Transcript Work, Energy and Power

Work, Energy
and Power
Work and Energy
 Work
is defined as the transforming or
converting from one form of energy into
another form of energy.
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Every time you flip on a light switch, work is
being done. It is changing electrical
energy in to heat (light).
 Energy
is the ability to do work or effect
change.
Different Forms of Energy
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Kinetic energy (motion)
Elastic energy (springs)
Electrical energy (batteries)
Thermal energy (fire)
Radiation energy (light)
Gravitational energy
Chemical energy (food)
Wind energy
Sound energy (sound waves)
Hydraulic energy (waterfalls)
Nuclear energy (atomic nuclei, the sun)
Potential Energy vs Kinetic
Energy
 Potential
Energy – is energy that can be
stored in an object.

A battery has potential Energy as it has a lot
of energy stored in it (electrical) that is just
waiting to be used.
 Kinetic
Energy - is the energy that an
object has because of its motion.
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When I move I release energy that I had
stored up in my body.
Potential vs Kinetic Energy
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Ex. When you throw a ball up in the air, you
give it kinetic energy (it moves).
This energy is transformed into gravitational
potential energy as the ball rises.
At the balls highest point, all the kinetic
energy has been transformed into potential
energy.
As the ball comes back down, gravitational
potential energy is transformed into motion
which is kinetic energy.
Potential vs Kinetic Energy
 Kinetic
energy has a relationship between
mass and velocity. The greater the mass
or the speed, the greater the kinetic
energy.
 KE = 1mv2
2
 Where
KE is Kinetic Energy measured in
joules, m is mass measured in kg and v is
velocity is measured in m/s.
Work and Energy
W
= KEfinal – KEinitial
W = ∆KE
 Where W is work and it is measured in
joules and KE is kinetic energy measured
in joules.
Law of Conservation of Energy
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Work can then be deduced as a change of
energy from one type to another.
Thus W = ∆E
This is known as the Law of Conservation of
Energy - Energy can be neither created nor
destroyed; it can only be transferred or
transformed.
So the amount of energy needed to turn on a
light bulb derived from the potential electrical
energy passing through it.
Power and Watts
 Power
is the rate at which energy is
transformed or, the rate at which work is
done. It is measured in Watts.
 P = ∆E
∆t
Where P is power and is measured in Watts,
E is energy measured in Joules and t is time
in seconds.
1watt = 1J/s
Power and Watts
 Since
we know that W = ∆E we can
substitute W for ∆E
P=W
∆t
Where P is power in watts, W is work done in
Joules, and t is the time interval in seconds.
Kilowatt Hours
 Energy
is commonly measured in Joules
(J), but can also be measured in kilowatt
hours (KWh).
1 kWh = 3 600 000 J
 KWh
is used by B.C. Hydro to calculate
your energy bill.
Example #1
A
stereo has a power rating of 200W & is
used for 1800 seconds. How much energy
is used?
E = P•t
E = (200)•(1800)
E = 360,000 J
360,000 Joules of energy were
used to work the stereo
Example #2
A
video game has a power rating of
175W & is used for 3600 seconds. How
much energy is used?
E = P•t
E = (175)•(3600)
E = 630,000 J
630,000 Joules of energy were
used to work the video game
Example #3
A
hair dryer has a power rating of 425W &
is used for 10 minutes. How much energy
is used?
E = P•t
E = (425)•(600)
E = 255,000 J
255,000 Joules of energy were
used to work the hair dryer
Example #4
A
stereo plays for 2 hours & 30 min (9000
seconds) & consumes 1, 800, 000 J of
energy. What is the amount of wattage
used?
P = ∆E
∆t
P =1,800,000 J
9000s
P= 200 J/s or 200 Watts
Example #5
A
kettle is used for 5 min & has a power
rating of 24 Watts. How much work is
being done by the kettle?
P=W
∆t
W = P∆t
W = (24 watts)(300s)
W = 7,200 J
Example #6
A
100 W light bulb is left on for 3 hours.
How much energy did it use? Express your
answer in scientific notation.
3 h (3600 s) = 10,800 s
1h
P = ∆E
∆E = P(∆t)
∆t
∆E = 100 W(10,800 s)
∆E = 1,080,000
∆E = 1.1 x 106
Challenging Question
A
circuit has a resistance of 5Ω. If the
battery powering the circuit has a voltage
of 10V, how much power is used to run
this circuit?
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R=V/I
I= q/t
V=E/q
P=E/t
V=P/I
R = 5Ω
V = 10 V
P= ?
I=?
Step 1
R=V/I
5 = 10 / I
5 I = 10
I = 10/5
I=2A
Step 2
V=P/I
10 = P/2
10 * 2 = P
P = 20 watts