Question

The photoelectric plate is kept vertically above the emitter plate. Light source is put on and a saturation photo-current is recorded. An electric field is switched on which has a vertically downward direction, then

• A. the photo-current will increase
• B. the kinetic energy of the electrons will increase
• C. the stopping potential will decrease
• D. the threshold wavelength will increase

Hint:

If electric field accelerates electrons toward collector, it increases their kinetic energy, but saturation current remains unchanged.

Answer 1

The Correct Option is B

Step 1: Understand arrangement.
Emitter plate is below and collector plate is above.
Electrons move upward from emitter to collector.
Step 2: Direction of applied electric field.
Electric field is vertically downward.
Force on electron is:
\[ \vec{F} = -e\vec{E} \] So if \(\vec{E}\) is downward, electron force is upward.
Step 3: Effect on electrons.
Since force is upward, electrons accelerate more while moving to collector.
Hence their kinetic energy increases.
Step 4: Eliminate other options.
Saturation current does not increase because it depends on number of emitted electrons (intensity).
Stopping potential depends on maximum kinetic energy at emission, not after acceleration.
Threshold wavelength depends on work function, not field.
Final Answer: \[ \boxed{\text{the kinetic energy of the electrons will increase}} \]