```Electrical Power
09/10/2015
Lesson Objectives:
• To know what the ‘power’ of a device means.
• To be able to calculate the power of a device.
Electrical Power
09/10/2015
Power is defined as ‘the amount of energy transferred each second.
The unit of power is known as the ‘watt’.
The watt is the same as one joule per second
1 watt = 1 joule per second.
Remember, KILO just means 1000. If we say a KILOWATT (kW) it means 1000W.
3.6kW is equal to 3600W
0.2kW is equal to 200W
Formula Triangles
Formula triangles help you to rearrange formula, the
triangle for Power is shown below:
Whatever quantity you are trying to find cover it up
and it will leave you with the calculation required.
…and you are left
with the sum…
So if you were
trying to find
power (P)…..
…you would
cover P
up…
P = E
t
E
P
t
Calculating Power
The formula we use to calculate power is
Power (W) = energy transferred (J)
time taken (s)
E = Energy (in joules)
E
P = Power (in watts)
t = Time (in seconds)
P
t
Calculating Power
Worked Example:
An electric motor transfers 20,000J of energy in 30 seconds. What is its power?
Step 1.
Write the equation
Power (W) = energy transferred (J)
time taken (s)
Step 2.
Calculate the solution
Power = 20,000
30
= 666.67W
Calculating Power
Using the formula for calculating power, find the power of the appliances below. Show
your working.
1. If a light bulb transfers 1,250J of energy in 25 seconds what is the power of the light bulb?
2. An electric motor transfers 15,000J of energy in 20 seconds. What is its power?
3. What is the power of an crane motor that transfers 250,000J of energy in 5s?
4. An escalator transfers 32,000J of energy over 10 seconds. What is the power of the escalator
in kW?
5. An electric motor transforms 36kJ of electrical energy into kinetic energy in 3 minutes. What
is the power output of the motor?
Re-arranging the Equation
Sometimes we already know the power of a device but want to know how much
energy is transformed over a period of time. To do this, we rearrange the equation.
So, using our formula triangle, if we want to find the energy, following
the cover-up rule, we see that Energy = Power x Time
Worked Example:
A lightbulb has a power rating of 100W. How
much energy, in kJ is transferred over 45
minutes?
E
From rearranging the equation, we know that
E=Pxt
P
t
E = 100 x 2700 = 270000J
= 270kJ
Calculating Power
Using the formula triangle answer the following questions on power. Show your
working.
1.A crane has a power rating of 75kW. How much energy is transferred in half an hour? Give
your answer in kJ.
2.A weight lifter has a power rating of 600W. In a 25 minute weights session, how much energy,
in joules is transferred?
3.A lift motor has a power rating of
transfer?
35,000W. In 90 seconds, how much power does it
4.A train motor has a power rating of 1,000,000. How many seconds will it take to transfer
62,000kJ?
5.If a microwave has a power rating of 800W, how many seconds does it take to transfer
32,000J?
Power Ratings
Here are some typical values of power ratings for different energy transfer appliances
& ‘devices’
Device
Power Rating
Torch
1W
Electric light bulb
100W
Electric cooker
10,000 W (10kW)
Electric train
1,000,000 W (1MW)
Large power station
10,000 MW
World demand for power
10,000,000 MW
The Sun
100,000,000,000,000,000,000 MW (
A zettawatt!)
How many 100W light bulbs would use the same amount of power as a 10kW electric
cooker? What about the sun?
‘Watt’ is the power rating?
100W 0.65kW 15W
900W
110W
1200W
1kW
09/10/2015
Questions
Remember, you will need to rearrange the equation for some of these.......
1)
What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds?
2)
What is the power of an electric fire that transfers 10,000J of energy in 5 seconds?
3)
Toby runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what
is his power rating?
4)
How much energy does a 150W light bulb transfer in a) one second, b) one minute?
5)
Little Johnny’s brain needs energy supplied to it at a rate of 40W. How much energy does it
need during a physics lesson?
6)
Ralph’s brain, being more intelligent, only needs energy at a rate of about 20W. How much
energy would his brain use in a normal day?
7)
Investigate the term ‘horsepower’. Give a brief description of the term and include how
many watts are in one horse power.
CALCULATING ELECTRICAL
POWER
The unit of power is the Watt.
Power = voltage X current.
(Watts) =
(volts)
X
(amps) or A
.
What does this mean ?
Question
The following information is stamped on a
kettle.
Electrical Supply 230v Fit with a 13 A fuse.
When the kettle is working correctly a current
of 10A flows through it What is the power
used?
Power = 2,300
230 Xwatts
voltage
10.
X current.
Try this for yourself
1. An electric shower runs at 230v and has a
current of 25A. What is the power of the
shower?
Power = voltage X current.
Power = 230 X 20.
Power = 4,600 watts
460 light
bulbs
How many 100 Watt light bulbs would it take
to use 4,600 watts of power?
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