Equations and confused of when to use what

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Vincent Leong 2B
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Joined: Fri Aug 09, 2019 12:15 am

Equations and confused of when to use what

Postby Vincent Leong 2B » Wed Oct 16, 2019 2:08 am

So primarily talking about the E= hv, c= λv, r = h/n^2, Ek = 1/2mv^2, λ = h/mv

So in essence, ryberg equation, energy of a photon equation, speed of light equation, de broglies, etc.

When do we use such equations? Which equations should I primarily memorize or understand the most? What are the special restrictions to each equation? When can I use one equation but not the other? What are the correct units for each equation?

Jocelyn Thorp 1A
Posts: 103
Joined: Wed Sep 18, 2019 12:20 am

Re: Equations and confused of when to use what

Postby Jocelyn Thorp 1A » Wed Oct 16, 2019 2:39 am

Vincent Leong 1A wrote:So primarily talking about the E= hv, c= λv, r = h/n^2, Ek = 1/2mv^2, λ = h/mv

So in essence, ryberg equation, energy of a photon equation, speed of light equation, de broglies, etc.

When do we use such equations? Which equations should I primarily memorize or understand the most? What are the special restrictions to each equation? When can I use one equation but not the other? What are the correct units for each equation?


Honestly the easiest way to look at it is what you're trying to find vs what you're given. Keep in mind that E=hv and c= λv have v as frequency, whereas in Ek = 1/2mv^2 and λ = h/mv v is velocity.

trying to find the energy of a photon of a certain frequency, or the frequency of a photon with a certain amount of energy? E=hv

c= λv is just universally useful for converting between frequency and wavelength.

r = h/n^2; I'm not sure where this specifically comes up, but the closely related En=-hR/n2 is useful for comparing various energy states (n=1,n=1...) or calculating the difference (either the energy absorbed or emitted when the state is changed).

Ek = 1/2mv^2 is the universal equation for kinetic energy, we've used it in combination with the work function. The full equation was Ek= 1/2mv^2 = hv - (work function). Of course, you'd use this if supplied the work function, or asked to calculate the work function. This function works because of the law of conservation of energy, that the energy used to perform the work to remove a photon must not be destroyed, and is instead converted to the kinetic energy of the photon (Ek).

λ = h/mv is connecting wavelength to momentum, so if you need to find the wavelength of a particle when given its mass and velocity (or honestly just any of the variables when given 2/3 of them) this is the equation for you. Take notice that this v=VELOCITY not frequency. (honestly it if there's mass you're probably dealing with velocity, but I can't guarantee that's true for all equations going forward.

As long as you know what you're given and looking for, I'm sure you can figure out what equation (or combination of equations) to use. Please be careful not to mix up frequency and velocity, though!

Chetas Holagunda 3H
Posts: 51
Joined: Thu Jul 11, 2019 12:17 am
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Re: Equations and confused of when to use what

Postby Chetas Holagunda 3H » Wed Oct 16, 2019 11:25 am

If some equations have variables in similar, like frequency, they can be substituted into each other to solve for the problem if the other variables in a certain formula aren't provided.

Audrie Chan-3B
Posts: 50
Joined: Thu Jul 25, 2019 12:17 am

Re: Equations and confused of when to use what

Postby Audrie Chan-3B » Thu Oct 17, 2019 11:26 am

Typically, I find that drawing things out or seeing what units I have to end with and therefore finding ways to manipulate equations in order to give me desired results will get me the correct answer.


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