Download 8.4 Energy changes

8.4 Energy changes
Both bullet and egg change energy
Energy and Work – when the force move
something, work is done!
Examples for work
Work?
Calculation of work
If we apply a force to move a mass a certain distance, we do work, we transfer
energy. Both work and energy are measured in joules (J). Work and force
are linked by a simple equation:
Work (J) = force used (N) × distance moved in the direction of the force (m)
Written as an equation:
W=Fxd
If there is a force but nothing is
moving, is work done?
If there is a force but nothing is
moving, is work done?
How fast energy is used : POWER
If you're interested in developing a
micro-hydro system, you'll first need
to determine the water rights and laws
surrounding the water source. From
there you can determine the flow rate
of the river or stream and the "head",
or vertical drop from the intake to the
turbine. These last two factors will
determine the power output of your
system.
Kinetic Energy – of moving object
Units – E [joules], mass [ kg], speed [m/s]
Speed and injury risk
Speed and injury risk
http://w3.unisa.edu.au/researcher/issue/2005may/footy.asp
Potential energy is energy which results
from position or configuration.
Stored energy
Types of potential energy
Elastic potential energy
A force acting on an object may cause the shape of an
object to change. Elastic objects can store elastic potential
energy if they are stretched.
Elastic objects can also
store elastic potential
energy when they are
squashed
Potential Gravitational Energy
EP = mgh
where
· EP = Energy (in Joules)
· m = mass (in kilograms)
· g = gravitational acceleration of the earth (9.8 m/sec2)
· h = height above earth's surface (in metres)
Conservation of energy
Energy flow diagrams
Energy flow diagrams show what is
happening to a particular type of energy as
it is used or changed in some process or
situation.
Energy flow diagrams can be extended to
include other forms of energy, such as light
energy, heat energy and sound energy.
The potential energy of a cherry on a tree
can be changed into kinetic energy when
the cherry falls. When it hits the ground, this
energy is changed into sound energy and
some heat energy as it crunches up.
1. Describe the changes in energy that occur when a basketball is thrown
up to get a goal.
2. Sketch an energy flow diagram for a device that changes kinetic
energy into electrical energy.
3. Sketch an energy flow diagram for a rocket taking off. You should
mention its chemical energy, heat energy, light energy, sound energy,
kinetic energy and potential energy.
4. Why does an aeroplane speed up as it is descending even though the
pilot cuts back on the engine power?
5. Where does the kinetic energy of the car go when the brakes are
applied and the car stops? (Hint: Feel the brakes on the wheels.)
Law of conservation of Energy: "Energy can neither be created nor it is destroyed,
however energy can be converted from one form energy to any other form of
energy"
Kinetic +
potential = Total
Energy
ENERGY CONVERSION EFFICIENCY
The purpose of a device determines its useful energy output. For example,
we want light from a lamp, but we get mostly heat; only 5% of the energy
input (electricity) is converted into light, so the efficiency of a conventional
incandescent light bulb is 5%.
The “service” that needs to be
provided in
this example is keeping coffee warm.
→ This task can be achieved either
by
applying energy (hotplate on the right)
→ or by avoiding the loss of heat in
the
first place (thermos flask on the left).
While there is still minor heat loss,
it is so small that there is no need for
the hotplate.
The illustration shows the basic principle of improved energy efficiency
using an example.
It becomes clear why energy efficiency not only “saves some energy”,
but can also replace
the provision of energy to a great extent.
Wireless charging is making some serious headway in public
mindshare. Once confined to toothbrushes and other simple
household items, it's now a handy means of powering phones (HP's
Touchstone, for example) and media players, and it'll soon be ready
to charge up our notebooks and cameras at our command. Are we
sure this is the future, or will this just be a passing fad? Why should
it matter? Read on after the break to get the air-conditioned answers
to those burning questions