# Video: GCSE Physics Higher Tier Pack 1 • Paper 1 • Question 3

GCSE Physics Higher Tier Pack 1 • Paper 1 • Question 3

07:32

### Video Transcript

The National Grid is the UK’s electrical power transmission network. If consumer demand for electrical power increases, the output from power stations in the UK is increased to meet the demand. Figure one shows the total demand for electrical power in the UK over a 24-hour period. At what time is the demand for electrical power highest?

Okay, so here in figure one, we’ve been given a graph that shows the total power demand against time. And we can see that the total power demand varies widely over the course of the day. It starts at very low here around midnight, but then rapidly increases at around eight o’clock in the morning. And it continues varying throughout the rest of the day as well. Now, we’ve been asked to find the time at which the demand for electrical power is highest. In other words, what’s the highest point on our graph.

Well, we can actually judge this by eye. We can see that this is roughly the highest point on our graph. And this peak power demand happens to be just over 30 gigawatts. But that’s not actually what we’re looking for. What we’re actually looking for is the time at which this occurs. And so, we need to actually drop down to the horizontal axis to tell us at what time this occurs. Now, this point here is roughly halfway between eight o’clock and 12 o’clock. We don’t need to be too precise here. A ballpark figure will do. And so, we can say that the demand for electrical power is highest at 10 o’clock in the morning.

As a side note by the way, it’s important to notice that we’re looking at times using the 24-hour clock on the horizontal axis. In other words, 00:00 refers to midnight, 04:00 refers to four o’clock in the morning, and so on and so forth. Anyway, so let’s move on to looking at why the total power demand is low at certain times of the day.

Suggest one reason for the shape of the graph between 00:00 and 04:00.

In other words, we’re trying to find out the reason for the shape of the graph between this time and this time. Now, as we already discussed, this time is midnight and this time is four o’clock in the morning. And we can see that at this time, the total power demand is actually very low. It’s only about 15 gigawatts, as opposed to the massive 30 plus gigawatts that we saw at 10 o’clock in the morning. As well as this, the shape of the graph of the total power demand is roughly flat between midnight and four o’clock in the morning. It may be decreases slightly over time, but not by much.

So why is the total power demand so low at these times of day? Well, between these times, most people are usually sleeping. And if they’re sleeping, then they’re not using electrical appliances such as TVs or lights or computers. And so we can say that that’s one of the main reasons that the power demand is low during these times. Now, in this question, we have been asked to give one reason. But we can think of another one. And that reason is that most businesses are also closed overnight.

Now, businesses can use huge amounts of electricity to power their offices or their warehouses or their shops or whatever they have. And most of these are closed at night. So these are going to be using much less electricity as well. And so we could say that our second reason is that many business close overnight, so they use less electricity. And therefore, we can choose either one of these points to give as an answer to our question. Now, let’s move on to considering different types of power station.

Certain types of power station are either kept off or operating well below their maximum power output, so that if demand suddenly rises, these power stations can be started up to meet that demand. Give two types of power station that can start up rapidly to meet sudden increases in demand.

Okay, so what we’ve been told in this question is that there are a few types of power station that run continuously at full power, so that they provide all of the electricity to the country when the demand for power is low. And then, there’s a few other types of power station, the ones that we’re actually discussing here, that are kept completely switched off or operating well below their maximum power output when the demand is low. This is done so that when the demand increases, these power stations that were initially kept off can be switched on, so that suddenly there’s enough power available to match that demand.

And it’s important that this is done quickly. The power stations need to be switched on very fast because the demand for power can increase really quickly as well. Think about, for example, when people are coming home from work and suddenly switch on all their kettles. Now, this can happen anywhere between, let’s say, 4:30 in the afternoon and six in the evening. But if we think about it, that’s actually quite a small window of time for the power demand to increase rapidly. Lots of kettles being used, lots of people turning on their TVs after work or going on their computer, et cetra, et cetra.

So what are these types of power station that can start up rapidly to meet sudden increases in demand? Well, one type of power station is a gas-fired power station. Now, there are a couple of reasons for this. And to understand them, let’s compare a gas-fired power station to a bog-standard coal-fired power station. Now, a coal-fired power station is one of the types of power station that’s kept on constantly, to provide power during the entire day. This is because it’s one of those types of power station that’s not easy to start up quickly. So why is this?

Well, firstly, a gas-fired power station, you guessed it, combusts gas whereas the coal-fired power station combusts coal. Well, duh! But, this is important because gas is really easy to transport from where it’s being stored to where it needs to be combusted. Because it’s a gas, it can be transported along pipes to where it needs to go. Whereas, coal is actually a solid. Therefore, we can’t squirt it along pipes. It needs to be transported from where it’s been stored to where it’s been combusted. However, in a gas-fired power station, it’s very easy to go: “Oh, look! The power demand is increasing. Quick, turn on the taps for the gas.” And the gas within seconds or minutes can be transported to where we’re combusting the gas. So this is why a gas-fired power station is quicker than a coal-fired power station to get started.

But there is also another reason. In a gas-fired power station, the gas enters the combustion chamber. And then, the gas itself is combusted. Now, it’s the exhaust gases from the reaction that actually exit the combustion chamber and then go on to turning a turbine which results in the power being generated. However, this is different to a coal-fired power station. Because in a coal-fired power station, the coal, as we said earlier, is solid. Therefore, the energy from this coal needs to be transferred to some water. That’s the water coming in here. And then eventually, the water heats up, gets converted to steam, and then it exits the combustion chamber. And then, it’s this steam that goes on to turn a turbine.

So in other words, in a coal-fired power station, because coal is not a gas, we need to faff around with a lot of water. Evaporating the water takes time, and certainly much longer than just combusting the gas and having the exhaust gases escape and turn the turbine. And so, this is the second reason why a gas-fired power station can be started up very quickly and provide power very quickly. So we know one of the answers is a gas-fired power station. But what’s the other answer?

Well, the second answer is a hydroelectric dam. Now, in a hydroelectric dam, essentially, what we have is a large reservoir high up in some mountains. Now, this reservoir basically contains a lot of water that’s been pumped up there when the demand for electrical power is low. And a dam stops that water from flying back down again. Or, at least, until we need the water to flow down. Because as soon as the demand for electrical power increases, the dam can be opened. At which point, the water starts flowing downhill again and into a turbine. Thus, turning it and generating power.

Now, this is a fairly rapid process. Opening the dam can take anywhere between seconds and minutes. And so, all of the gravitational potential energy that the water had when it was in the reservoir gets converted into electrical energy via the turning of the turbine. So it’s quite a clever system. When the demand for electrical power is low, take all of the water that’s downstream, pump it up hill into the reservoir, and close the dam. Then, open the dam as soon as a large amount of power is needed. And we’ve got a power station that can meet the demand for power very quickly. So two types of power station that can start up rapidly to meet sudden increases in demand are gas-fired power stations and hydroelectric dams.