(a) zero.
(b) constant independent of disc size.
(c) increasing radially in the outward direction.
(d) decreasing radially in the outward direction.
Answer: (c) increasing radially in the outward direction.
Explanation:
CONCEPT: Motional EMF in a Rotating Disc
- When
a conducting disc rotates in a magnetic field, the free
electrons move with the disc.
- Due
to motion in a magnetic field, electrons experience the Lorentz
force.
- This
causes charge separation, leading to an induced electromotive
force (EMF).
Lorentz Force Explanation
- The
force acting on a moving charge is given by:
Where:
= charge
= velocity of
charge (tangential)
= magnetic field
(vertical)
- Since:
- Electron
velocity is tangential
- Magnetic
field is vertical
- The
resulting Lorentz force acts radially (towards or away
from the center).
Charge Separation
- This
radial force pushes electrons:
- Towards
the rim or center (depending on
rotation direction).
- This
creates a potential difference between the center and the
rim of the disc.
Formula for Induced EMF
- The
induced EMF between the center and edge of a disc of radius
, rotating with angular
velocity 
in magnetic
field 
, is:
Radial Dependence
- From
the formula:
- This
shows that the induced EMF:
- Increases
with the square of the radius
- Is minimum
at the center
- Is maximum
at the rim
Final Answer
✅ Correct option: (c)
The induced EMF increases radially outward
Key One-Line Exam Statement
In a rotating metallic disc, the induced EMF increases
radially outward and is proportional to 
.
Important Exam Notes
- This
phenomenon is known as Faraday’s disc generator
- EMF
depends on:
- Magnetic
field strength
- Angular
velocity
- Square
of radius
- Used
to demonstrate motional EMF