 # First Law of Thermodynamics Questions

## What is Thermodynamics?

Thermodynamics is a branch of physics that deals with heat, temperature and work and their relation with energy, physical properties of matter and radiation.

The First Law of Thermodynamics states that thermodynamic processes are subject to the principle of conservation of energy, and heat is a form of energy too. This concludes that heat energy can neither be created nor be destroyed. However, it can be converted from one form to another form of energy and can be relocated from one location to another.

Heat, internal energy, and work are related to the first law of thermodynamics.

The first law of thermodynamics, also recognised as the law of energy conservation, states that energy can be changed from one form to another. Still, it can neither be created nor destroyed.

According to this law, some heat is used to change the internal energy given to the system, while the remaining heat is used in doing work by the system.

Mathematically it can be represented as

 ΔQ=ΔU+W

Were,

• ΔQ = heat given or lost
• ΔU is the change in internal energy
• W is the work done

## Important Questions on the First Law of Thermodynamics

1) Isothermic process means that ΔU=0. True or False?

a) True

b) False

Correct Option: (a)

Explanation: ΔU=0 indicates that the process will have a constant temperature since internal energy is a function of temperature only. Therefore, it is isothermic.

2) Which among the following laws is the basis of the first law of thermodynamics?

a) Law of conservation of mass

b) Law of conservation of energy

c) Law of conservation of work

d) Law of conservation of momentum

Correct Option: (b)

Explanation: The first law of thermodynamics deals with work done and heat energy supplied or removed from a system. Therefore, it is based on the conservation of energy. This law says that energy supplied to a system is conserved.

3) Find the change in internal energy of a system if 415cal of heat is given to the system, and the system does 35cal of work.

a) 290cal

b) 380cal

c) 0cal

d) 200J

Correct Option: (b)

Explanation: According to the first law, ΔU=ΔQ – ΔW

= 415 – 35

= 380cal.

Since the system does the work, and the heat is supplied to the system, it is positive.

4) If a gas of volume 30m3 is contained in a rigid container and 60J of heat is provided to it. Then, calculate the change in internal energy. Assume that the gas exerts 1atm of pressure on the walls.

a) 40J

b) 0

c) 60J

d) 80J

Correct Option: (c)

Explanation: Change in internal energy is equal to all the heat supplied. Since the container is rigid, there will be zero volume change; therefore, the work done is zero.

Thus, ΔU = ΔQ – ΔW = 60 + 0 = 60J.

5) During the process of photosynthesis, which reaction takes place?

a) Chemical reaction

b) Endothermic reaction

c) Exothermic reaction

d) Redox reaction

Correct Option: (b)

Explanation: By absorbing heat and energy from the surroundings, photosynthesis takes place. A reaction in which the system absorbs heat from its surroundings is known as an endothermic reaction. During photosynthesis, plants absorb heat and energy from their surroundings; therefore, it is an endothermic reaction.

6) Which of the following shows the equation of the first law of thermodynamics equation?

a) Heat added into work done = Internal Energy

b) Heat rejected into work done = Internal Energy

c) Heat added divided by work done = Internal Energy

d) Heat added plus work done = Internal Energy

Correct Option: (d)

Explanation: A relationship between internal energy, heat and work done is given by thermodynamic expression. Work done by the system will be negative, and the work done on the system will be positive. Joule is the standard unit of all these quantities.

7) The temperature developed during a fluid flow is due to _____.

a) Change in density

b) Increase in pressure

c) Fluid level

d) Translational kinetic energy

Correct Option: (d)

Explanation: The molecules tend to collide with each other when there is a high rate of fluid flow. The average translational kinetic energy of the particles at this state increases. And due to this increase in the kinetic energy, the temperature developed is called the Kinetic temperature.

8) The average kinetic energy equation is given by _____.

a) 0.5 KT

b) 1.5 KT

c) 2.5 KT

d) 3.5 KT

Correct Option: b

Explanation: Since the equation for kinetic energy is 1/2mv2, this equation agrees to 1.5 KT, where K=Boltzmann’s constant and T = Temperature.

9) Due to the increase in enthalpy, the internal energy _____.

a) Increases

b) Decreases

c) First decreases, then increases

d) No change

Correct Option: (a)

Explanation: The number of molecular interactions increases when the temperature increases. Hence, according to the equation from the first law of thermodynamics, with the increase in temperature.

10) What is the SI unit of enthalpy?

a) kgK/Joule

b) Joule/K2

c) Joule/kg

d) kg/k

Correct Option: (c)

Explanation: The measurement of energy in a thermodynamic system is known as enthalpy. It is equal to the product of volume and pressure plus the internal energy. Therefore, its unit is Joule/kg.

### Practice Questions

1) What is the first law of thermodynamics?

2) How many laws of thermodynamics are there?

3) What is the latent heat of vaporisation?

4) Define enthalpy.

5) What are the limitations of the first law of thermodynamics?

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