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A gas contained in a cylinder fitted with a frictionless piston expands against a constant external pressure of 1 atm from a volume of 5 litres to a volume of 10 litres. In doing so it absorbs 400 J of thermal energy from its surroundings. Determine the change in internal energy of system.
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$$\triangle V =10-5=5 \space litre=5×10^3 m^3$$

$$Q=400J \space ,\space \space P=1atm =1.013×10^{-5} pa$$ W=p.dV=$$1.013×10^5 ×5 ×10^{-3}$$

=506.5 J

$$\triangle U=Q-W$$ =400-506.5 J

=-106.5 J

$$|\triangle U|=106.5 J$$

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