Physics thermodynamics conceptual questions
Define a thermodynamic system and its environment
A thermodynamic system is an object or set of objects considered to be a distinct physical entity to or from which energy is added or removed. The surrounding of the system is its environment
Rewrite the equation for the first law of thermodynamics for each of the following special thermodynamic processes A- an isothermal process B- an adiabatic process C- an isovolumetric process
A- an isothermal process In this case the temperature doesn't change which means that the internal energy of a system remains constant. Delta U = 0. So 0=Q-W. So W=Q. B- an adiabatic process In this case there is no change of heat between the system and the environment so Q= 0. So delta U =-W C- an isovolumetric process In this process there is no change in volume so delta V = 0. This means that the work done by the system is 0 so W=0. So delta U=Q
In which of the following systems is entropy increasing? A- An egg is broken and scrambled B- a cluttered room is cleaned and organized C- a thin stick is placed in a glass of sugar saturated water and sugar crystals form on the stick
A- entropy increases B- entropy decreases C- entropy decreases
For each of the following, which thermodynamic quantities (delta u, q, w) have values equal to zero? A- an isothermal process B- an adiabatic process C- an isovolumetric process
A- since the temperature doesn't change the internal energy of the system doesn't change. Delta U=0 B- no heat is added or removed from the system Q=0 C- the volume of the system doesn't change so there is no work done on or by the system. W=p delta v= 0
In each of the cases, trace the chain of energy transfers (as heat or work) as well as changes in internal energy A- you rub your hands together to warm them in a cold day, and they soon become cold again B- a hole is drilled into a block of metal. When a small amount of water is placed in the drilled hole, steam rises from the hole
A-When you rub your hands, you perform work against friction and during that process heat is generated. Thus work is converted into heat energy. After that heat is transferred to the surroundings by conduction and convection of air and the hands become cold. B- during the drilling process the drill machine preforms work on the block of metal which converts into heat and increases its temperature. This temp is typically about 100 degrees celsius, the boiling point of water. Now if a small amount of water is placed in the hole, the water will absorb the heat and some amount of water will become vapor. In this case heat is transferred from the metal plate to the water molecules and the internal energy of water increases.
Paint from an aerosol can is sprayed continuously for 30 s. The can was initially at room temperature but now feels cold to the touch. What type of thermodynamic process occurs for a small sample of gas as it leaves the high pressure interior of the can and moves to the outside atmosphere.
As a small sample of gas comes out of the aerosol can from high pressure region to the outside atmospheric pressure, its volume increases rapidly. The process happens so fast that the gas doesn't get chance to exchange any heat with the surroundings. Thus it is an adiabatic process. Since the process is adiabatic and it's volume increases so that W=P delta V >0 from the first law of thermodynamics we have delta u = Q- W < 0. Thus the internal energy of the gas decreases which means that the temperature of the gas will go down. This is why the can feels cold after it is sprayed
Write the equation for the first law of thermodynamics and explain why if is an expression if energy conservation
Delta U = Q-W Or Q=W+ delta U The expression tells us that the heat energy absorbed by a system is used up in two ways- the system some work on its environment and the rest of the heat increases its internal energy and thus temp. There is no loss of energy and thus it is an expression for conservation of energy
A bomb calorimeter is placed in a water bath and a mixture of fuel and oxygen is burned inside it. The temp of the water is observed to rise during the combustion reaction. The calorimeter and the water remain at constant volume. if the reaction products are the system, which thermodynamic quantities - delta U, Q, or W - are positive and which are negative
During the combustion reaction, heat is produced by the mixture of fuel and oxygen. This heat then slowly is transferred to the water bath until the temperature of the reaction product and the water bath becomes same. Since the temp decreases so too does the internal energy. Since heat is transferred from the reaction product to the water bath Q<0. There is no change in volume so W= 0 while delta u and q are negative.
A bomb calorimeter is placed in a water bath and a mixture of fuel and oxygen is burned inside it. The temp of the water is observed to rise during the combustion reaction. The calorimeter and the water remain at constant volume. If the water bath is the system, which thermodynamic quantities - delta U,Q, or W - are positive and which are negative?
During the combustion reaction, heat is produced by the mixture of fuel and oxygen. This heat then slowly is transferred to the water bath until the temperature of the reaction product and the water bath becomes same. Since the temp of the water bath increases so too does the internal energy so delta U is positive. Since heat is transferred from the reaction product to the water bath, the water bath absorbs heat so Q is positive. There is no change in volume so W=0
What conditions are necessary for a heat engine to have an efficiency of 1.0?
In order to have an efficiency of 1, one must have Qc=0 which means that no heat energy can be removed from the engine. In other words all of the energy added to the engine has to be converted to work. But this is in contradiction to the second law of thermodynamics so no heat engine can have an efficiency of 1.
Which of the thermodynamic values (delta U, Q or W) would be negative for the following system The interior of a closed refrigerator (system)
In this case the compressor of the fridge takes heat out of the interior so Q is negative. As a result the temp of the interior will decrease which means that delta U is negative. Since the volume of the air inside the fridge doesn't change W=0. So delta u and Q are negative
Which of the thermodynamic values (delta U, Q or W) would be negative for the following system The helium in a thermally insulated weather balloon (system) expands during inflation
In this case the weather balloon is thermally insulated which means that no heat can be exchanged between the helium and its environment so Q=0. Since the volume of the balloon increases W is positive. The first law of thermodynamics tells us that in its case delta u is negative.
A salt solution is placed in a blow and set in sunlight. The salt crystals that remain after the water has evaporated are more highly ordered than e randomly dispersed sodium and chloride ions in the solution. Has the requirement that total entropy increase been violated?
Initially in the salt solution sodium and chloride ions are randomly dispersed. When it is placed under sunlight, the water molecules absorb heat from sunlight and start evaporating. Since gas phase of a substance is a more disordered state than its liquid state, in this process the entropy of the water molecules increase. The amount of entropy of the salt molecules decreased in this process actually is less than the increase in entropy of the water molecules. Thus the requirement of increasing entropy is not violated in this process.
Why is it not possible for all of the energy transferred as heat from a high temp source to be expelled from an engine by work
It is not possible for all pf the energy transferred as heat from a high-temperature source to be expelled from an engine by work because it is against the second law of thermodynamics. The second law of thermodynamics demands that a fraction of energy must be removed from a heat engine in order to perform work and thus the efficiency always remain less than unity.
How is energy conserved if more energy is transferred as heat from a refrigerator to the outside air than is removes from the inside air of the refrigerator?
Let the heat removed from the inside air and the heat transferred to the outside air denoted by Qr and Qt respectively. Now for the fridge to remove heat from the inside it has to preform some amount of work. This amount if energy comes from the electricity and finally is converted into heat. This heat adds up with Qr and thus Qt is always greater than Qr. If the work done by the fridge compressor is W then Qt=Qr+W
The can if spray paint in the previous question is set aside for an hour. During this time the contents of the can return to room temperature. What type of thermodynamic process takes place in the can during the time the can is not in use?
The cold aerosol can when left aside, heat in transferred into it from its surrounding due to the temperature difference. In this process the temperature of the can slowly increases and finally the can reaches the room temperature. Since during the process the volume of the gas doesn't change, it is an isovolumetric process.
Suppose the waste heat at a power plant is exhausted to a pond of water. Could the efficiency of the plant be increased by refrigerating the water in the pond?
The efficiency of a heat engine depends upon the temperature of the source and the sink. The more is the temperature difference, the more is the efficiency. Thus by refrigerating the water of the pond, efficiency of the pond can be increased.
The first law of thermodynamics states that you cannot obtain more energy from a process than you orginally put in it. The second law states that you cannot obtain as much usable energy from a system as you put into it. Explain why these two statements do not contradict each other.
The first law of thermodynamics is basically a law of energy conservation which states the amount of energy put into a system is used up in two ways - some part of it increase the internal energy of the body and the rest is used to perform work. Clearly, on can never obtain energy more than what is originally put in. But if we consider the case that there is no change in internal energy involved, then the first law says that one can have the occasion of full conversion of heat to work done. But the second law putsch bound to it by stating that no heat engine can convert the energy transferred as heat into fully work. There will always be some heat lost thus the two laws don't contradict.
If a cup of very hot water is used as an energy source and a cup of cold water is used as an energy "sink" the cups can in principle be used to do work as shown below. if the contents are mixed together and the resulting lukewarm contacts are separated into 2 cups no work can be done. Use the second law thermodynamics to explain this. has the first law thermodynamics been violated by mixing and separating the content of the 2 cups
The second law of thermodynamics states the cyclic process that convert heat entirely into work are possible. In other words heat engines must have a source and a sink in order to absorb heat energy and then dump some amount of energy after doing work. Not in the first case the hot water acts as a source and the cold water acts as a heat sink so that the engine can dump the released energy into the cold water. But in the second case water temperature are at the same temp and heat cannot flow unless there is a temp difference. Thus there is no sink for the engine and it cannot preform any work. No, the first law of thermodynamics is not violated in the mixing process bc no energy is lost or gained during this process
Which of the thermodynamic values (delta U, Q or W) would be negative for the following system A steel rail (system) undergo slow thermal expansion on a hot day displaces the spikes and ties that hold the rail in place
The steel rail absorbs heat from the sunlight so Q is positive. As a result the temp of the rail will increase which means that delta U will be positive. It also expands and displaces the spike and ties which means that it performs positive work W>0 so none of the thermodynamic quantities are negative
In what two ways can the internal energy of a system be increased
This means that internal energy of a system can be increased by adding heat into the system or by performing work on the system.
Use a discussion of internal energy and entropy to explain why the statement "energy is not conserved in an inelastic collision" is not true
This statement is true if we consider the amount of mechanical energy associated with the system before and after the collision. Now we know that entropy of the universe increases in any spontaneous process. In the case of an inelastic collision the temp of the colliding bodies slightly increases due to the heat generated by the collision this in turn increases the internal energy of the bodies. Thus if we include the fact of increasing internal energy then the statment is not true.
When an ideal gas expands adiabatically it does work in its surroundings. Describe the various transfers of energy that take place
When an ideal gas expands adiabatically, there is no transfer of heat so Q=0. Using the first law of thermodynamics: delta U=Q-W. The work done by the gas is positive which implies that the change in internal energy is negative so in this case the internal energy of the gas is transferred to its surroundings