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Learning Outcomes

• Distinguish between Power and Energy. And use the equation Energy = Power x Time

• Use the equations: • Units Used (kWh) = Power (kW) x Time (h)• Cost (p) = Units used x Cost per unit (p)

• Use data to make comparisons of different sources of domestic energy.

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Key Words:

• Power: The rate of flow of energy

• Energy: The ability to do work

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Water heater25%

Fridge/ freezer 15%

Bulbs25%

Oven5%

Tumble dryer3.5%

Television 2.5%

Dish washer1.5%

Washing machine

<1%

Computer <1%

Others2%Drag the appropriate labels

to the correct boxes in order to complete the graph showing the percentage use of electricity in the home.

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Energy Power

Definition Energy is the ability to change your

environment. In order to heat something up,

make something move, glow, or stretch etc. you

need energy!

Power is the rate of flow of energy. To make something move faster, glow

brighter etc. Requires more

power. Power is the energy per second

Unit Joule Joule/Second(Watt)

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Power

Now rearrange this equation using

an equation triangle!

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The power rating of a bulb is 60W.This means that 60 Joules of electrical energy is used every second to produce light and heat energy.

The power rating of a kettle is 2kW.This means that 2000 Joules of electrical energy is used every second to produce heat energy and to boil the water.

Remember that there are1000W for every 1kW. So,

2kW = 2000W = 2000J/s0.1kW = 100W = 100J/s3.7kW = 3700W = 3700J/s0.02kW = 20W = 20J/s

The unit of power is the Watt (W), and

1W is equivalent to 1J of energy per

second.

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You will need to be able to use:

Power (W) = voltage (v) current (A)

Electrical power is the measure of how fast electrical energy can be transformed into

other energies.

Power (W) = current (A) voltage (V)

= 4.2 240

= 1008W

( about 1kW )

Power(W) = current (A) voltage (V)

= 0.4 240

= 96W

The current flowing through a toaster is 4.2A,The mains voltage is 240V. What is the power of the toaster?

If the mains voltage is 240V, what is the power of a sound system, if there is 0.4A of current flowing through it?

Answer Answer

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This small power station is producing 250kW of electricity.

This step-up transformer is increasing the voltage to 25,000V.What will the current be through the cables?

Step-up-transformer

Answer

Power (W) = current (A) voltage (V)

250,000 = current 25,000

250,000 = current = 10A

25,000

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I pay my electricity bill every month by

direct debit

I pay my electricity bill every quarter

How much energy do we use?

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100W Power Power is the rate of doing work, or the rate of energy transfer and is measured in Watts.

There are 1000W in every 1kW

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Highest value

Lowest value

100W

0.4kW

30W

20kW

650W

1kW

0.05kW

Click here for the answer

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Highest value

Lowest value

100W

0.4kW

30W

20kW

650W

1kW

0.05kW

20,000W

1,000W

650W

400W

100W

50W

30W

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energy transfer = power time

There are 1000W in every 1kW

So, a 100W light bulb will transfer 100J of electrical energy into a total of 100J of light and heat energy per second.

In fact, 1W = 1J/s

And a 2kW kettle will transfer 2,000J of electrical energy into a total of 2,000J of heat energy per second.

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Units used (kWh) = power (kW) time (h)

kWh

391M 4158478

This was the type of electric meter that would have measured the amount of electricity used in your home.

It measured the electrical energy used in units of 1kWh (equivalent to a 1000W device running for 1 hour). You are then billed on the number of units used in total for a given period.

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Units of Energy used (kWh) = power (kW) time (h)

These days, digital electric meters are the norm.

But although it looks different, the amount of electricity used is still measured in kWh.

0567

kWh

Cost(£/p) = units used(kWh) cost per unit(£/p)

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Below are the meter readings taken at particular times during the year for a 3 bedroom house.

0567

kWh

2207kWh

3519

kWh

September 9th

(2011)January 9th

(2012)May 9th (2012)

1. Calculate the number of units (KWh) used during each period.

4517

kWh

September 9th

(2012)

2. Calculate the total number of units (KWh) used during the year.

3. During what period was the most electricity used, and can you explain why?

4. If one unit (1kWh) costs 8.3p, what is the average cost per day for this house?

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If a single 100W light bulb was switched on for a total of 4 hours, and the cost of a unit of electricity (1kWh) is 8p then…..

Units used (kWh) = power (kW) time (h)

= 0.1 4

= 0.4

cost = units used cost per unit

= 0.4 8

= 3.2p

This doesn’t seem much, but this is for only one 100W light bulb that is on for 4 hours – how many light bulbs do you have switched on in your home during the day?

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Different ShoesHow would today’s lesson be different if it was taught by:

A third world (LEDC) farmer

A climate change scientist A money saving expert