# Compute lorentz force from charge

1. Oct 22, 2011

### tifa8

1. The problem statement, all variables and given/known data

A raindrop carrying a charge of 25x10-12 coulombs falls at a velocity of 5 m/s between the poles of a permanent magnet. It passes through a 50 mm long region in which the magnetic field density of 0.9 T is directed horizontally.

1) determine the lorentz force of the raindrop
2) ESTIMATE THE HORIZONTAL COMPONENT OF THE RAINDROP VELOCITY AFTER IT PASSES THROUGH THE MAGNETIC FIELD (DIAMETER DROP=1MM)

Joined, diagram of how i interpreted the text

2. Relevant equations

F=q*(vector v x vector B)=qvb in magnitude

3. The attempt at a solution

1) by applying the formula I got F=25.10-2*5*0.9=1.125 N

However I do not understand question 2. For me the horizontal velocity component is equal to zero (going down) so it doesn't make sense to me.

Can someone explain it to me ? Thanks

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• ###### diagram.png
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2. Oct 22, 2011

### tifa8

sorry F=1.125.10-10

3. Oct 22, 2011

### vela

Staff Emeritus
Won't the force cause the drop to accelerate? What will happen to its velocity?

4. Oct 23, 2011

### tifa8

the velocity will increase and change slightly direction i think. But even though there is not any horizontal component , right ? I really cannot see it...

5. Oct 23, 2011

### vela

Staff Emeritus
Try drawing it from the point of view where the magnetic field points into or out of the page. Which way does the force point?

6. Oct 23, 2011

### I like Serena

Hi tifa8!

The raindrop is accelerated horizontally by the Lorentz Force F.
To find the horizontal acceleration you need to estimate the mass of the raindrop (F=ma).
To find the horizontal component of the velocity you need to calculate the time the magnetic field exerts its influence (v=at).

7. Oct 23, 2011

### tifa8

Oh ok thanks. Now I can visualize the problem. Thank you I like serena and vela :)