Download WELCOME TO PHYSICS 1103

WELCOME TO 1103
PERIOD 8
Homework Exercise #7 is due today.
Pick up the handout about Midterm 1 next
Wednesday at 6:30 pm and the midterm review
session next Tuesday at 6:30 pm
PHYSICS 1103 – PERIOD 8
•How is energy conserved?
•How can gravitational potential energy be used to
do work with a water wheel?
•What is the efficiency of multiple energy
conversion processes?
•Remember to put away your phone.
No calls or texting during class.
Law of conservation of energy
• Energy can be neither created nor destroyed.
• The total amount of energy we have now in the
Universe is the same amount that existed at its
beginning.
• All the energy that goes into a conversion process
must come out – no energy can be lost or destroyed.
• In every energy conversion, some energy is wasted.
• Wasted energy is often in the form of thermal energy
from the force of friction.
Kinetic energy
The kinetic energy of a moving object is:
E kin =
½Mv2
E kin = kinetic energy (joules)
M
= mass (kilograms)
v
= velocity (meters/sec)
What happens to the kinetic energy of the two carts?
Work
Work is done when a force moves an object over some
distance in the direction of the force.
Work = Force x Distance
W = FxD
W = work (joules or foot-pounds)
F = force applied (newtons or pounds)
D = distance moved in the direction of the
force (meters or feet)
Work and energy are measured in units of joules.
1 joule = 1 kg m2/s2
Work done against friction
Work is done to move the box horizontally against
the force of friction.
The work done goes into thermal energy.
Applied
Force
Force of
friction
W=FxD
Efficiency
The fraction of the work input that is useful work
output is the efficiency of the process:
Efficiency =
Total
Work In
Useful work out
Total work in
Conversion
Process
Wasted Work Out
Useful Work
Out
Work done against gravity
Work is done to raise an object vertically against the force
of gravity.
The work done goes into gravitational potential energy.
Applied Force equals the
weight of the box = Mg
Force of gravity
W=Mgh
h = height the center
of gravity of the
object is raised
Work to raise an object vertically
The work required to lift an object equals the object’s
weight times the vertical distance it moves.
W = Mgh
W = work (joules)
M = mass of object (kilograms)
g = acceleration of gravity = 9.8 m/s2
h = change in height (meters)
Neglecting the energy wasted by friction, the gain in
gravitational potential energy from raising an object
equals the work done to raise it.
Epot = M g h
Efficiency of the water wheel
Efficiency =
Epot
Gravitational
potential
energy in
Useful work out
Total work in
= Mgh
Conversion
Process
Wasted energy out
Kinetic
energy out
Efficiency of multiple energy conversions
Efficiency = Useful energy out
Total energy in
The overall efficiency of a series of energy conversions
is the product of the efficiencies of each step.
Overall Efficiency = Eff1 x Eff2 x Eff3 x …..
Example: 3 energy conversions steps with efficiencies
of 40%, 50% and 20% have an overall efficiency of….
0.40 x 0.50 x 0.20 = 0.04 = 4%
America Revealed: Electric Nation
1) What energy processes are required to provide coal
from the Powder River Basin in Wyoming to a power
plant in Ohio?
2) A power plant burns coal to turn water into steam, which
turns a turbine that generates electricity. What energy
processes are required?
3) What is the main difference between a coal burning
power plant and a nuclear plant? Which is more
expensive to build? Why?
4) What energy processes are required to produce
electricity from methane gas at the Blue Spruce dairy?
5) How does conservation of energy apply to each of the
processes described above?
BEFORE THE NEXT CLASS…
Read textbook chapter 9
Complete Homework Exercise 8
Bring a blank Activity Sheet 9 to class.
Study for midterm 1
Note: midterm 1 review session next Tuesday,
9/25, 6:30 – 7:30 pm in 2005 Smith Lab