Video: Domestic electricity review

﻿Hello everyone. And welcome to today's Science lesson with me, Miss Walrond. Today, we're going to be continuing with our physics topic, electricity and we're going to be completing a review of domestic electricity. Please make sure that your phone is now turned to silent and that you've removed any distractions from around you. Where possible safe you can find yourself a nice, quiet place to work. And when you're ready, let's get started In today's lesson we're going to be doing a recap of everything that we've studied so far in the domestic electricity topic. We're going to start off with a quick reminder of what we've covered in this topic. Then we're going to move on to do a recap and some revision of electricity in the home, power and energy transfers, and then the National Grid. Lastly we'll be having a look at a worked example of an exam question, which is a bit more Synoptic and ties in lots of different areas of this topic. For today's lesson you're going to need a pen and some paper, and also a calculator. If you don't have any of these items, please pause the lesson now, while you go and collect them. Okay, let's start off then by thinking about what we have learned so far in this topic. We started off the domestic electricity topic by looking at electricity in the home. In this topic we learned about alternating and direct current, the three pin plug, fuses and double insulation. We then moved on and had to look at power. Here we learned two equations for power, power equals current multiplied by potential difference and power equals current squared multiplied by resistance. We then continued to have a look at energy transfer, and we described some of these energy transfers, but also looked at ways to calculate the energy that's an appliance is transferring. We did this in two ways. We use energy transfer is equal to power multiplied by time and energy transfer is equal to charge flow multiplied by potential difference. On that section was then the National Grid. Here we looked at the structures of the National Grid, the purpose of transformers and why the National Grid is such an efficient system. Lastly, we looked at the relative advantages and disadvantages of overhead and underground cabling. So now let's do a bit of a review about each of these sections. We'll start off with electricity in the home. You remember that we learned about direct and alternating current. Direct current is where the current travels in one direction only, the electrons move around the circuit in just one direction. Examples of sources, which create direct current are cells and batteries. And when we look at a potential difference versus time graph like this one here, we normally see that we have a nice day straight line. This graph is representing a three volt cell, and we can see this as we have a horizontal line telling us we've got the direct potential difference. The second type of current that we looked at was alternating current. An alternating current is where the direction of the current continually changes, keep swapping over. We see that the electrons will move both forwards and backwards in a socket. A source of alternating current comes from our mains electricity supply, and this is produced from power stations. If we were to plot a graph of potential difference versus time for an alternating supply, we'd see something like this. Here we can see that the potential difference or voltage has both positive and negative amperes. When the potential differences in the positive direction, this tells us that the current is travelling forwards, when the potential difference is in the negative direction, the current is travelling backwards. We also had a look at the three pin plug and you'll remember this diagram from our previous lessons. Each of the numbered sections on this plug are very important. Number one is our cable grip. This holds all of the wires in place and is made to an insulating material like plastic. Number two, this is where our neutral wire attaches to the three pin plug. Remember the neutral wire is our blue wire and it's held at zero volts to complete the circuit. Number three is where our earth wire connects. Remember the earth wire is our safety wire. The earth wire is a green and yellow striped wire and is attached both to the outer case of the appliance and the ground. Then we have number four. Number four is our live wire. The live wire is the most dangerous of the wires and is brown. This wire held at the high potential of 230 volts. This is where the energy enters the appliance. And number five is our safety device, the fuse. The fuse is designed so that it will melt If the current becomes too high. Each fuse has a rating, and that will be the value that it melts that. We always want to set a fuse at a current rating just higher than the normal operating current of the appliance. Here's the symbol that we would use in a circuit diagram for a fuse. We also had to look at double insulation and you'll remember this image which tells us that an appliance is double insulated. Double insulation just means that all of the electronics inside the appliance are coated in plastic. Then we have the outer case. When something has this double insulation, it doesn't need an earth wire. Right, let's do a quick recap then on all of this section, which we covered in our previous lesson about electricity in the home. For each of these questions, please select the correct answer from the boxes below. I'll give you a count of three to do this. Please point to the correct answer and say out loud. Here's our first question. Is the mains electricity a.c or d.c? Three, two, one, brilliant. It's a.c it comes from our power station. Next question. What is the approximate potential difference of the main supply? Three, two, one, brilliant. It is 230 volts. We saw this as the potential of our live wire. What is the name of the green and yellow striped wire? Three, two, one. Brilliant, It is the earth wire, this is our safety wire. Which part of the plug is made of brass? Can you remember from our previous lessons? Three, two, one. That's right, well done. It's made of brass. And brass is a good conducting material, but it's also very strong, so it won't bend. Which part of the plug is made of plastic? Three, two, one. That's right, It is our cable grip. Okay, everyone. Now it is time for an independent task. Here's a past examination question. Please pause the lesson now while you complete these questions. Well done everyone. Please pause the lesson again now, while you mark your answers. We're now going to look at Power and Energy transfer. In this section, we learned about four different equations. Please don't make any notes while I'm talking through this section, just watch your screen. Then we'll have an independent task at the end. So here are the four equations that we covered in this topic about power and energy transfer. Pause the lesson now, while you read through these and remind yourselves of the equations. Important things to remember about these equations is that the unit for power is the Watt. This is written as a capital W. The unit for energy is joules. And this is a capital J. You'll also remember that we talked about how charge flow is represented in equations by the letter Q, but it has a unit of coulombs, which is the capital C. It's important to remember these little details to go with each of the different equations. Let's complete some quick fire questions to help us with these equations, before we reach our independent task. So, first question, what is the unit of power? Three, two, one. Brilliant, It is the Watt. And you might remember that I said at the time, this is my favourite question. Which equation links power, current and resistance? Three, two, one. Well done, If you said power equals current squared multiplied by R or P equals I squared R. What does this equation equal? Charge flow multiplied by potential difference. Three, two, one. Brilliant, you said energy transferred. Remember E equals Q multiply by V. QV equals. What does this equation equal? Three, two, one. Lovely, It is the energy transferred again. This is the same equation. Power times time equals, three, two, one. This is the energy transferred too. we've now looked at both of those equations for energy transfer. Now it's time for your independent task. Please pause the lesson now, while you complete this examination question. Excellent work, everyone. Please pause the lesson again now so you can mark your answers. Each of the lines in pink here are worth one mark. So it's one mark for showing the calculation and one mark for your final answer. Okay, let's do another quick recap then of the National Grid. Again, please don't write anything in this section until we reach the worked examples. And I'll give you some time to write these down after the worked example. And also then we'll get to an independent task. So here is a diagram or a flow chart of how the National Grid works. Pause the lesson now, so you can read through this diagram carefully. Great, so you'll remember that our mains electricity is first produced in the power station and here it has a potential difference of 25,000 volts. It then goes through a step up transformer and this step up transformer is there to increase the potential difference. And it increases the potential difference up to 400,000 volts. At the same time now, it produces our current. When the electricity travels through the power cables, it has the potential difference of 400,000 volts, but at a minimum current, and you'll remember, this is really important because the lower the current, the less energy that's dissipated through the cables, this means we're not losing as much energy is not being transferred to the thermal store of the surrounding so much, which means more of that energy reaches you in your house. It makes the system more efficient, but we can't have a potential difference of 400,000 volts entering your home. That would be too dangerous. So we have a step down transformer after the power cables and before your home, this reduces the potential difference and increases the current, at this stage, we have a potential difference now of 230 volts. Let's take a look at some quick fire questions about this section. Okay, first question. What does a step-up transformer do? Three, two, one. Brilliant, It increases the potential difference. Well done, If you said that. Next question. What does a step-down transform do? Three, two, one. Well done, It decreases the potential difference. Next question. In a step up transformer, the potential difference increases and the current... Fill in the gap for me. Three, two, one, brilliant. The current decreases. In a step down transformer the potential difference decreases and the current... Three, two, one. Lovely, the current will have to increase. Electricity enters your house at what potential difference? Three, two, one, it's 230 volts, brilliant. This is our main's power supply potential difference. Now we're going to have a look at a worked example together. Make sure for this part, you are listening and watching your screen. Then I'll give you some time at the end to make any notes that you need to. Let's take a look at this worked example from an examination question. Pause the video here while you read the question. In the next part of the question, we've then shown a comparative circuit. This diagram shows the comparative circuit. Here we can see that the teacher has added in two transformers before and after the resistance wires. These transformers are found here T A and here T B. Here, we can see in our second circuit that the lamp now glows brightly. We have to explain these observations and how this type of arrangement is used to reduce energy losses in the National Grid. To gain a top bound or level three answer here. We need to make a comparative statement between those two different circuits. The one without the transformers, where we have the dim light and the one with the transformers that gives us a bright light. We can assume that our first transformer T A will be a step up transformer, increasing the potential difference through the resistance wire and T B is going to be a step down transformer, decreasing the potential difference again, as it reaches the 12 volt lamp. This means that we're going to be reducing the energy losses through these resistance wires as we'll have a much lower current travelling through the wires. So we have less resistive heating. This means that there is more energy per coulomb remaining. When those electrons reach the lamp. Let's have a look at how we can structure our answer then. We'll start off by stating that T A will be a step up transformer, which means that the potential difference is increased. If the potential difference is increased then our current will decrease. We can then comment on how this lower current is going to provide less energy loss or less power loss in the second circuit. To gain a top-hand answer, we'd need this to be a comparative statement. So let's start by saying in circuit B, there is a lower current through the resistance wire and that this means there will be less power loss due to the equation P equals I squared R. The last thing we then need to do is to link this to the lamp glowing brightly. If we have less power loss in the resistive wires, this would mean that we have got less energy wasted. So the lamp can glow more brightly. The very last thing we need to do is also to explain how this type of arrangement is used to reduce energy losses in the National Grid. Here, we have basically explained this. So all we now need to add in, I'm going to pop an asterisk here and just move up the top here as I've run out of space. But all we need to say here is that transformers are used in the same way in the National Grid to reduce energy losses. Okay, pause the lesson now, while you write down this worked example. Now is your time to try an examination question. Please pause the lesson now, while you answer the question on your screen. Great work everybody. Please pause the lesson again now and mark your answers. Okay everyone. Our last section of our lesson today is about Synoptic Questions. I'm going to go through a worked example with you, which is what we call Synoptic. This means that we're taking two aspects of the course and merging them together in a question. In this example, we're going to be looking at power and fuses. For this section again, please don't write anything down while I'm talking. I will give you an opportunity to write your notes at the end. Then there'll be some opportunities for you to try some examination questions in the worksheet that follows this lesson. Let's take a look. Here we have a Synoptic style question. We are needing to link the ideas of fuses with power. Pause the lesson here while you read this question carefully. We're now going to have a look on the next slide, how we can structure our answer to this question. So the question asks us to use calculations. That means we're going to have to write an equation and do some calculations to explain why the original fuse wire melts and explain which size fuse is the safest to use. In the books just here, we can see that we have got three different fuses that we can select. The first thing we're going to need to do is to actually calculate the current that flows through the electric drill. To do this, we're going to use the equation. Power equals current, multiplied by potential difference. Then we're going to substitute in the values that we know, that will be 750 Watts is equal to our current I multiplied by the potential difference of 230 volts, the mains power supply. We'll now need to divide both sides by 230 and that will leave us with 750 divided by 230 is equal to our current, which should come out as 3.26 amps. We can therefore start off by saying that there is a current of 3.26 amps. And that this current is the normal operating current. We then need to discuss what would happen to the three amp fuse, which fuse is the best to use this. And then for the one other fuse, we haven't chosen why that is the case. So we know that the three amp fuse will melt for normal operation. The current is higher than the rating of the fuse. Let's write that here. You can see I've added the three amp fuse will melt as it has a lower rating than the operating current. We know that we should use a five amp fuse because this is just above the normal operating current. We know though that the 13 amp fuse is unsuitable, because a very large current would be needed before it would melt. Let's add that last point in. Okay, so I'll find the answer includes a calculation for the operating current and it then says the drill has a current of 3.26 amps. This is the normal operating current. The three amp fuse will melt as it has a lower rating than the operating current. The correct fuse would be the five amp fuse as it has a rating just above the operating current. The 13 amps fuse is unsuitable because of very high current is needed before it would melt. Pause the lesson now, while you write down this worked example. Great work today, everybody we have now reached the end of our lesson. Today we have managed to recap all of domestic electricity. So that's included alternating and direct currents. The three pin plug, fuses, double insulation, power and energy equations and the National Grid. Fantastic work. All this left you to do now is to complete the worksheet and the exit quiz. I hope you have a great rest of your day and take care. Bye everyone.

Worksheet: Domestic electricity review

Exit Quiz: Domestic electricity review

Completed: Domestic electricity review

In this lesson we will be reviewing the ideas of electricity in the home, power and the national grid. We will also be looking at synoptic style questions.

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Domestic electricity review

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