First law of Thermodynamics C ontents: Basic Concept Example Whiteboards.

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First law of Thermodynamics C ontents: Basic Concept Example Whiteboards

First law of Thermodynamics TOC 1. Energy is conserved. (heat is also energy) KE and PE can be turned to heat Heat can be turned to KE and PE Car brakes Bike brakes on mountain roads Hammers hitting nails Demo - steel balls and paper

Example TOC Myron Wondergaim lets a 54.1 kg weight fall, turning a stirrer in an insulated container holding.872 kg of water initially at 20.0 o C. What distance has the mass descended at a constant speed when the water reaches a temperature of 30.0 o C? c w = 4186 J/kg/ o C potential energy = Heat gained mgh = m w c w  T w (54.1 kg) (9.8 N/kg)h = (.872 kg)(4186 J/kg/ o C)(10.0 o C) h = 68.7 m

First law problems 11 | 22 TOC

Potential energy = Heat gained mgh = m Pb c Pb  T Pb (4.5)(9.8)(32) = (4.5)(130)  T Pb  T Pb = 2.4 o C 2.4 o C W Hugo Idonuana drops a 4.5 kg bag of lead shot from a height of 32 m. What is the change in temperature of the shot assuming that all of the potential turns into heat absorbed by the shot? (c Pb = 130 J o C -1 kg -1 ) (Does the answer depend on the mass of the lead shot?)

Kinetic energy = Heat gained 1 / 2 mv 2 = m Fe c Fe  T Fe 1 / 2 (2.5)v 2 = (.015)(450)(.55) v = 1.7 m/s 1.7 m/s W Jenny Rator hits a 15 gram nail with a 2.5 kg hammer. What is the velocity of the hammer if the nail rises in temperature.55 o C with each stroke of the hammer? (assume the nail absorbs all the heat evolved) (c Fe = 450 J o C -1 kg -1 )