# Planck's constant and speed

1. Dec 19, 2007

### fk378

This is a general question..

For the equation c=hv
where c=speed of light, h=Planck's constant, and v=frequency...

Can u (speed of a particle/electron) be substituted in for c? ie, if given the frequency of an electron and we need to find its speed, can we use this equation (u=hv) or must we get to the answer by using other equations?

2. Dec 20, 2007

### Vagrant

Isn't this equation E=hv?

3. Dec 20, 2007

### fk378

Oh sorry, I meant to refer to the equation c=lambda(v)

4. Dec 20, 2007

### chemisttree

Soooo, what is the question?

5. Dec 20, 2007

### fk378

For c=(lambda)(frequency)
where c=the speed of light, can you replace c with u (speed of a particle)? Say, you're given the frequency and the wavelength of a particle and you need to find its speed, can you use this equation, or must you find it from another equation(s)?

6. Dec 20, 2007

### Hootenanny

Staff Emeritus
No, the equation you have quoted there is called the 'Dispersion relation for EM-waves in a vacuum' and is therefore only valid for EM-waves in a vacuum. You would have to use other equations, namely the de Broglie relations.

Last edited: Dec 20, 2007