Answer:
D) the second at the doorknob
Explanation:
The torque exerted by a force is given by:

where
F is the magnitude of the force
d is the distance between the point of application of the force and the centre of rotation
is the angle between the direction of the force and d
In this problem, we have:
- Two forces of equal magnitude F
- Both forces are perpendicular to the door, so 
- The first force is exerted at the midpoint of the door, while the 2nd force is applied at the doorknob. This means that d is the larger for the 2nd force
--> therefore, the 2nd force exerts a greater torque
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Answer:
Part a)

Part b)

Explanation:
Part a)
Electric potential is a scalar quantity
so here we can say that total potential due to a ring on its center is given as

here we know that



now we have


Part b)
Potential on the axis of the ring is given as


