An ideal monatomic gas is confined in a cylinder by a spring-loaded piston of cross-section `8.0xx10^-3m^2`. Initially the gas is at 300K and occupies a volume of `2.4xx10^-3m^3` and the spring is in its relaxed (unstretched, unompressed) state, fig. The gas is heated by a small electric heater until the piston moves out slowly by 0.1m. Calculate the final temperature of the gas and the heat supplied (in joules) by the heater. The force constant of the spring is `8000 N//m`, atmospheric pressure is `1.0xx10^5 Nm^-2`. The cylinder and the piston are thermally insulated. The piston is massless and there is no friction between the piston and the cylinder. Neglect heat loss through lead wires of the heater. The heat capacity of the heater coil is negligible. Assume the spring to be massless.

Answer

B

Explanation

The final pressure on the gas
=atm pressure +pressure due to compression of spring
`P_2=P_(atm)+(kx)/A`
`rArrP_2=10^5+(8000xx0.1)/(8xx10^-3)=2xx10^5 Nm^2`

The final volume,
`V_2=V_1+xA`
`=2.4xx10^-3+0.1xx8xx10^-3=3.2xx10^-3m^-3`
Applying `(P_1V_1)/(T_1)=(P_2V_2)/(T_2) rArr T_2=(P_2V_2T_1)/(P_1V_1)`
`rArrT_2=(2xx10^5xx3.2xx10^-3xx300)/(10^5xx2.4xx10^-3)=800K.`
Heat supplied by the heater is used for expansion
of hte gas, increasing its temperature and storing potential
energy in the spring.
`:. Heat supplied `
`=PDeltaV+nC_vDeltaT+1/2kx^2`
`=10^5[0.8xx10^-3]+(P_1V_1)/(RT_1)C_vDeltaT+1/2kx^2`
`=80+(10^5xx2.4xx10^-3)/(2xx300)xx3/2xx2xx500+1/2xx8000xx0.1`
`=720J`