Video: Understanding the Transmission of Electrical Power across Large Distances

The national grid transmits electrical power at a very high potential difference and a very low current. Why is this?


Video Transcript

The national grid transmits electrical power at a very high potential difference and a very low current. Why is this?

Okay, so when we talk about the national grid, we’re speaking of a very large-scale electrical network. This network has three main sections or parts to it. These parts are generation, where electricity is generated, transmission, where the electrical energy travels over large distances, and then distribution, where the electrical energy is delivered to its end users. Now, our particular question is asking about the transmission phase of this process.

During this phase, electrical energy is transmitted over large distances up to hundreds of kilometers. During this process, one of the most important considerations is the efficiency with which electricity can get from one point to another. That is, of all the electrical energy that goes into the transmission wires in our network, we want as much of that energy as possible to make it out the other end at the distribution phase. In other words, during transmission, we want to minimize the energy lost.

Now, if we picture a wire in our transmission network, we know that electrical current travels through that wire. And we also know that the more current flows in the wire, the more the wire heats up. Now, this heat is not a way that we want this electrical energy to be used. We prefer the energy make it to the distribution phase where it can be used in homes and businesses. So during the transmission phase, we would like our wire heating to be a small as possible. And the way to do that is to make the current running through these wires as small as possible. So the smaller our current 𝐼 is that runs through these transmission wires, the less the wires heat up, and therefore the less energy is lost due to heating.

But interestingly, because of the properties of electricity, if we want to change the current 𝐼, there’s another property we’ll need to change, and that’s the potential difference 𝑉. The reason for this comes down to a mathematical expression for electrical power. This equation tells us that power is equal to potential difference or voltage multiplied by current 𝐼. The electrical power moving through the transmission lines of our national grid depends on the power plant that was used to generate that electricity. For a given power plant, we can consider the electrical power 𝑃 in the transmission lines to be a constant. In other words, 𝑃 in this equation is fixed. That’s something that we’re given rather than something we can change.

This means that if we want to decrease the current running through our transmission lines and we do because that will minimize energy losses, then in order to do that, we’ll have to increase the potential difference of our electricity. It’s only by doing both of these things that we can decrease the current as well as keep the power constant. And so we now see why the national grid transmits electrical power both at very high potential difference and at very low current. We can write out that reason this way. We can say that while high current heats cables and loses energy, using less current and, therefore, more potential difference wastes less energy. So the reason for transmitting electrical power this way is ultimately to minimize energy loss.

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