Second law, reversible and irreversible processes
- A Carnot engine has an efficiency of 50% when its source is at a temperature 327^° C. The temperature of the sink is : Answer: 2. 27^° C Carnot efficiency: η=1-T₂/T₁ with temperatures in kelvin. T₁=327+273=600 K, η=0.5⇒ T₂=0.5×600=300 K = 27^° C. The answer i... 1
- A carnot engine having an efficiency of 1/10 as heat engine, is used as a refrigerator. If the work done on the system is 10 J, the amount of energy absorbed from the reservoir at lower temperature is : Answer: 1. 90 J Key idea: as a refrigerator, coefficient of performance =Q₂/W=1-η/η . η=tfrac110⇒Q₂/W=9/10/1/10=9 . Q₂=9×10=90 J absorbed fr... 1
- The efficiency of an ideal heat engine working between the freezing point and boiling point of water, is Answer: 1. 26.8% Efficiency of an ideal (Carnot) engine is η=1-T₂/T₁ , with temperatures in kelvin. T₂=273 K (freezing point), T₁=373 K (boi... 1
- The temperature inside a refrigerator is t_2°C and the room temperature is t_1°C. The amount of heat delivered to the room for each joule of electrical energy consumed ideally will be Answer: 1. t₁+273/t₁-t₂ The refrigerator works as a heat pump. Ideally its coefficient of performance for the heat delivered to the room is... 1
- A refrigerator works between 4^° C and 30^° C. It is required to remove 600 calories of heat every second in order to keep the temperature of the refrigerated space constant. The power required is : (Take 1 cal = 4.2 Joules) Answer: 3. 236.5 W Key idea: for a refrigerator, COP=T₂/T₁-T₂=Q₂/W . T₂=277 K, T₁=303 K, so COP =277/26≈10.65 . Heat removed per second =600... 1