How to find kinetic energy from velocity

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Natalie Phan 3G
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How to find kinetic energy from velocity

Postby Natalie Phan 3G » Thu Oct 15, 2020 12:52 am

Can someone please explain to me what the steps are to find the kinetic energy of the ejected electron given the velocity of the ejected electron and the work function? I was confused to what formulas I should use/rearrange/plug in.

Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 10^5 m.s^-1. The work function for sodium is 150.6 kJ.mol^-1. What is the kinetic energy of the ejected electron?

Thank you!

Chem_Mod
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Re: How to find kinetic energy from velocity

Postby Chem_Mod » Thu Oct 15, 2020 1:10 am

The formula for kinetic energy is 1/2mv^2 where m is the mass of the electron in kilograms, and v is velocity in m/s. The question gives you the velocity of the ejected electron, and the mass of the electron will be given on the formula sheet. Therefore, you can plug in those values to find the kinetic energy in Joules (J).

Sera Aintablian 2E
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Re: How to find kinetic energy from velocity

Postby Sera Aintablian 2E » Thu Oct 15, 2020 1:58 am

Kinetic Energy =
(mass of Na = 22.990 amu)
so,...
KE = (1/2)(22.990)((6.61x10^5)^2) = 5.0224 x 10^12 J (if my calculations are correct!)

With this value and the given work function, you can also solve for the Energy(photon) by manipulating the equation,
E(photon) = work function/threshold energy + kinetic energy !

jessicasilverstein1F
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Re: How to find kinetic energy from velocity

Postby jessicasilverstein1F » Thu Oct 15, 2020 10:43 am

the kinetic energy formula is (1/2)(m)(v^2) where m is mass and v is velocity

Justin Zhang_1A
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Re: How to find kinetic energy from velocity

Postby Justin Zhang_1A » Thu Oct 15, 2020 11:27 am

The formula to calculate kinetic energy with velocity is 1/2 * m * v^2.

Chance Herbert 3A
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Re: How to find kinetic energy from velocity

Postby Chance Herbert 3A » Mon Oct 19, 2020 11:48 am

The formula for Kinetic Energy is 1/2(m)(v^2). In this problem, the velocity is provided as 6.61x10^5 m/s. Here you would not use molar mass as you are considering the velocity for a sole electron and this would not provide you with the correct units for Kinetic Energy (keep in mind Joules are kgm^2/s^2). You would use the mass of an electron which is 9.109x10^-31 Kg, which would give you the kinetic energy! 1/2(9.109x10^-31Kg)((6.61x10^5m/s)^2)=1.99x10^-19 Kgm^2/s^2 or 1.99x10^-19 J.

905290504
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Re: How to find kinetic energy from velocity

Postby 905290504 » Mon Oct 19, 2020 12:36 pm

to find kinetic energy from velocity use the Ek = 1/2mv^2 equation!


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