Threshold frequency(quantum phenomena)

help?

Start by calculating the work function, and see if you can go from there (or there is a shortcut because of the values they have used)

so i did this

$E_{k\ max}= hf- \phi$

$E_{k\ max}= \left( 6.63 \times 10^{-19} \times 5.6 \times 10^{14}\right) - \left( \dfrac{5.6 \times 10^{14}}{2}\right)$

Think about how the work function relates to the threshold frequency

work function is half the value of frequency isn't it?

no. E=hf

work function is half the value of frequency isn't it?

but it asks for max kinetic energy....and E=hf doesn't have $\phi$ in it :/

but it asks for max kinetic energy....

and E=hf doesn't have $\phi$ in it :/

but it asks for max kinetic energy....

and E=hf doesn't have $\phi$ in it :/

but it asks for max kinetic energy....

and E=hf doesn't have $\phi$ in it :/

Ekmax = work function - hf (where f is threshold frequency)
thus for this question:
first calculate work function
workfunction = hf (where f is threshold frequency of 5.6 * 10^14)
thus workfunction is something you can calculate (my calculator is under my bed xD)
therefore
Ekmax = work function - (2*5.6 × 10^14) - since frequency is twice the threshold frequency

You're pretty much there
$E_k=hf-\phi$
You know $2f=f_0$ right? so $f=2 \times 5.6 \times 10^{14}$
$E=hf$ is the best start, calculating the energy of the incoming photons.
$E=6.63 \times 10^{-34} \times 2 \times 5.6 \times 10^{14}$
$\therefore E=7.426 \times 10^{-19} J$. This is the total energy of the incoming photons yes?

Now not all this energy goes into the kinetic energy of the electron. Remember, some of it goes into liberating it. The energy needed for this is the work function energy.
$E=hf_0$ for the energy needed at the work function, we know $\phi = hf_0$ where $f_0$ is the threshold frequency.
$\therefore \phi = 3.71\times 10^{-19}$
I think this is where you are confused. Work function of a metal is the energy required to liberate an electron. Not the threshold frequency, which is just the minimum frequency of an incoming photon to have the energy equal to the work function.

Back to the first equation:
$E_k=hf-\phi$ therefore
$E_k=7.426 \times 10^{-19} - 3.71 \times 10^{-19} =[b] 3.72 \times 10^{-19}[/b]$

True, but this only works in specific questions. I find it nicer to run through the set algorithm for kinetic energy shortcuts loose me marks all the time

Work function relates to the threshold frequency by
E (work function) = h (planck's constant) * f0 (Threshold Frequency)

Work function is the minimum amount of energy required to liberate a photon, and we know that the threshold frequency is the minimum frequency required to liberate a photon. Therefore thew ork function =h x threshold frequency

i'm pretty sure E=hf
is
where
E= energy of photon in joules <--- how does that become the work function?? i don't understand
any formula working out and substitutions required pls i don't understand :/

i'm pretty sure E=hf
is
where
E= energy of photon in joules <--- how does that become the work function?? i don't understand
any formula working out and substitutions required pls i don't understand :/

Okay so basically... What is work function. What is threshold frequency
Threshold frequency is the minimum frequency of light required for electrons to be emitted from the metal.
Work function is the minimum amount of energy required for an electron to be emitted from the surface of a metal.
So if you link the two... The threshold frequency provides the work function right?
Thus the photon that is absorbed must have energy equal to the work function for the electron to be emitted. and we know work function and threshold frequency are linked
So we can say that the energy of the photon must have a minimum energy = h * threshold frequency i.e. E = hf. And we know that the minimum energy is the work function so:
work function = h*threshold frequency. This is a general rule. Learn this formula.

You're pretty much there
$E_k=hf-\phi$
You know $2f=f_0$ right? so $f=2 \times 5.6 \times 10^{14}$
$E=hf$ is the best start, calculating the energy of the incoming photons.
$E=6.63 \times 10^{-34} \times 2 \times 5.6 \times 10^{14}$
$\therefore E=7.426 \times 10^{-19} J$. This is the total energy of the incoming photons yes?

Now not all this energy goes into the kinetic energy of the electron. Remember, some of it goes into liberating it. The energy needed for this is the work function energy.
$E=hf_0$ for the energy needed at the work function, we know $\phi = hf_0$ where $f_0$ is the threshold frequency.
$\therefore \phi = 3.71\times 10^{-19}$
I think this is where you are confused. Work function of a metal is the energy required to liberate an electron. Not the threshold frequency, which is just the minimum frequency of an incoming photon to have the energy equal to the work function.

Back to the first equation:
$E_k=hf-\phi$ therefore
$E_k=7.426 \times 10^{-19} - 3.71 \times 10^{-19} = 3.72 \times 10^{-19}$