Grade 12 Physics Chapter 11

Part 2: Study Material - Experimental Study of Photoelectric Effect

1. The Experimental Setup The study is conducted in an evacuated glass tube containing two electrodes: a Photosensitive Plate (Emitter) and a Collector Plate. Light is allowed to fall on the emitter plate through a quartz window.

2. Effect of Intensity (At Constant Frequency)

• Intensity represents the number of photons hitting the surface per second.

• Observation: The photoelectric current increases linearly with light intensity.

• Key Point: Intensity does not affect the kinetic energy of the electrons, only the number of electrons emitted.

3. Effect of Potential (At Constant Intensity and Frequency)

• Saturation Current: As the positive potential of the collector plate increases, the current increases until it reaches a maximum level where all emitted electrons are collected.

• Stopping Potential (V0): The negative potential applied to the collector plate that is just enough to stop the most energetic photoelectrons (current becomes zero).

• Relationship: K_max = e multiplied by V0.

4. Effect of Frequency (At Constant Intensity)

• Observation: Higher frequency light results in a higher (more negative) stopping potential.

• Key Point: Stopping potential is independent of intensity but depends on frequency.

• Threshold Frequency (v0): The minimum frequency required to start the emission process.

Part 2: MCQ Question Bank (15 Questions)

Part 2: MCQ Question Bank (15 Questions)

In a photoelectric experiment, if the intensity of incident light is doubled, the photoelectric current will:

A. Be halved

B. Remain same

C. Be doubled

D. Become four times

Answer: C

The maximum value of photoelectric current is called:

A. Threshold current

B. Stopping current

C. Saturation current

D. Leakage current

Answer: C

Stopping potential of a metal depends on:

A. Intensity of incident light

B. Frequency of incident light

C. Number of incident photons

D. Distance between source and metal

Answer: B

At the stopping potential, the photoelectric current is:

A. Maximum

B. Minimum but not zero

C. Zero

D. Equal to saturation current

Answer: C

If the frequency of incident radiation is increased, the stopping potential:

A. Increases (becomes more negative)

B. Decreases

C. Remains constant

D. Becomes zero

Answer: A

For a given frequency, the stopping potential is:

A. Proportional to intensity

B. Inversely proportional to intensity

C. Independent of intensity

D. Proportional to the square of intensity

Answer: C

The maximum kinetic energy of photoelectrons is equal to:

A. e multiplied by V0

B. e / V0

C. V0 / e

D. h multiplied by V0

Answer: A

When the collector plate is at a high enough positive potential, the current stops increasing. This state is:

A. Threshold

B. Saturation

C. Retardation

D. Stopping point

Answer: B

Which graph represents the relation between photoelectric current (I) and intensity (L) of light?

A. Parabola

B. Hyperbola

C. Straight line through origin

D. Circle

Answer: C

In the graph of current vs potential, two different intensities of the same frequency will have:

A. Different stopping potentials

B. Same stopping potential

C. Same saturation current

D. Zero saturation current

Answer: B

The stopping potential is a measure of:

A. Average kinetic energy of electrons

B. Minimum kinetic energy of electrons

C. Maximum kinetic energy of electrons

D. Total number of electrons

Answer: C

As the distance of the light source from the photoelectric cell increases, the saturation current:

A. Increases

B. Decreases

C. Remains constant

D. First increases then decreases

Answer: B

The slope of the graph between stopping potential (V0) and frequency (v) for a metal is:

A. h

B. h / e

C. e / h

D. Work function

Answer: B

Two different metals will have graphs of stopping potential vs frequency that are:

A. Intersecting

B. Parallel

C. Coincident

D. Perpendicular

Answer: B

If light of frequency less than threshold frequency is used, the stopping potential will be:

A. Positive

B. Negative

C. Zero

D. Not defined (no emission)

Answer: D

Part 2: Explanations

1. Explanation: Photoelectric current is directly proportional to intensity as more photons knock out more electrons per second.

2. Explanation: Saturation occurs when all emitted electrons are being successfully collected by the anode.

3. Explanation: Stopping potential is tied to the maximum kinetic energy of electrons, which is determined by frequency.

4. Explanation: Stopping potential is defined as the voltage required to reduce the photoelectric current to zero.

5. Explanation: Higher frequency photons have more energy, creating faster electrons that need more negative voltage to be stopped.

6. Explanation: Intensity changes the number of electrons, not their individual energy or speed.

7. Explanation: The work done to stop the electron (eV0) must equal its kinetic energy (K_max).

8. Explanation: Saturation current is reached when the potential is high enough to attract all emitted electrons.

9. Explanation: The relationship is linear; intensity and current increase at the same rate.

10. Explanation: Since the frequency is the same, the electrons have the same max energy and thus the same stopping potential.

11. Explanation: It specifically measures the energy of the fastest (maximum kinetic energy) electrons emitted.

12. Explanation: Intensity follows the inverse square law with distance; as distance increases, intensity and current decrease.

13. Explanation: Based on the equation V0 = (h/e)v - (Phi/e), the coefficient of frequency is h/e.

14. Explanation: All metals share the same slope (h/e) because h and e are universal constants.

15. Explanation: No electrons are emitted below threshold frequency, so there is no current to stop.