First Law of Thermodynamics PDF

LET'S LEARN! LAWS OF THERMODYNAMICS First Law of Thermodynamics (LAW OF CONSERVATION OF ENERGY)  Founder William Thomson also known as Lord Kelvin and Baron Kelvin. He was a British mathematician, mathematical Physic...

LET'S LEARN! LAWS OF THERMODYNAMICS First Law of Thermodynamics (LAW OF CONSERVATION OF ENERGY)  Founder William Thomson also known as Lord Kelvin and Baron Kelvin. He was a British mathematician, mathematical Physicist, and engineer born in Belfast. Professor of Natural Philosophy at the The University of Glasgow for 53 years, he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics and did much to unify the emerging discipline of physics in its contemporary form. He received the Royal Society's Copley WILLIAM THOMSON Medal in 1883, was its President 1890–1895, and in JUNE 26, 1824 – DECEMBER 17, 1892 1907 was the first British scientist to be elevated to the House of Lords.  First Law The first law of thermodynamics states the of Thermodynamics general principle of the conservation of (LAW OF CONSERVATION OF ENERGY) energy. “Energy is neither created nor destroyed, but only gets transformed from one form to another”.  First Law The total energy of the universe is a of Thermodynamics constant. (LAW OF CONSERVATION OF ENERGY) Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa. When a system undergoes a thermodynamic cycle then the net heat supplied to the system from the surroundings is equal to net work done by the system on its surroundings. SUM OF ENERGY The Sum of Energy Entering is equal to the Sum of Energy Leaving. where : PE = potential energy KE = kinetic energy H = U + PV H = enthalpy PE1 + KE1 + H1 +Q is equal (=) PE2 + KE2 + H2 + W  THE FIRST LAW OF THERMODYNAMICS FORMULA Noted: all are measured in Joules (J) Joule’s Experiment: Figure shows the experiment for checking the first law of thermodynamics. The work input to the paddle wheel is measured by the fall of weight, while the corresponding temperature rise of liquid in the insulated container is measured by the thermometer. INTERNAL ENERGY It is the heat energy stored in a gas. If a certain amount of heats supplied to a gas the result is that temperature of gas may increase or volume of gas may increase thereby doing some external work or both temperature and volume may increase ; but it will be decided by the conditions under which the gas is supplied heat. If during heating of the gas the temperature increases its internal energy will also increase. We do not know how to find the absolute quantity of internal energy in any substance ; however, what is needed in engineering is the change of internal energy (ΔU). INTERNAL ENERGY The “internal” energy of a system (U) (for a container of ideal gas, U = kinetic energy of the molecules) can be changed by transferring heat to and from the environment and/or performing work on or by the environment. ΔU = Q - W Positive Q = heat input to the system from the environment Negative Q = heat output from the system to the environment Positive W = work done by the system on the environment Negative W = work done on the system by the environment Note: The combined energy of the system and the environment is conserved; energy merely transferred to and from system and environment APPLICATION When a process is executed by a system, the change in stored energy of the system is numerically equal to the net OF FIRST heat interactions minus the net work interaction during theprocess. LAW TO A ∴ E2 – E1 = Q – W PROCESS ∴ ΔE = Q – W [or Q = ΔE + W ] where E represents the total internal energy. If the electric, magnetic and chemical energies are absent and changes in potential and kinetic energy for a closed system are neglected, the above equation can be written as ∴ Q – W = ΔU [ΔU = U2 – U1]  Generally, when heat is added to a system its temperature rises and external work is performed due to increase in volume of the system. The rise in temperature is an indication of increase of internal energy. Heat added to the system will be considered as positive and the heat removed or rejected, from the system, as negative.  PERPETUAL MOTION MACHINE OF THE FIRST KIND—PMM 1 The first law of thermodynamics states the general principle of the conservation of energy. “Energy is neither created nor destroyed, but only gets transformed from one form to another”. There can be no machine which would continuously supply mechanical. Work without some form of energy disappearing simultaneously (Fig. 4.3). Such a fictitious machine is called a perpetual motion machine of the first kind, or in brief, PMM 1. A PMM 1 is thus impossible.  Sample perpetual motion machines illustration: Capillary Bowl: Perpetual Wheel by Da Vinci : ENERGY An isolated system is one in which there is no interaction of the OF AN system with the surroundings. ISOLATED For an isolated system, SYSTEM ∴ dQ = 0, dW = 0 The first law of thermodynamics gives ∴ dE = 0 or E = constant The energy of an isolated system is always constant Specific The specific heat of a solid or liquid is usually defined as the heat required to raise unit mass Heats through one degree temperature rise. For small quantities, we have ∴ dQ = mcdT Where: m = mass, c = specific heat, and dT = temperature rise. Specific For a gas there are an infinite number of Heats ways in which heat may be added between any twotemperatures, and hence a gas could have an infinite number of specific heats. However, only two specific heats for gases are defined. Specific heat at constant volume, cv. Specific heat at constant pressure, cp.  Sample Problems 1. Find the change in internal energy of a system that absorbs 500 KJ of heat and at the same time does 400 KJ of work. A. 900 KJ B. 450 KJ C. 100 KJ D. No work is done on the system Solution 2.  Work Work is a means for transferring energy. Accordingly, the term work does not refer to what is being transferred between systems or to what is stored within systems. Energy is transferred and stored when work is done. ΔU = Q + W ΔU is the total change in internal energy of a system, Q is the heat exchanged between a system and its surroundings, and W is the work done by or on the system. Work is also equal to the negative external pressure on the system multiplied by the change in volume: W = − p ΔV From a thermodynamic perspective, the following apply: When a gas expands, the energy is transferred to the system’s surroundings. Work is done by the gas on the surroundings. Here the work is negative (-W) with respect to the system (gas), as energy is released from the system. When a gas is compressed, energy is transferred from the surroundings to the gas. Work is done on the gas by the surroundings. Hence the work is positive (+ W) with respect to the system (gas). If the work done is being considered with respect to the surroundings, then the sign in the equation becomes positive. Work done becomes positive when the gas is expanded, while the work done is negative when the gas is compressed. Sample Solved Problem A gas in a system has constant pressure. The surroundings around the system lose 62J of heat and does 474J of work onto the system. What is the internal energy of the system? Solution To find internal energy, ΔU, we must consider the relationship between the system and the surroundings. Since the First Law of Thermodynamics states that energy is not created nor destroyed we know that anything lost by the surroundings is gained by the system. The surrounding area loses heat and does work onto the system. Therefore, q and w are positive in the equation ΔU = q + w because the system gains heat and gets work done on itself. ΔU = 62J + 474J = 536J A system has constant volume (ΔV=0) and the heat around the system increases by 45J. a. What is the sign for heat (q) for the system? b. What is ΔU equal to? c. What is the value of internal energy of the system in Joules? Solution Since the system has constant volume (ΔV=0) the term –PΔV = 0 and work is equal to zero. Thus, in the equation ΔU = q + w, w = 0 and ΔU = q. The internal energy is equal to the heat of the system. The surrounding heat increases, so the heat of the system decreases because heat is not created nor destroyed. Therefore, heat is taken away from the system making it exothermic and negative. The value of Internal Energy will be the negative value of the heat absorbed by the surroundings. a. negative (q