Transcript Conservation of Energy

SECTION 2: Conservation of Energy
WARM-UP:
 All living things need energy to stay
alive. How do people obtain energy?
Where does this energy come from?
LEARNING GOALS
 Describe how energy can be transformed
from one form to another.
 Explain how the mechanical energy of a
system is the sum of the kinetic and
potential energy.
 Discuss the law of conservation of energy.
ENERGY CHANGING FORMS
 Energy is constantly
changing forms.
ENERGY CHANGING FORMS
 Electrical energy is one of the main
types of energy transformations that
occur today.
 What are some examples that you have
used so far today?
ENERGY CHANGING FORMS
 Fuel stores chemical potential energy in its
chemical bonds.
 In a car:
 A spark plug fires, converting chemical energy into
thermal energy.
 The hot gas expands and moves parts of the car
(mechanical energy).
MECHANICAL ENERGY
 Mechanical energy: the total amount
of potential and kinetic energy in a
system.
 ME = potential energy + kinetic energy
FALLING OBJECTS
 The objects on the shelf have
potential energy.
 The objects also have mechanical
energy.
FALLING OBJECTS
 When the objects fall, they lose
potential energy, but gain
kinetic energy.
 No energy is truly “gained” or “lost”. The
energy is transformed.
 The amount of mechanical energy stays
the same.
EXAMPLE:
 A swing has a potential energy of 100 J and a
kinetic energy of 55 J at a certain spot. What
is the mechanical energy of the swing?
PROJECTILE MOTION
 As a projectile is launched upwards, it
has mostly kinetic energy.
 As the velocity decreases, kinetic energy
decreases and GPE increases.
 As it falls, GPE decreases and kinetic
energy increases again.
CONSERVATION OF ENERGY
 Law of Conservation of Energy:
energy cannot be created nor
destroyed, only transferred or
transformed.
 The amount of energy in the universe has
always been the same!!
CONSERVATION OF ENERGY
 As you swing on a swing, air resistance and
friction slow the swing down.
 This doesn’t mean that energy is “lost”, it has
only been converted to thermal energy.
 Friction always converts energy into thermal
energy.
MASS TO ENERGY
 During some processes, mass is
converted into energy.
 This is represented by Einstein’s famous
2
equation E = mc .
MASS TO ENERGY
 Two examples of this are :
 nuclear fusion (like in the Sun)
MASS TO ENERGY
 nuclear fission (like in a nuclear power plant)
ENERGY CONVERSIONS
 You also obey the law of conservation of energy:
 Your body obtains energy from the food that you eat.
 You digest it and break it down to get the energy out.
 Your body either uses the energy immediately or stores
that energy as fats and carbohydrates in your body.
 When you require energy, your body converts those
energy stores to other forms of energy.
ENERGY CONVERSIONS
 The food Calorie is a unit used by
nutritionists to measure how much
energy you get from various foods.
 1 Calorie (Cal) = 4,184 J.
ENERGY CONVERSIONS
 Every gram of fat provides 9 Cals of
energy – how many J?
ENERGY CONVERSIONS
 Every gram of carbohydrates or protein
provides 4 Cals of energy – how many J?
ENERGY CONVERSIONS
 We all depend on electricity every day.
 Have you ever wondered where that
electricity comes from?
ENERGY CONVERSIONS
 Power Plants convert other types of
energy into electrical energy.
 Coal Power Plant: Chemical  Electrical
 Oil and Natural Gas: Chemical  Electrical
 Solar Power: Electromagnetic  Electrical
ENERGY CONVERSIONS
 Nuclear Power: Thermal  Electrical
 Hydroelectric: Mechanical  Electrical
 Wind: Mechanical  Electrical
ENERGY CONVERSIONS
 How it all works:
1. Energy is needed to make a turbine
spin.
a. Mechanical energy (wind or water)
b. Thermal energy (steam)
2. The rotating turbine spins an
electric generator which produces
electricity.
CHECK-IN:
 A roller coaster is at the top of a hill and
rolls to the top of a lower hill. If
mechanical energy is conserved, on the
top of which hill is the kinetic energy of
the roller coaster larger?