Calculating max. kinetic energy (photoelectric)

[NATSALANE]

given the cutoff potential (0.25 V) and wavelength (578 nm), how do i find the maximum kinetic energy of the electrons ejected from photoelectrif surface, in both eV and J?

my guess is,
KE = e V^o
= (1.6 x 10^-19) x (0.25 V)
= 4.0 x 10^-20 J

but if i convert that to eV, i get the same value as the cutoff potential - that can't be right.

can someone help me?

 

[NATSALANE]

also, how do i graph the maximum kinetic energy-vs-frqeuency of photons graph? i only have one value of frequency and kinetic energy. i know that the slope must be the same as the value of plank's constant, but assuming i do not know the constant, how can i find more points on the line to connect?

 

[alecsing]

According to the photoelectric equation,
Energy of light radiation = Kinetic Energy of photoelectron + Work function

Here, you are actually calculating the work function and thinking that it is the Kinetic Energy. Calculate the energy of the light radiation [hc/(wavelength)], where h is Planck's constant and c is the speed of light.

Then, subtract work function from the calculated energy to obtain the max. K.E. of the photoelectron.

 

[alecsing]

Max K.E. = h(v-v_o) = hv - Work function
where h is Planck's constant, v is frequency of radiation and v_o is the cutoff frequency. As you have already calculated the work function, therefore you can create the graph (which will be a straight line).

 

[NATSALANE]

According to the photoelectric equation,
Energy of light radiation = Kinetic Energy of photoelectron + Work function

Here, you are actually calculating the work function and thinking that it is the Kinetic Energy. Calculate the energy of the light radiation [hc/(wavelength)], where h is Planck's constant and c is the speed of light.

Then, subtract work function from the calculated energy to obtain the max. K.E. of the photoelectron.

all i am given is the wavelength and cutoff potential, so in order to find the maximum kin energy, don't i use this equation: Kinetic energy = speed of electron * cutoff potential ?

how does that equate to work?

 

[alecsing]

all i am given is the wavelength and cutoff potential, so in order to find the maximum kin energy, don't i use this equation: Kinetic energy = speed of electron * cutoff potential ?

how does that equate to work?

There is a previous thread:- https://www.physicsforums.com/archive/index.php/t-208429.html
. After viewing it, you will be able to understand the meaning of cutoff potential more clearly. It is used to calculate the work function and not the kinetic energy.

 

[NATSALANE]

okay then.

so E_K = (hc)/λ - eV_o

i got the same answer as W but opposite sign (negative)...