is wrong but cant see why.

6. Jun 4, 2010

### RoyalCat

Given a uniform electric field, all of those answers are wrong. The particle's path will be a parabola.

If we're discussing a uniform magnetic field, then the particle will follow a counter-clockwise helix upwards.

The right hand rule has absolutely nothing to do with the motion of a charged particle in an electric field.

7. Jun 4, 2010

### ghostbuster25

what do i need to do to work that our RoyalCat? it is a magnetic field

8. Jun 4, 2010

### RoyalCat

Ah, be very careful with that. Magnetic fields and electrical fields affect moving charges very differently!

The Lorentz force on a charge moving with velocity $$\vec v$$ is given (In SI units) by:
$$\vec F = q(\vec E + \vec v \times \vec B)$$

The magnetic force, as evident by its statement using the vector cross product, will always act perpendicular to the plane determined by the velocity and magnetic field vectors, with the direction being specified by the right hand rule.

Since the cross product derived force only acts on the the velocity component perpendicular to the magnetic field, the component of the velocity parallel to the magnetic field (This means either in exactly the same direction as the field, or exactly opposite it) remains unchanged. Superposing the two motions, one circular, and the other straight, makes for helical motion.