Lesson_1.3_Printable_PPT

Lesson_1.3_Printable_PPT - First Law of Thermodynamics The...

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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: 1. It increases the internal energy of the system ( U ) . results in the system doing work ( Q = U + W 2. It results in the system doing work ( W ) If Q is the quantity of heat added to a system, then:

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This statement of the first law of thermodynamics establishes a direct relationship between heat and mechanical work. This statement suggests that mechanical energy 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. For a small quantity of heat dQ added to dQ = dU + dW system, the law can be stated as:

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Example 1 : If 1.68 x 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? dQ = 1.68 × 10 6 J dW = 8.0 × 10 5 J dQ = dU + dW dU = dQ – dW = 1.68 × 10 6 – 8.0 × 10 5 = 8.8 × 10 5 J
Example 2: 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. U = mgh Let us consider m kg of water at the top of the fall. Its potential energy U = mgh. Potential energy at the bottom is U = 0 U = 0 The change in energy U = mgh U = mgh

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U = mgh If ∆θ is the change in temperature of water, its increase in internal heat energy is: dQ = m c w ∆θ ∆θ This change in energy becomes internal heat energy. = mgh h .8 50 U = 0 U = mgh w gh C θ ∆ = 9.8 50 4180 × = = 0.12 K
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 o C and L f for lead is 2.47 x 10 4 J.kg -1 , specific heat of lead c L = 128 Jkg -1 K -1 et the speed of the bullet be nd its mass . Let the speed of the bullet be v and its mass m. The kinetic energy

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Lesson_1.3_Printable_PPT - First Law of Thermodynamics The...

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