Download Chapter 13: Work and Energy - South Kingstown High School

CHAPTER 13: WORK AND ENERGY
SECTION 4: CONSERVATION OF ENERGY
ENERGY TRANSFORMATIONS
 Energy readily changes from one form to another form.
 Potential energy can become kinetic energy
 Kinetic energy can become potential energy
 Mechanical energy can change to other forms of energy  heat, sound
 Whatever form energy takes during a transformation, the TOTAL AMOUNT of
energy ALWAYS REMAINS THE SAME
 We know where it goes.
 Can’t always use the new energy form, but it is counted
LAW OF CONSERVATION OF ENERGY
 Energy cannot be created or destroyed
 Although energy may change from one form to another, the total amount of energy
in the universe never changes
 Energy does not appear or disappear
 Also called the First Law of Thermodynamics
 For any system, the net change in energy equals the energy transferred as work and as
heat
EFFICIENCY
 Only a portion of the work done by a machine is useful work
 Some useful energy is lost due to friction as heat energy.
 Efficiency  measure of how much useful work a machine can do
𝑢𝑠𝑒𝑓𝑢𝑙 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡
𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
x 100%
𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡
EFFICIENCY CONTINUED
 Efficiency is usually expressed as a percentage.
 100% efficient machine means that the machine uses all the work put into it as work done by the
machine
 This is impossible in the real world, because of friction
 This means that perpetual motion machines are impossible.
 Machines need energy input