Transcript Measuring Energy Input and Output

Measuring Energy Input
and Output
Power
Is the rate at which a device converts energy
Unit of power is Watt (W)
=which is equal to one joule per second
The faster the device converts energy, the greater its power
rating
P=IxV
I=P
V
V=P
I
P = Power (W)
I = Current (A)
V = Volts (V)
Example: A hair dryer has a power rating of 1000 W. It
is plugged into a 120-V outlet. What is the current
flowing through the hair dryer?
Steps to Solving the Problem
Solution
Information and
1. Identify known quantities
Power (P) = 1000 W
Voltage (V) = 120 V
2. Identify the unknown
quantity
Current (I) = x
I=P / V
3. Use the correct formula
4. Solve the problem
I = 1000 W / 120 V
I = 8.33 V
Energy
Is the amount of energy a device uses over a period
of time.
Measured in joules (watts per second)
E=Pxt
P=E
T
t=E
P
E = Energy (J)
P = Power (W)
t = time (s)
Example: A microwave has a power rating of 800 W. If you
cook a roast in this oven for 30 min at high, how many
joules of electrical energy are converted into heat by the
microwave?
Steps to solving the Problem Information and Solution
1. Identify known quantities
Power (P) = 800 W
time (t) = 30 min
2. Identify the unknown
Energy (E) = x
quantity
3. Use the correct formula
4. Solve the problem
E=Pxt
E = 800 W x 30 min
E = 800 W x 1800 s
E = 1 440 000 J = 1.4 MJ
Kilowatt hours
 It does not take an electrical device
long to consume a large number of
joules, so a kilowatt hour is often used
 The energy calculation is the same,
except that hours are substituted for
seconds, and kilowatts (kW) are
substituted for watts
800 W used in 30 minutes
E = 0.8 kW x 0.5 h
= 0.4 kW h
Law of Conservation of Energy
Energy cannot be created or destroyed,
Energy does not just appear or disappear,
only transformed from one form to
another
•
However we usually find that the output
energy of a device is smaller than the
input energy
The missing energy is lost or dissipated
as heat.
Do you Remember the Conservation of Energy Song???
If no one remembers we will listen to it… Someone has to start singing it…
Efficiency
 The ratio of useful energy that a
device produces verses the amount
of energy that went into the
electrical device
 The more input energy that a
device converts into useable
output energy, the more efficient
the device is.
Percent efficiency = joules of useful output x 100
joules of input energy