19/04/23Electrical Power

Lesson Objectives:

• To know what the ‘power’ of a device means.

• To be able to calculate the power of a device.

Electrical PowerPower 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.

19/04/23

Remember, KILO just means 1000. If we say a KILOWATT (kW) it means 1000W.

3.6kW is equal to

0.2kW is equal to

3600W

200W

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.

So if you were trying to find power (P)…..

…you would cover P up…

…and you are left with the sum…

P = E

t

Formula Triangles

E

tP

Calculating Power

E = Energy (in joules)

P = Power (in watts)

t = Time (in seconds)

E

tP

The formula we use to calculate power is

Power (W) = energy transferred (J)

time taken (s)

Calculating PowerWorked 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 PowerUsing 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 EquationSometimes 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

E

tP

Worked Example:

A lightbulb has a power rating of 100W. How much energy, in kJ is transferred over 45 minutes?

From rearranging the equation, we know that E = P x t

E = 100 x 2700 = 270000J

= 270kJ

Calculating PowerUsing 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 35,000W. In 90 seconds, how much power does it transfer?

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 RatingsHere 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 110W 1kW

900W 1200W

QuestionsRemember, 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.

19/04/23

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.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 = voltage X current.Power = 230 X 10.Power = 2,300 watts

Try this for yourself1. 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

How many 100 Watt light bulbs would it take How many 100 Watt light bulbs would it take to use 4,600 watts of power? to use 4,600 watts of power?

460 light bulbs