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Q1. Discuss the application of the first law of thermodynamics to Isothermal process.

Solution

Isothermal process :  Since temperature remains constant in an isothermal process, dU = 0 From the first law of thermodynamics, dQ = dU + dW Therefore, dQ = dW = PdV (a)  Therefore, if a gas is to expand isothermally, then an amount of heat equivalent to the work done by the gas will have to be supplied to it from the external source. (b)  During isothermal compression, dV will be negative.  This implies that during an isothermal compression, the amount of heat equivalent to the work done on the gas will have to be removed from it. (c)  During isothermal compression or expansion, there is no change in the internal energy of the gas.  

Q2. What thermodynamic variable is defined by (a)  Zeroth Law? (b)  First Law?

Solution

(a) The thermodynamic variable defined by the zeroth law is temperature. (b) The thermodynamic variable defined by the first law is internal energy.

Q3. Discuss the application of the first law of thermodynamics to Boiling process :

Solution

Boiling process : When a liquid is heated, it starts boiling at a particular temperature called its boiling point.  At its boiling point (pressure is kept constant), the unit mass of the liquid requires a definite amount of energy to change from the liquid to the vapour state.  The amount of heat supplied is known as the latent heat of vaporisation of the liquid. Consider a liquid of mass m at its boiling point.  Let P be the external pressure and L be the latent heat of vaporisation. Therefore, Amount of heat required to convert the whole of the liquid into vapour, dQ =mL.  From the first law of thermodynamics, dQ = dU + dW dU = U_f - U_i and dW = PdV = P(V_f - V_i) where the subscripts i and f denote the initial and final values respectively. Therefore, mL = (U_f - U_i) + P(V_f -V_i) or (U_f - U_i) = mL - P(V_f - V_i) Thus, knowing m, L, V_f, V_i, the increase in the internal energy can be calculated.

Q4. A piece of lead is hammered.  Does its internal energy increase?  Does the heat enter the lead from outside?

Solution

Yes,  internal energy of lead increases.  No heat energy from outside enters the lead.

Q5. What does a heat engine do? Name two types of heat engines. Give one example of each.

Solution

It is a device used for converting heat energy into mechanical energy. (i)  External combustion engine - where fuel is burnt in a separate unit, outside the working portion of the engine, e.g., steam engine. (ii)  Internal combustion engine - where fuel is burnt within the working portion of the engine, e.g., petrol engine and diesal engine.

Q6. State the conditions which must be fulfilled to utilize heat into useful work.

Solution

Following conditions must be fulfilled to utilize heat for a useful work: (i)  A device called engine with a working substance is essential. (ii)  The engine must work in a reversible cyclic process. (iii)  The engine must operate between two temperatures.  It will absorb heat from a hot body ( called the source), convert a part of it into useful work and reject the rest to a cold body (called the sink).  

Q7. State whether the following processes are reversible or irreversible : (i)  Heat production by friction (ii)  The growth of a planet (iii)  Diffusion of gases (iv)  Electrolysis (v)  Sudden unbalanced expansion of a gas (vi)  Carnot's cycle (vii)  Joule's heating in an electrical resistance.

Solution

(i)  Irreversible (ii)  Irreversible (iii)  Irreversible (iv)  Reversible (v)  Irreversible (vi)  Reversible (vii)  Irreversible

Q8. 200 joule of work is done on a gas to reduce its volume by compressing it.  If this change is done under adiabatic conditions, find out the change in internal energy of the gas and also the amount of heat absorbed by the gas.

Solution

In adiabatic changes dQ = 0 Therefore, dQ = dU + dW = 0 dU = - dW = - ( - 200 J ) = 200J Internal energy increases by 200J.  Heat absorbed is zero.

Q9. How does the Second Law of Thermodynamics help in the working of a refrigerator?

Solution

A way of stating the second law of thermodynamics is that work must be done to get heat to flow from a cold object to a hot object. In a refrigerator, there is a cycle that is carried on continuously. A liquid refrigerant substance vaporizes in the cooling coils inside the fridge. The fluid absorbs heat from its surroundings to vaporize. This cools the interior of the fridge. The gas thus formed is pumped to the exterior of the fridge where it is compressed into a liquid. Work is done on the gas to compress the gas, causing the gas to release heat. This heat is lost to the air surrounding the fridge. Hence, heat is moved from the inside of the fridge to the outside.

Q10. State two conditions of a reversible process.  What is a quasi-static process?

Solution

(i)  The process should take place very slowly, so that it satisfies the following requirements : (a)  The system should be in thermal equilibrium. (b)  The system should be in mechanical equilibrium. (c)  The system should be in chemical equilibrium. (ii)  There should be no fricitional losses. A process in which the system passes through a continuous sequence of equilibrium states is said to be quasi-static, i.e., apparently static.