Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] [**External**] Photoelectric effect: electron current vs. frequency

Nice distinction. Probably a common misconception



Sent from my iPhone

On Nov 10, 2021, at 11:07 AM, John Denker via Phys-l <> wrote:

On 11/10/21 8:26 AM, Philip Keller via Phys-l wrote:

Very much not an expert, but: if the frequency is higher the
photoelectrons come out with higher speed. Then,as long as the voltage is
below the stopping potential, wouldn't a larger number of the electrons
happen to be moving in a direction that lets them make it across the gap to
the other electrode? So though there are not more electrons being
liberated, we do see a higher current?

That's not a winning argument.

Let's suppose, for simplicity, that electrons are popping out
at the same rate, i.e. the same number of electrons per second.
That is the *definition* of current: charge per unit time.
The velocity has got nothing to do with it.

The same definition applies to cars on the road: The current is
the number of cars per second. If you know the number of cars
per second, you don't need to know the speed.

This may conflict with your intuition about the flow of water,
because the water has constant density (unlike cars, unlike
electrons in a vacuum). Note the contrast:

-- IF (big IF) you have constant density, then yes, sure,
increasing the speed increases the flux.

-- If you have constant current, then increasing the speed
just means the cars are farther apart. Less density. Each
one travels farther down the road before the next one enters.


This is an almost-typical OTBE problem. "Other Things Being Equal".

The problem with OTBE is that other things are generally not equal.
At the very least, you have to be careful to specify *which* other
things are being held equal.

There's an unforgettable OTBE diagram here:

If you pull up on W, does Y go up, or does it go down?
Forum for Physics Educators