Confusion on equations

Alexis Robles 2k
Posts: 100
Joined: Wed Sep 18, 2019 12:18 am

Confusion on equations

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)

DTingey_1C
Posts: 55
Joined: Fri Aug 30, 2019 12:16 am

Re: Confusion on equations

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

Kevin Liu 1J
Posts: 53
Joined: Thu Jul 25, 2019 12:16 am

Re: Confusion on equations

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!

Maya Pakulski 1D
Posts: 105
Joined: Thu Jul 11, 2019 12:17 am

Re: Confusion on equations

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.

Osvaldo SanchezF -1H
Posts: 122
Joined: Wed Sep 18, 2019 12:21 am

Re: Confusion on equations

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.

Ziyan Wang 3J
Posts: 51
Joined: Wed Sep 18, 2019 12:22 am

Re: Confusion on equations

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.

Jingyao Chen 4H
Posts: 49
Joined: Fri Sep 27, 2019 12:19 am

Re: Confusion on equations

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.

Veronica_Lubera_2A
Posts: 106
Joined: Sat Sep 14, 2019 12:16 am
Been upvoted: 1 time

Re: Confusion on equations

An equation that you could separate from the rest is DeBroglie's equation, which is only used for particles (electrons/protons/neutrons).

Qilan Li 4I
Posts: 54
Joined: Mon Jun 17, 2019 7:23 am

Re: Confusion on equations

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.

Posts: 125
Joined: Sat Aug 17, 2019 12:17 am

Re: Confusion on equations

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.