Energy & Its Impact on Global Society Jerome K. Williams, Ph.D. Saint Leo University Dept. Mathematics & Sciences
Chapter 2: Energy Mechanics Energy & Work Heat First Law Thermodynamics Examples of Work & Energy Power & Electrical Applications
Energy & Work Energy is the “capacity to do work.” Work is the product of a force in the direction of motion times the object’s displacement. W = F x d
Motion
Energy & Work Units (Energy & Work) – SI System:Newton x meters = Joules (J) – English System:Foot-pounds = ft-lb Table 2.4(Units in Mechanics)
Energy & Work Work is one way of transferring energy to an object W = Δ(KE + PE) Thermal Energy (TE) of object is a function of its temperature
Energy & Work Thermal Energy (TE) of object is a function of its temperature; it is equal to total of all the microscopic energies of the molecules that comprise an object. W = Δ(KE + PE + TE)
Heat Addition of heat is another way of transferring energy to a system Heat is the energy transferred as a result of a temperature difference between two objects. W = Δ(KE + PE + TE)
First Law of Thermodynamics Thermal energy of an object can be changed by doing work (W) as well as adding heat (Q). First Law of Thermodynamics: total energy of a system can be increased by doing work on it or by adding heat. W + Q = Δ(KE + PE + TE)
The First Law: Equations & Math PE G = weight x height = mgh – g = acceleration due to gravityg = 9.8 m/s 2
Examples of Work & Energy A man pushes a box across the floor by exerting a force of 150 N on it in the direction of motion. If the box moved 3 m, how much work (W) did he do? Answer450 J
Examples of Work & Energy What is the kinetic energy of a 80 kg person walking with a speed of 1 m/sec? Answer40 J
Examples of Work & Energy What is the potential energy does a 50 kg person standing on the roof of a house 4 m off the ground? Answer1960 J
Power & Electrical Applications Another basic concept of energy mechanics is power. Power is the rate of doing work or the rate at which energy is used, produced, or transferred.
Power & Electrical Applications
Units for Power are energy divided by time Watt is SI unit for power (1 Watt = 1 Joule/sec) More common unit is kilowatt (kW) which is the same as 1000 Watts.
Power & Electrical Applications Units for Power are energy divided by time English system uses ft-lb/sec as unit for power Another common unit is horsepower (hp) 1 hp = 746 Watts and 1 hp = 550 ft-lb/sec
Power & Electrical Applications Average power expended per person per year in US is about 12 kW In most countries, higher standards of living (measured by GDP per capita) are matched by higher levels of energy consumption per capita
Power & Electrical Applications Rate of producing electricity is electrical power measured in watts. A modern-size power plant produces electrical energy in billions of watts or 1000 megawatts (1000 MWe) where e signifies electrical power output
Power & Electrical Applications Many power plants in US still rely on coal to operate; however, newer plants are relying on renewable energy sources Hydropower plants provide about 10% of US total electrical energy needs
Power & Electrical Applications If it takes 2 seconds to raise an 8 kg block a vertical height of 1 m, what is the power expended? Answer39 W
Power & Electrical Applications Ascending to the top of Muir Pass in the Sierra Nevada Mountains in California, Robert climbed 2000 feet (vertically) in a time of 2.5 hours. What was his average power expended (in watts)? Robert plus his backpack weigh 180 lb. Answer54 W
Power & Electrical Applications An electric heater has a power rating of 1500 watts (1.5 kW). If the heater is run for 6 hours, and electricity costs $.12 per kWh, how much will it cost to run the heater for this time interval? Answer$1.08
Suggested Problems 3, 4, 16, 19, 27, 30, 32