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DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET PDF Download

Introductory Exercise 18.1

Ques 1: A gas in a cylinder is held at a constant pressure of 1.7 × 105 Pa and is cooled and compressed from 1.20 m3 to 0.8 m3. The internal energy of the gas decreases by 1.1 × 105 J.
(a) Find the work done by the gas.
(b) Find the magnitude of the heat flow into or out of the gas, and state the direction o f heat flow.
(c) Does it matter whether or not the gas is ideal ?
Sol: 
(a) Work done by gas under constant pressure is
W = P(Vf + Vi)
= (1.7 × 105)(0.8 - 1.2) 

= -6.8 × 104J
(b) Q = W + ΔU
= -6.8 × 104 -1.1 × 105
or
DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET
As Q is negative. So, net heat flow is out of the gas.

Ques 2: A thermodynamic system undergoes a cyclic process as shown in figure.
DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET
(a) over one complete cycle, does the system do positive or negative work.
(b) over one complete cycle, does heat flow into or out of the system.
(c) In each of the loops 1 and 2, does heat flow into or out of the system.
Sol: 
(a) W1 = + ve, as cycle is clockwise
W2 = - ve as cycle is anticlockwise
DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET
∴ Net work done by the system is positive.
(b) In cyclic process, Qnet = Wnet
Since Wnet is positive.
So, Qnet is also positive. So, heat flows into the system.
(c) W1 = Q= + ve, so, into the system.
W2 = Q2 = -ve, so, of the system.

Ques 3: A well insulated box contains a partition dividing the box into two equal volumes as shown in figure. Initially the left hand side contains an ideal monoatomic gas and the other half is a vacuum. The partition is suddenly removed so that the gas now is contained throughout the entire box.

DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET
(a) Does the temperature of the gas change?
(b) Does the internal energy of the system change?
(c) Does the gas work?
Sol:  
Q = 0 as the box is insulated.
Wgas = 0 as on the right hand side there is vacuum.
ΔU = Q - W = 0
ΔT is also zero, as U ∝ T

Ques 4: How many moles of helium at temperature 300 K and 1.00 atm pressure are needed to make the internal energy of the gas 100 J?
Sol: 
Helium is monoatomic. So, degree of freedom f = 3.
DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET
= 2.67 × 10-2 mol

Ques 5: A 1.0 kg bar of copper is heated at atmospheric pressure (1.01 × 105 N/m2). If its temperature increases from 20°C to 50°C, calculate the change in its internal energy. α = 7.0 × 10-6 per°C, ρ = 8.92 × 103 kg/m3 and c =387 J/kg-°C
Sol:  

DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET
= (1.12 × 10-4)(2.1 × 10-5)(30) = 7.056 × 10-8m3
W = PΔV
= (1.01 × 105)(7.056 × 10-8) = 7.13 × 10-3J
DC Pandey Solutions: First Law of Thermodynamics - 1 | Physics Class 11 - NEET

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FAQs on DC Pandey Solutions: First Law of Thermodynamics - 1 - Physics Class 11 - NEET

1. What is the first law of thermodynamics?
Ans. The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed in a closed system. It can only be transferred or converted from one form to another.
2. How does the first law of thermodynamics relate to the given article on DC Pandey solutions?
Ans. The first law of thermodynamics is fundamental to understanding and solving problems related to thermodynamics. In the context of the given article on DC Pandey solutions, the first law helps in analyzing and explaining the concepts and problems related to the first law of thermodynamics as discussed in the textbook.
3. Can you provide an example that demonstrates the application of the first law of thermodynamics?
Ans. Sure! Let's consider a simple example of a gas-filled balloon. When the balloon is heated, the gas molecules inside gain energy, causing an increase in temperature and pressure. According to the first law of thermodynamics, the energy supplied to the system (balloon) in the form of heat is used to increase the internal energy of the gas molecules, resulting in an increase in temperature and pressure.
4. What are some common misconceptions about the first law of thermodynamics?
Ans. One common misconception is that the first law allows for the creation of energy. However, the first law states that energy cannot be created or destroyed, only transferred or converted. Another misconception is that the first law applies only to mechanical systems, whereas it is applicable to all forms of energy, including heat, work, and internal energy.
5. How is the first law of thermodynamics related to the concept of energy conservation?
Ans. The first law of thermodynamics is closely related to the concept of energy conservation. It states that the total energy of a closed system remains constant, meaning that energy cannot be created or destroyed. This principle aligns with the broader concept of energy conservation, which states that the total energy in the universe is constant.
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