# Calculating max. kinetic energy (photoelectric)

1. Jul 29, 2008

### 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 Vo
= (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?

2. Jul 29, 2008

### 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?

3. Jul 29, 2008

### 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.

4. Jul 29, 2008

### alecsing

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

5. Jul 29, 2008

### NATSALANE

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?

6. Jul 29, 2008

### alecsing

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.

7. Jul 29, 2008

### NATSALANE

okay then.

so EK = (hc)/λ - eVo

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

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