# How does centripetal force affects the path?

1. Jan 16, 2012

### jaobyccdee

When there's magnetic field, a charged particle moves in a circle. I would like to ask why won't the circle goes smaller even when there's a force pointing inward (towards the center of the circle). Thank you!

2. Jan 17, 2012

### The_Duck

Any force that acts perpendicularly to the direction of motion doesn't change a particle's speed, only the direction. And the magnetic force always acts perpendicularly to the direction of motion.

3. Jan 17, 2012

### jaobyccdee

So when it changes the direction of the path, why will it become a perfect circle instead of a path that circles around closer and closer to the center? Sorry i don't know the word to describe that path...

4. Jan 17, 2012

### rcgldr

The word you're looking for is spiral. The reason the path is a circle in this case is because the force (and acceleration) are constant as well as perpendicular to the path (velocity). If the amount of force varies with time, then just about any path would be possible, with the only constraint that speed is constant. The force could be adjusted to create a spiral, an ellipse, a parabola, a hyperbola, a sine wave, ... , any path that is possible with constant speed and only direction changes. For example, think of the possible paths your car could follow while moving at constant speed with just steering inputs, however if you hold the steering wheel in one position, then the car's path will be a circle (or a straight line) (assuming it doesn't slide).

Last edited: Jan 17, 2012
5. Jan 17, 2012

Thank you:)