g A rigid air cylinder with a volume of 100 cm3 is punctured with a hole having a crosssectional area of 0.3 mm2 . The original pressure and temperature of the air inside the cylinder are 800 kPa and 35 C. As the air leaves the hole in the cylinder, it reaches a pressure of 100 kPa and a temperature of 5 C. The velocity of the air as it escapes through the hole is 100 m/s. Calculate the original mass of air inside the tank and the mass in the tank 5 seconds after it is punctured, assuming the exit conditions of the air remain independent of time g

Solution :

The volume of the rigid cylinder =

Initial pressure inside the cylinder,

Initial temperature inside the cylinder,

Final temperature inside the cylinder,

Final pressure inside the cylinder,

Area of the hole, A =

Velocity of the air through the hole, V = 100 m/s

The final pressure and the temperature inside the cylinder will be the condition same as the ambient conditions.

At initial state, from the equation of state,

PV = mRT,    where R = 287 J/kg-K for air

∴  

Since the exit condition does not change with time, we have ,

At ambient condition, and .

Therefore, we can find the density of the air

Mass flow rate of air from the cylinder =

can be written as

=

= kg/s

Mass escaped from the cylinder in 5 seconds

Mass of air remaining in the cylinder after 5 seconds :

    = 0.7225 grams

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia

3. dark matter

empty space could also be a simple vacuum