Equations and Formulas

[005388369]

How do we know when to use which formula? For example, there's 1) wavelength = c/frequency, 2) wavelength = hc/E, and 3) wavelength = h/(mass)(velocity). Someone please explain, thank you.

[Trinity Vu 1D]

You use different equations depending on if you are calculating values for photons (electromagnetic radiation/light) or particles (ie electrons). Particles have mass whereas photons don't. You would use equations 1 and 2 for photons/electromagnetic radiation calculations (you can also tell because both equations use c which is the constant for the speed of light. Equation 3 is used for particles.

[Amy Pham 1D]

Use the above commenter's guidelines, and just remember that the third equation is the De Broglie Equation for calculating wavelengths of particles, proving that all matter has wavelike properties. When you see particles and you are asked to find their wavelength, you know to use lambda=h/(mass x velocity).

[Katie Bart 1I]

If you're dealing with momentum, you will use the third equation.

[Justin Quan 4I]

Just to add on, a good trick you may use to better figure out which equations you need is to look at the units. For example, if the problem gives you m/s, you probably have to use c=lambda(v) because c is measured is m/s. Sometimes you need to use several equations to convert the units into the units you actually need to solve for.