# Homework Help: Delta dirac

1. Feb 21, 2010

### alejandrito29

if $$\phi$$ is a angular coordinate , between ($$-\pi,\pi$$)

¿how much is $$\delta(\phi-\pi)$$ with $$\phi=-\pi+\epsilon$$????

2. Feb 21, 2010

### SpectraCat

The Dirac delta function yield zero unless it's argument is zero, in which case it yields 1 (this is an oversimplification, but it should do for the present discussion). In your case, the argument of the delta function is $$-2*\pi + \epsilon$$, so it should be zero. Did you mean to type, $$\phi=\pi + \epsilon$$? In that case, the argument of the delta function would be just $$\epsilon$$, and then you need to get a bit more specific about how you are defining the delta function. Have you looked at this thread? https://www.physicsforums.com/showthread.php?t=73447

3. Feb 21, 2010

### alejandrito29

then the dirac delta evaluated in (-2pi+epsilon) is 0 or infinite??????

4. Feb 21, 2010

### alejandrito29

specifically, i need to find T, in the follows equation:

$$\delta(\phi-\pi)+k=T\delta(\phi-\pi)$$
where $$\phi$$ is between ($$-\pi,\pi$$)

5. Feb 22, 2010

### haushofer

With that specific range the delta distribution is zero; phi can't become equal to pi. So you get the equation 0 + k = T*0.

6. Feb 22, 2010

### alejandrito29

but, if i do:

$$\int^{\pi-\epsilon}_{-\pi+\epsilon}\delta(\phi-\pi)d\phi+\int^{\pi-\epsilon}_{-\pi+\epsilon} k=T\int^{\pi-\epsilon}_{-\pi+\epsilon}\delta(\phi-\pi)$$

is correct????????????

pd: $$\phi$$ can be equal to $$\pi$$.......$$\phi$$ is between $$(-\pi,\pi)$$

Last edited: Feb 22, 2010
7. Feb 22, 2010

### Matterwave

You want to integrate from pi-epsilon to pi+epsilon. That will include the relevant range of the dirac delta.