Lesson video

Hello, I'm Dr. de Mello and I'll be guiding you through today's lesson.

Today's lesson is about mains electricity and it comes from the resistance and parallel circuits unit.

Our outcome today is I can compare the power of different appliance and work out how much they cost to use.

Let's look at today's keywords.

The first keyword is power, and this is the amount of energy that is transferred every second.

Power is measured in watts, that's the next keyword, and so the unit of power is watts.

We also have kilowatts, which is a larger unit of power.

One kilowatt equals 1,000 watts.

Then we have joules, and this is the unit of energy.

And finally, for household usage, kilowatt-hours is the unit we use.

One kilowatt-hour is one unit of energy used.

It's one kilowatt running for one hour.

If you'd like, pause the video now, have a look at these keywords and then look out for them in the lesson.

This lesson on mains electricity has two parts.

The first part is what is electrical power?

The second part is energy use and cost.

Let's start with what is electrical power.

In our homes, we have different electrical appliances and devices that help us to do work.

We have things like vacuum cleaners, drills, kettles, toasters, and washing machines.

Each one uses different amounts of energy every second, depending on the job they do.

The amount of energy used each second is equal to the power of the device.

The power rating of each appliance or device is usually found on a sticker on the back or the underside of it.

This is the sticker for a desk lamp, and you can see that it has a power of 25 watts.

This loudspeaker has a power rating of 50 watts.

It won't always be working at 50 watts.

Sometimes the sound will be softer and it'll use a lower power, but it can go up to 50 watts, and here is the sticker for a toaster.

It has a power rating between 860 and 1,050 watts depending on the setting that it's at.

Be sure to look out for the power ratings of the different appliances and devices you use.

Remember, W is the symbol used for the unit of power.

It stands for watts.

Let's do a check for understanding.

The power rating of three devices are shown below.

Which of the devices has the greatest power rating?

Pause the video now, make a choice and then come back to see how you've done.

Welcome back, if you chose device A, that's correct.

It has a power rating of between 2,520 and 3000 watts.

B has a power rating that goes up to 24.

0 watts, and C seems to have a power rating of 1,300 watts, but it also does say 350 watts suction power, so answer A had the highest power rating.

Well done if you got that right.

Power remember is measured in watts.

You may also see power values given in milliwatts, kilowatts, megawatts, and gigawatts.

Milliwatts are thousandths of a watt.

Kilowatts are thousands of watts.

Megawatts are millions of watts, and gigawatts are billions of watts.

Let's look at the equivalent values of power with these different prefixes.

1000 milliwatts equals one watt, so 1000 watts would equal one kilowatt.

1000 kilowatts equals one megawatt and 1000 megawatts equals one gigawatts.

The number changes by a factor of 1000 each time.

Here's a check for understanding.

The power of a kettle is shown below.

How many kilowatts does the kettle use when operating at maximum power rating?

Is it answer A, three kilowatts, B, 60 kilowatts C, 240 kilowatts or D, 3000 kilowatts?

Pause the video now, make your choice and then come back to see how you've done.

Welcome back, if you chose answer A, three kilowatts, that's correct.

3000 watts equals three kilowatts.

Well done if you got that right.

A laser pointer is rated at less than one milliwatt or 1000th of a watt, and you can see this here on the label.

One milliwatt equals 0.

001 watts.

Even though this is a very low power, lasers are dangerous and you should be very careful using them.

Be careful not to point them into your eyes.

A typical mobile phone is rated at up to 10 watts depending on what the phone is being used for.

Normally this is a lot less 'cause you won't be using all the apps at once.

A kettle or toaster is rated at up to 3000 watts or three kilowatts.

This is more than the power a professional cyclists can generate, which is less than two kilowatts.

Normal cycling would be a couple of 100 watts.

A single wind turbine can generate a few megawatts.

The bigger the wind turbine, the more it can generate.

A fossil fuel power station can generate a couple of gigawatts.

Remember, Mega stands for a million and Giga stands for a billion.

Here's a check for understanding how many one megawatt wind turbines will be required to replace a one gigawatt coal fired power station?

Would it be A, 10, B, 100, C 1000, or D, 10,000?

Pause the video now, make a choice and then come back to check how you've done.

Welcome back, if you chose 1000, that's correct, you'd need 1000, one megawatt wind turbines to replace one gigawatt of coal fired power.

Let's look at how to convert between watts and kilowatts.

1000 watts equals one kilowatt, so 100 watts equals 0.

1 kilowatts.

10 watts would equal 0.

01 kilowatts and so one watt equals 0.

001 kilowatts.

To convert watts into kilowatts, you divide by 1,000.

Let's look at how to go in the reverse direction.

To convert kilowatts into watts, you multiply by 1000.

We can now convert between the different prefixes.

To go from watts to kilowatts, you divide by 1000 and then to go from kilowatts to megawatts, you also divide by 1000 and from megawatts to gigawatts, you also divide by 1000.

To go from gigawatts to megawatts, you multiply by 1000 Likewise, megawatts to kilowatts is multiplying by 1000 and kilowatts to watts is multiplying by 1000.

When you're converting, for example, watts to gigawatts, you can either go in steps or do it all in one go, it's up to you.

Let's do a check for understanding.

Which of the following powers is equivalent to 200,000 watts?

That's about the maximum power an electric car can generate.

Is it answer A, 200 kilowatts, B, 2000 kilowatts, C, two megawatts or D, 0.

2 megawatts?

Pause the video now, make your choice and then come back to see how you've done.

Welcome back, if you chose A, 200 kilowatts, that's correct, 200,000 watts divided by 1,000 equals 200 kilowatts.

If you also chose D, 0.

2 megawatts, that's also correct.

200 kilowatts divided by 1,000 equals 0.

2 megawatts.

Well done if you got that right.

When we light our homes more efficient light bulbs with a lower power erecting will give the same brightness.

An old incandescent bulb would be rated typically at something like 60 watts.

A fluorescent bulb of the same brightness would typically be rated at something like 20 watts, so it's using a lot less power about a third of the power that an incandescent light bulb would use.

An LED bulb of the same brightness would typically be three or four, maybe five watts.

All these bulbs would give the same brightness.

They just use different amounts of power.

If you've ever used all three of these bulbs, you'll notice that the incandescent bulb gets really hot.

A lot of the electrical power is being transferred into heat.

The fluorescent bulb doesn't get quite as hot, and the LED bulb is the coolest of all three.

It's the most efficient.

Here's a check for understanding.

Which of the following sets of bulbs would require more power?

A fluorescent 20 watt bulb or 4 x 4 watt LED bulbs.

Pause the video now, make a choice and then come back to see how you've done.

Welcome back, if you chose the fluorescent bulb, that's correct, four times four equals 16 watts and 20 is greater than 16, so the 20 watt fluorescent bulb would use more power.

We've come to the end of the section.

Now is the chance for you to practice what you've learned.

Number one, convert the following powers into kilowatts.

We have A, 240,000 watts, B, 0.

030 megawatts, C, 0.

000055 gigawatts and D, 150,000 milliwatts.

Number two, in a home, the following devices are turned on, four LED bulbs each rated at five watts, an oven rated at 1,050 watts, a fridge rated at 400 watts and a wifi router rated at 0.

02 watts.

What's the total power required by all of these devices if they're all turned on together?

Pause the video now, write out your answers and then come back to check how you've done.

Welcome back.

Let's look at the possible answers you could have got.

For number one, A, you divide 240,000 by 1000, so you get 240 kilowatts.

Answer B, 0.

030 megawatts multiplied by 1,000 gives 30 kilowatts.

Answer C, 0.

000055 gigawatts multiplied by 1,000, gives 0.

055 megawatts, and if you multiply by 1,000 again to convert to kilowatts, you get 55 kilowatts.

You could have multiplied by a million in one go and got the same answer.

Answer D, 150,000 milliwatts divided by 1,000 gives you 150 watts.

Divide by 1,000 again, and you get 0.

15 kilowatts.

Number two, the total power would be worked out in the following way.

First of all, for the four bulbs each of which is five watts, you'd multiply by four, so five times four is 20 watts for all four bulbs.

You then add 1,050 watts of the oven and then 400 watts of the fridge and finally last, a smaller amount of power, 0.

02 watts for the wifi.

All of these added together would give you 1470.

02 watts.

Notice how the oven uses the most power.

Things that get hot often use the highest levels of power.

Well done if you got that right.

We're now going to move on to the second part of this lesson, energy use and cost.

Power is the amount of energy in joules transferred each second.

This four watt LED bulb show will use four joules every second when it's lit.

If it is lit for one minute, 60 seconds, it'll use four times 60 or 240 joules.

In one hour, 60 times 60 seconds, it'll use four times 60 times 60, which is 144,000 joules.

This is a large number.

Here's a check for understanding, how many joules of energy would a 60 watt incandescent bulb use if it's left on for 30 minutes?

Pause the video now, work out the answer, make a choice, and then come back to see how you've done.

Welcome back, if you chose answer C, that's correct.

The time in seconds is 30 minutes times 60 seconds, so 60 watts times 30 times 60 equals 108,000 joules.

Well done if you got that right.

For domestic energy bills, we are charged for the energy we use in units called kilowatt hours.

When we measure power in kilowatts and the time in hours, the energy used is found using the equation, energy used equals power times time.

The energy used has units of kilowatt-hours.

The power has units of kilowatts and the time is in units of hours.

Energy used is measured in kilowatt-hours.

Which of the following is the correct unit of domestic energy use?

Is it A, megawatt minutes, B, kilowatt hours, C, gigajoule hours, or D, kilojoules?

Pause the video now, make a choice and then come back to see how you've done.

Welcome back, the correct unit for domestic energy use is kilowatt hours.

There's other possible units, but this is the one that we use on our meters and our electricity bills.

One kilowatt hour equals one kilowatt times one hour.

This is 1,000 watts times 60 minutes, which is 1,000 watts times 60 times 60 seconds.

This is 3,600,000 joules.

Measuring energy in kilowatt-hours is more appropriate for energy bills as the equivalent value in joules is much greater and makes it more difficult to work out the cost of using it.

Joules can still be used for smaller amounts of energy.

An electricity meter shows how much energy has been used in units of kilowatt hours.

You can see these two meters, an old traditional meter and a smart meter displaying kilowatt hours as the unit.

Smart meters can also show the cost, the amount of energy used in a given time and allow you to analyze your energy use.

Electricity supply companies charge people according to how many units they use.

Here's a typical electricity bill.

Let's try and understand the different numbers.

It can look a bit confusing, but we'll take it step by step.

The first thing to look out for is the units used, so the units used are calculated by looking at the latest reading, 82,669 minus the previous reading, which is 82,516 in this case.

Subtract the two and you get 153 units were used between the two readings.

This is shown on the bill.

Beside the units used figure, there's the kilowatt hour rate.

This is the rate of the electricity company charges for each kilowatt-hour, and in this case it's 24 pence.

Calculating the charge, we have 153 units used multiplied by 0.

24 pounds.

This comes out as 36 pounds and 72 pence.

In addition to this charge, there's a standing charge.

The standing charge is a fixed cost for the electricity company to provide you electricity.

Adding these two together gives you the total charge for this period, so 36.

72 plus 15.

75 gives you a total of 52 pounds and 47 pence.

Do have a look at your electricity bill.

It's useful to understand how you're being charged and how much you have to pay.

Let's do a check for understanding.

How many units have been used between the two meter readings shown?

The initial reading is 82,376.

The final reading is 82,489.

Pause the video now, choose your answer and then come back to see how you've done.

Welcome back, if you chose D, 113 units, that's correct, 82,489 minus 82,376 equals 130 units.

Well done if you calculated that correctly.

Let's look at some typical energy usage.

So if a 2000 watt kettle is used for 15 minutes or a quarter of an hour, the energy equals power times time, so the power is 2000 watts and the time in hours is one quarter of an hour or 0.

25 hours.

This equals 500 watt hours.

We convert this to kilowatts by dividing by 1,000, and that is 0.

5 kilowatt-hours.

If one kilowatt-hour costs 20 pence, the cost of using the cattle is 0.

5 times 0.

20 pounds, which equals 0.

10 pounds or 10 pence.

Let's do a check for understanding.

Work out the cost of one kilowatt hour using the information shown on the smart meter.

Is it A, 23.

5 pence, B, 4.

25 pence, C, 12.

19 pence or D, 7.

55 pence?

Pause the video, make a choice and then come back to see how you've done.

Welcome back, if you chose A, 23.

5 pence, that's correct.

2.

32, which is the cost shown divided by the number of kilowatt-hours, 9.

87 equals 0.

235 pounds, which equals 23.

5 pence.

Well done if you got that right.

Smart meters can also be used to see how much energy we use so we can be more energy efficient.

The meter shows the power and the cost now while it's being used so you can monitor different devices.

Controlling how we use appliances and devices that use a lot of energy will help to reduce bills and improve our energy use.

Using less electrical energy saves money and reduces carbon emissions.

This could be done by things such as using more energy efficient devices such as LED light bulbs rather than incandescent ones, switching off electrical devices when they're not needed, keeping track of energy use with a smart meter and making changes to reduce it.

There's many other ways you can reduce energy use and they're a good idea.

Let's do a check for understanding.

How much energy is used by a kettle boiling water for seven seconds, if its power rating is 2000 watts?

Kettles boil water for a few seconds before automatically turning off.

Pause the video now, make a choice and then come back to see how you've done.

Welcome back, if you chose answer D, 14,000 joules, that's correct.

For small time periods, it's better to use joules rather than kilowatt hours.

We use the equation, energy equals power times time.

The power is 2000 and the time is seven.

2000 times seven gives you 14,000 joules.

Well done if you got that right.

We've come to the end of the section.

Here's a practice task to see how you're doing.

Sam and Jacob are designing a poster to help people use less energy and reduce their environmental impact.

Sam says, "I would tell people to switch off a kettle just as it begins to boil rather than leaving it to automatically switch off on its own.

" Remember kettles keep going for a bit before they switch off.

Jacob says, "I would tell people to switch off flights when they're leaving a room empty.

" List some other ways of saving energy for the poster and then compare Sam and Jacob's ideas by calculating how much energy each of their methods would save.

Pause the video now, go ahead and carry out the tasks and then come back to see how you've done.

Welcome back, here's some suggested answers to saving energy.

You could upgrade to more energy efficient devices, but avoid buying them without checking if using the less efficient device until it stops working, costs less than the cost of buying and using the new device for the same time.

Switch off devices like TVs that are often left on standby overnight.

Use a lower temperature wash cycle on washing machines.

Avoid heating homes and water more than is necessary and use a smart meter to identify devices that use a lot of energy.

There are other ways of saving energy.

These are just a few ideas.

You may have written other things as well.

Comparing Sam and Jacob's ideas, you may have done similar calculations to these.

Looking at Sam's idea, switching a kettle off as it starts to boil for seven seconds can save up to 2000 times seven joules, which is 14,000 joules in one go.

With Jacob's idea, not leaving a five-watt LED bulb on for one hour can save five times 60 times 60 joules, which is 18,000 joules.

If you have an incandescent bulb, which is 40 watts worth, not leaving it on for an hour can save 40 times 60 times 60 joules, which is 144,000 joules.

Obviously, changing to LED bulbs is a big saving on energy and that's something that's recommended to be done.

Well done if you've got similar answers.

We've reached the end of the lesson.

Let's summarize what we've learned.

Electrical appliances and devices use different amounts of energy each second as indicated by their power rating.

Power is a measure of the amount of energy used by or supplied to a device in one second, and it's measured in watts.

The energy used by appliances and devices in our homes is measured in kilowatt-hours and one kilowatt-hour equals one unit of energy.

The equation for energy use is energy used equals power times time.

Power is measured in kilowatts and time is measured in hours.

Remember, one milliwatt equals 0.

001 watts.

One kilowatt is 1,000 watts, one megawatt is a million watts and one gigawatt is a billion watts.

Well done on completing this lesson.

I hope to see you again soon.