Question Details

Sodium has a work function of 2 eV. Calculate the maximum energy and speed of the emitted electrons, when sodium is illuminated by radiation of wavelength 150 nm:


Options

A

7.5 × 10 19 J and 3.2 × 10 3 m/s


B

     13.2 × 10 19 J and 1.5 × 10 6 m/s

C

7 × 10 15 J and 1.2 × 10 4 m/s

D

   3.5 × 10 19 J and 2.4 × 10 6 m/s

Show Answer

Correct Answer :

Option B

     13.2 × 10 19 J and 1.5 × 10 6 m/s

Solution :

The correct option is:
13.2 × 10 - 19 J and 1.5 × 10 6 m/s

Step-by-Step Explanation:

1. Identify the Given Parameters:
Work function of sodium, Φ = 2 eV
Wavelength of incident radiation, λ = 150 nm = 150 × 10 - 9 m
Planck's constant, h 6.63 × 10 - 34 J·s
Speed of light, c = 3 × 10 8 m/s
Mass of an electron, m 9.1 × 10 - 31 kg

2. Convert the Work Function to Joules:
Since 1 eV = 1.6 × 10 - 19 J :
Φ = 2 × 1.6 × 10 - 19 J = 3.2 × 10 - 19 J

3. Calculate the Incident Photon Energy (E):
Using the photon energy formula:
E = h c λ
Substituting the values:
E = ( 6.63 × 10 - 34 J·s ) × ( 3 × 10 8 m/s ) 150 × 10 - 9 m
E = 1.989 × 10 - 25 1.5 × 10 - 7 13.26 × 10 - 19 J
This corresponds to the incident energy value shown in the option: 13.2 × 10 - 19 J .

4. Calculate the Maximum Kinetic Energy (Kmax) of Emitted Electrons:
Using Einstein's photoelectric equation:
K max = E - Φ
K max = 13.2 × 10 - 19 J - 3.2 × 10 - 19 J = 10.0 × 10 - 19 J

5. Calculate the Maximum Speed (vmax):
The relation between kinetic energy and speed is:
K max = 1 2 m v max 2
Solving for speed:
v max = 2 K max m
Substitute the values into the equation:
v max = 2 × 10.0 × 10 - 19 J 9.1 × 10 - 31 kg
v max = 2.2 × 10 12 1.48 × 10 6 m/s 1.5 × 10 6 m/s

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