Photoelectric Effect - DOC

Document Sample
Photoelectric Effect - DOC Powered By Docstoc
					                             Photoelectric Effect

1887 – Heinrich Hertz notices metallic surfaces lose their negative charge
when exposed to/illuminated by UV light.

When EMR strikes a metal surface, electrons are released from the plate.
This phenomenon – the interaction of photons and metals - was called the
photoelectric effect and the emissions were called photoelectrons.

Albert Einstein realized the significance of Planck’s quantum theory and
applied it successfully to the photoelectric effect.

Emission of electrons will only occur if the incoming light has a high enough
frequency – this suggests the existence of a threshold frequency (fo).

A single photon hitting the metal target transfers its energy to one electron,
giving that one electron the energy to move. The photon must have some
minimum amount of energy to snap the electron off the metal.

Imagine the situation if there was no minimum amount of energy.

This minimum amount of energy is called the work function of the metal,
since you needed to do work on the electron to break it off. Each metal has a
unique work function – chart on pg. 712 - different metals hold on to their
electrons with different strengths.

The work function is related to the threshold frequency. The formula for
this is a modification of Planck's formula:

                                                          W = work function (J)
                                                           h = Planck’s constant
                                                  fo = threshold frequency (Hz)
W=hfo Use this formula only when calculating things concerning electrons
being knocked off of metal.
E=hf When just calculating individual photon energy, use Planck’s formula

Eg. 1 Determine the threshold frequency of a material with a work function
of 9.6 eV.

Eg. 2 Determine the work function of a metal in Joules if the maximum
threshold wavelength is 1.25 x 10-7 m.

When the UV source was turned on, the ammeter showed a current flowing
through the circuit, despite the big gap between the plates. The electrons
fly from the zinc plate to the metal plate. The metal plate is negative (gains
electrons) and the zinc plate is positive (loses electrons).

Turn on the variable voltage source, making the metal plate negative and the
zinc plate positive. The electrons will now decelerate, and as the voltage is
increased, fewer and fewer electrons will reach the opposite terminal. The
photoelectric current will decrease.

The voltage prevents many electrons from making it all the way across the
gap – only those with enough kinetic energy (enough velocity) will make it.
(Like charges repel)

At some point, the voltage will become too large for even the fastest
electrons to get across the gap; the ammeter will read 0 – this is the stopping
                                     E = qV
                                  Ek max = qVstop

                     Ek max = kinetic energy of the fastest moving electrons (J)
                                                     q = charge of an electron (C)
                                 Vstop = voltage needed to stop the electrons (V)
Eg. 3 Determine the maximum kinetic energy of electrons emitted from a
zinc surface if they are stopped by a 14 N/C uniform electric field over a
distance of 2.8 cm.

With a high enough frequency, the incoming EMR will have enough energy to
break off the electron and give it the remainder of its energy. The
remainder of the energy supplied by a photon will show up as kinetic energy of
the photoelectron. Einstein’s Photoelectric Equation:

                                 hf = Ek max + W
                                hf = mv2 + hfo

This formula shows the conservation of energy.

The intensity of the light (brightness) is a measure of the rate at which
photons strike the surface, not the energy.

 greater intensity  more photons  more photoelectrons released 
greater current

Recall that the electron volt (eV) is a unit of energy.

                 1eV = 1.60 X 10-19 J
Eg. 4 If light of  =425 nm falls on cesium (W = 1.90 eV) determine
   a) the energy of a photon
   b) the max Ek of a photoelectron released
   c) the velocity of a photoelectron released
   d) the velocity of an incident photon
   e) the stopping voltage in this cell
   f) the threshold frequency.
   g) If  was decreased which values would change?

Eg. 5 UV light of wavelength 270nm is incident on a zinc electrode (W=3.31
   a) What is the stopping voltage in this cell?
   b) With what speed will an electron reach the opposite electrode, if a
      retarding potential of 1.10 V is applied to this cell?
The results of photoelectric experiments.
   A metal shows the photoelectric effect only if the frequency of the
     incident light is above a certain threshold frequency (fo).
   Above the threshold frequency, with greater intensity light (brighter),
     more current is produced.
   The threshold frequency is different for different metals used as
   For any given metal and light, the stopping voltage does not change with
     intensity. Bright or dim light produces the same maximum kinetic
     energy of the photoelectrons.
   For any given metal, the stopping voltage increases with increasing
     frequency. Thus the frequency of incident light determines the Ek of
     the emitted electrons. As well, if Ek is graphed against frequency, for a
     variety of metal surfaces, each graph has a similar slope (see page 774).
   Photoelectrons are emitted almost immediately (energy does not
     accumulate before the electron can escape).

Since Maxwell’s theory of EMR relates energy to intensity, most of the
observed results do not make sense (except #2)!