Grade 12 Physics Chapter 11

Part 1: Study Material - Electron Emission Basics

1. The Concept of Work Function (W0 or Phi)

The minimum energy required by an electron to escape from a metal surface is called the Work Function.

• Unit: Measured in electron-volts (eV).

• Conversion: 1 eV = 1.6 x 10^-19 Joules.

• Key Property: It depends only on the nature of the metal. For example, Caesium (Cs) has the lowest work function, while Platinum (Pt) has one of the highest.

2. Types of Electron Emission

To pull an electron out of a metal, we must provide energy equal to or greater than the work function. 

There are four ways:

• Thermionic Emission: Energy is provided by heating the metal.

• Field Emission: Energy is provided by applying a very strong electric field (around 10^8 V/m).

• Photoelectric Emission: Energy is provided by light (electromagnetic radiation) of a suitable frequency.

• Secondary Emission: Energy is provided by hitting the metal surface with high-speed primary electrons.

3. The Photoelectric Effect

When light hits a metal surface, electrons are ejected.

• Threshold Frequency (v0): The minimum frequency below which no electron emission happens.

• Threshold Wavelength (L0): The maximum wavelength above which no electron emission happens.

• Relationship: Work Function (Phi) = h x v0 (where h is Planck's constant).

4. Important Constants

• Planck's constant (h) = 6.63 x 10^-34 J s

• Charge of electron (e) = 1.6 x 10^-19 C

• Speed of light (c) = 3 x 10^8 m/s

MCQ Questions:

What is the SI unit of work function?

A. Volt

B. Joule

C. Ampere

D. Watt

Answer: B

One electron volt (eV) is equal to:

A. 1.6 x 10^-19 J

B. 6.6 x 10^-34 J

C. 3 x 10^8 J

D. 9.1 x 10^-31 J

Answer: A

Which of the following metals has the lowest work function?

A. Platinum

B. Gold

C. Caesium

D. Copper

Answer: C

Electron emission from a metallic surface by application of heat is known as:

A. Photoelectric emission

B. Field emission

C. Thermionic emission

D. Secondary emission

Answer: C

The work function of a metal depends on:

A. The nature of the metal only

B. The frequency of incident light

C. The intensity of incident light

D. The potential applied to the metal

Answer: A

To eject electrons using field emission, the required electric field order is:

A. 10^2 V/m

B. 10^4 V/m

C. 10^6 V/m

D. 10^8 V/m

Answer: D

If the energy of incident photon is less than the work function:

A. More electrons are emitted

B. Faster electrons are emitted

C. No electrons are emitted

D. Kinetic energy of electrons increases

Answer: C

The minimum frequency required to eject an electron from a metal surface is:

A. Resonant frequency

B. Threshold frequency

C. Stopping frequency

D. Maximum frequency

Answer: B

The process of ejecting electrons by hitting a metal surface with high-speed electrons is:

A. Primary emission

B. Secondary emission

C. Thermionic emission

D. Cold cathode emission

Answer: B

If the work function of a metal is 4.2 eV, what is its value in Joules?

A. 4.2 x 10^-19 J

B. 6.72 x 10^-19 J

C. 1.6 x 10^-19 J

D. 2.62 x 10^-19 J

Answer: B

Work function is also known as:

A. Kinetic barrier

B. Potential barrier

C. Current barrier

D. Power barrier

Answer: B

Which of the following is NOT a method of electron emission?

A. Thermionic emission

B. Field emission

C. Photoelectric emission

D. X-ray emission

Answer: D

Explanations for the MCQ Questions:

1. Explanation: Work function is a form of energy. The SI unit of any energy is the Joule (J).

2. Explanation: By definition, 1 eV is the energy gained by an electron moving through a potential of 1 Volt, which equals 1.6 x 10^-19 Joules.

3. Explanation: Caesium is the most photosensitive element and requires the least energy to lose an electron.

4. Explanation: Thermionic comes from "thermal" (heat). Heat energy is used to overcome the work function.

5. Explanation: Work function is an intrinsic property of the material itself; it does not change based on the light you shine on it.

6. Explanation: A very strong field of about 10^8 V/m is needed to overcome the attractive forces of the nucleus.

7. Explanation: If energy is less than work function, the electron doesn't get enough energy to break the potential barrier of the surface.

8. Explanation: Threshold frequency is the cutoff point for the photoelectric effect to begin.

9. Explanation: When high-speed electrons strike, they transfer momentum to surface electrons, causing secondary emission.

10. Explanation: Calculation: 4.2 multiplied by (1.6 x 10^-19) = 6.72 x 10^-19 J.

11. Explanation: It is called a potential barrier because it acts as a wall that prevents electrons from leaving the metal.

12. Explanation: X-ray emission is a result of electron transitions or deceleration, not a method used to extract electrons from a metal surface.