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Confusion on equations

Posted: Sun Oct 13, 2019 10:14 pm
I was wondering when do we each equation? Do each equation solve for something different. I know they all are switched around but I'm just confused about which ones to use for certain questions?

E=hv
lambda*v=c
E=h*(c/lambda)
lambda=(hc/E)

Re: Confusion on equations

Posted: Sun Oct 13, 2019 10:23 pm
E=hv
lambda*v=c
E=h*(c/lambda)
lambda=(hc/E)

Ultimately, you use equations to find unknown variables. So, if you know what the variables represent, and you understand how to find the variables out of the problem by manipulating other equations, you'll be fine.

Here is what the variables mean:

E = Energy
v: frequency
lambda: wavelength

The following are constants and you don't solve for them you just plug in the numbers they represent:

h: Plank's constant
c: Speed of light

Re: Confusion on equations

Posted: Sun Oct 13, 2019 10:26 pm
This is basically the same, but simply put, it is the same equation but is utilized based on the values given. If the wavelength is given, E = hc / lambda is ued. If the velocity is given, E = hv is used. Hope this helps!

Re: Confusion on equations

Posted: Mon Oct 14, 2019 9:23 am
Depending on what values you are given you also can rearrange each equation and combine them to find unknown variables. For instance if you were asked to solve for energy and you were only given wavelength not frequency, you can solve the equation lambda*v=c for v so that you can then plug your new equation(solved for v) into the energy equation, E=hv. It all depends on what you are given and how you can rearrange all of the equations to solve for what you need.

Re: Confusion on equations

Posted: Mon Oct 14, 2019 7:06 pm
Most of the equations given for energy are interchangeable and should yield that same value given on the variables needed. To find out which formula to use, then its good to set up a table of your knowns and one of your unknowns. This way you visually see what you are missing. After that just plug in the values for the variables given and you can combine 2 formulas if you have to.

Re: Confusion on equations

Posted: Mon Oct 14, 2019 10:53 pm
It just depends on what is given in the problem and what does the problem ask you to do. For example, given frequency, you can use lamda=c/v to get wavelength or E=hv to get energy.

Re: Confusion on equations

Posted: Thu Oct 17, 2019 9:56 am
For these equations, the E in the equations are all energy for the energy of a photon, so the equations are interchangeable because they apply to the same thing. Therefore, you can decide which equation to use based on the variables you are given and the variables that you want to solve.

Re: Confusion on equations

Posted: Thu Oct 17, 2019 10:53 am
An equation that you could separate from the rest is DeBroglie's equation, which is only used for particles (electrons/protons/neutrons).

Re: Confusion on equations

Posted: Thu Oct 17, 2019 11:40 pm
It all depends on what you are looking for and what the problem gives you. Like others have said, the formulas can be rewritten or combined to yield the correct results.

Re: Confusion on equations

Posted: Thu Oct 17, 2019 11:57 pm
005153851 wrote:I was wondering when do we each equation? Do each equation solve for something different. I know they all are switched around but I'm just confused about which ones to use for certain questions?

E=hv
lambda*v=c
E=h*(c/lambda)
lambda=(hc/E)

Think of the first three applied to light or a photon, while the DeBroglie equation applied to a particle (electron, car, etc.). It can get confusing in the questions as to which equations to use, but remembering this helps.