Everything Science Grade 12

16.3 CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER

• If the photon energy is greater than or equal to the work function (energy needed
  to escape from the metal), then the electron isemitted from the surface of the
  metal (the photoelectriceffect).

The work function is different for different elements. The smaller the work function, the

easier it is for electronsto be emitted from the metal. Metals with low work functions

make good conductors.This is because the electrons are attached less strongly to their

surroundings and can move more easily through these materials. Thisreduces the

resistance of the material to the flow of current i.e. it conducts well. Table 16.1 shows

the work functions for arange of elements.

Element WorkFunction (J)

Aluminium 6 , 9 × 10 −^19

Beryllium 8 , 0 × 10 −^19

Calcium 4 , 6 × 10 −^19

Copper 7 , 5 × 10 −^19

Gold 8 , 2 × 10 −^19

Lead 6 , 9 × 10 −^19

Silicon 1 , 8 × 10 −^19

Silver 6 , 9 × 10 −^19

Sodium 3 , 7 × 10 −^19

Table 16.1: Work functions of selected elements determined from thephotoelectric

effect. (From the Handbook of Chemistry and Physics.)

FACT

The electron volt (eV)
is the kinetic energy
gained by an electron
passing through a poten-
tial difference of one volt
(1V). A volt is not a
measure of energy, but
the electron volt is a unit
of energy. When you
connect a 1 .5V battery
to a circuit, you can give
1 .5eV of energy to ev-
ery electron.

Example 1: The photoelectric effect - I

QUESTION

Ultraviolet radiation with a wavelength of 250nm is incident on a silver foil

(work function φ = 6 , 9 × 10 −^19 ). What is the maximumkinetic energy of the

emitted electrons?

SOLUTION

Step 1 : Determine what is required and how to approach the problem

We need to determine the maximum kinetic energy of an electron

ejected from a silver foilby ultraviolet radiation.

The photoelectric effecttells us that:

Ek = Ephoton− φ

Ek = h

c

λ

− φ

We also have:

Work function of silver: φ = 6, 9 × 10 −^19 J