Lesson_1.3 - Lesson 1.3 First Law of Thermodynamics Lesson...

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Lesson 1.3 First Law of Thermodynamics Lesson Objectives: At the end of this lesson students will be able to use the laws of thermodynamics to explain the working of an internal combustion engine. They will also use the laws of thermodynamics to solve problems. 1. First Law of Thermodynamics The first law of thermodynamics is a statement of the law of conservation of energy that describes the behavior of systems when heat is either added or removed from them. Heat added to a system can result in two things: (i) it increases the internal energy of the system (ii) it results in the system doing work If a quantity of heat Q is added to a system, and as a result, if W is the work done by the system and U the increase in its internal energy, then according to the conservation of energy, Q = U + W Heat added to a system equals the change in the internal energy of the system plus the work done by the system. This statement is the first law of thermodynamics which establishes a direct relationship between heat and mechanical work. According to this, mechanical work can be converted to work and vice versa. While W is a measure of the work done by the system, -W is a measure of the work done on the system. The internal energy U is a function of the state of the system, just as P, V and T are functions of the state of the system. When a gas is compressed for example, its states namely, P, V, T and U will all change. Similarly when a gas expands, all these states change. For a small quantity of heat dQ added to system, the law can be stated as: dQ = dU + dW …3 Example 6 : If 1.68 × 10 6 J of heat energy is added to a gas that expands and does 800 kJ of work, what is the change in the internal energy of the gas?
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Solution: dQ = dU + dW 1.68 × 10 6 = dU + 8.0 × 10 5 dU = 8.8 × 10 5 J Example 7: At Niagara falls, the water drops 50 m. If the change in potential energy goes into internal energy of the water, compute the increase in its temperature. Solution: Let us consider m kg of water at the top of the fall. Its potential energy = mgh. If all this becomes internal heat energy, we have mgh = m C w ∆θ 9.8 50 4180 0.12 w o C gh C θ × = = = Example 8: A lead bullet initially at 30 o C just melts upon striking a target. Assuming that all of the initial kinetic energy of the bullet goes into the internal energy of the bullet to raise its temperature and melt it, calculate the speed of the bullet upon impact. (Melting point of lead is 327
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This note was uploaded on 05/01/2011 for the course PHY 2049 taught by Professor George during the Spring '11 term at Edison State College.

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Lesson_1.3 - Lesson 1.3 First Law of Thermodynamics Lesson...

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