Question

A glass bulb is balanced by a brass weight in a sensitive beam balance. Now the balance is covered by a glass-jar which is then evacuated. Then:
(A) The beam will remain horizontal
(B) The pan containing the bulb will go down
(C) The pan containing the bulb will go up
(D) None of the above

Answer 1

Hint:Here we have to see whether the mass of glass bulb or glass jar is high. Then we can compare the balances.

Complete step by step answer:
Given,
A glass bulb is balanced by a brass weight in a sensitive beam balance. Now the balance is covered by a glass-jar which is then evacuated.
After evacuating, the glass container would have no air in it. In order to maximise the mass of the glass bulb, since it contains air within the glass bulb. Then the pan that holds the bulb is going to go down. When the glass bulb is weighted in the air by a brass weight, both of them displace air, but the much larger volume of the glass bulb displaces greater air volume. When a bell jar surrounds the device, it is then drained. There will be no upthrust due to displaced air and the glass bulb will weigh more. So, the pan goes down.
Hence, option B is correct. The pan containing the bulb will go down.

Additional information:
In the range between $10\,mg$ and $1\,kg$, the beam balance is used to calculate weights. The accuracy and calculation resolution obtained depends on the strength and sharpness of the knife edge on which the pivot is made.
Mass of an object: An object’s mass is a measure of the inertial property of the object, or the volume of matter that it contains. An object’s weight is a measure of the force imposed by gravity on the object, or the force required to sustain it.

Note:Here we have to see how much of air volume is displaced by the glass bulb and brass weight. So we have to recall the buoyancy principle and apply here.