Shedding Light on Electricity Episode 2: Electric Circuits

The Shedding Light on Electricity series exposes the shocking truth about electricity. Yes, positively a bad pun to begin with, but we promise to conduct ourselves really well from now on. This series teaches students watts of stuff (sorry, couldn’t help ourselves) about electricity including how it’s produced, how it’s used in our homes, how it’s controlled, and how we keep ourselves safe from nasty shocks. This is high-voltage education that is hard to resist!
In Episode 2, Electric Circuits, we examine how lights, switches, and other electrical devices are all connected either in “series” or in “parallel” with each other. How do they wire up the lights in your house so that in some rooms the light switch turns on only one light but in other rooms the light switch switches on two (or more) lights? And how is everything wired up so as to allow us to turn the lights on and off in different rooms independently of one another. Well, don’t wait at the terminal, jump on board.

A 4-minute excerpt followed by a short trailer.

The Question Sheet for students.

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Part A: Introduction
Part B: A Very Simple Electrical Circuit
Part C: Circuit Symbols and Circuit Diagrams
Part D: Series and Parallel Connections

The Transcript
Part A: Introduction
Electricity generation is one of the greatest inventions ever.
Electricity has built our modern world. It powers the electric motors in fridges, cranes, trains and fans to name just a few examples, it powers our lights, it powers our communications equipment, and it powers a whole stack of other things.
Though there are millions of things that use electricity, they all follow the same basic rules. In our first episode we saw that there are four basic things that you need if you want to make use of electricity.
Firstly, you need a source of electricity, and we looked at things like power stations and solar farms. Secondly, you need a load, something that uses the electricity to produce light or movement, for example. Thirdly, you need wires of some description to connect the source of electricity to the load and fourthly, you need a switch so that you can easily control when the electricity flows.
So how are all these things connected? Here I have a battery, a switch, a light globe, and some wires. How should they all be connected so that when I press the switch the light globe turns on? And how would I connect a second light globe so that both turn on when I press the switch? Well, in this program we’re going to look at a variety of ways that electrical devices are connected. Let’s begin.
Part B: A Very Simple Electrical Circuit
Electrical devices are always connected in what in what is called an electrical circuit. The electric current has to be allowed to flow out of, in this case, the battery from this so-called “terminal”, through this wire, down, through this wire, up and through the light globe, down and through this wire, and then up and then through this wire and then back to this terminal of the battery.
It’s a continuous loop; a “circuit” in fact. If I connect just one wire, the electricity can’t flow at all.
Remember, electricity is just a flow of electrons through a wire of some sort. You can’t just connect one wire to a battery because the electrons just can’t move in a current if there’s nowhere for them to go.
Batteries are pretty complicated but when they’re connected, a bunch of chemical reactions occur which push the electrons around the circuit. But the chemical reactions can only occur if the electrons end up going back into the battery through the other terminal.
It’s the same with generators. The electrons in the coil of wire that is spinning between the two magnets get pushed out of the coil through the light globes and then back into the coil again. A generator is literally an electron pump and the electrons just move around and around in a continuous, unbroken circuit.
Now all light globes have two connections which are also called terminals.
On this light globe the two terminals are on the bottom, but on this light globe, one terminal is on the bottom and the screw itself is the other terminal.
So in the case of an incandescent light globe, when it’s connected, the current flows in through one of the terminals, up one of the supporting arms, across the filament, down the other supporting arm on the other side and out of the globe through the other terminal. The filament is so thin that it gets really really hot, hot enough to start giving off visible light. LED lights, motors, heaters, and in fact all electrical devices also have two terminals, and the electricity has to flow into them, through them and then out of them again.
So this is the simplest electrical circuit you can get. To turn the light globe off I can just disconnect the wire, but there’s a better way; I can use a switch. For the electric circuit to be complete, the electric current has to be allowed to flow from here to here, so I can place a switch between these two points. I’ll need another wire.
Now, when I press the switch, the electric current can flow through this wire, through this wire here, up and across this arm here to this connection here, through this wire, up and around through this wire, through the light globe, back through this wire and then back to the battery. When the switch is released, the circuit is interrupted and the electric current can’t flow any more.
Now this switch is just used for school experiments and demonstrations.
The switches that turn on lights in your house are basically the same but they’ve been designed to stay on or off and to be screwed into walls or door frames.
This simple electrical circuit is pretty much the same as the circuits in our houses and in buildings in general.
When a building is being built, electricians lay electrical cables inside the cavities that will be created once all the walls and ceilings are in place. We never really think about it much, but there are cables running all over the place inside buildings. The walls and ceilings are then put in and electricians connect the cables to switches, wall sockets, and light globes, and to the grid. Even though it may look pretty random initially, all of the circuits we use every day are basically the same as this one.
The electricity flows from power stations, through “transmission lines”, into our houses or schools or whatever, through a switch, through a light globe or whatever appliance, and then back through another set of wires.
All the electric circuits in our cars are the same as well. The current flows out of one terminal of the battery, through a set of wires, through a switch, and they all come in different shapes and sizes, through the load, in this case it’s the headlight, there are the connection there, and then back through another set of wires to the other terminal of the battery.
There are wires running from the battery to every electrical component of a car. These wires are connected to the light globes of what’s called the tail-light assembly.
Now if I was designing an electrical system of a car or a phone or whatever, I could draw the battery, the switches, the globes and any other components, but this can get pretty messy pretty quickly. Instead engineers and scientists use circuit symbols and circuit diagrams. They’re much easier to use. Let’s take a look.
Part C: Circuit Symbols and Circuit Diagrams
So instead of drawing diagrams of the electrical equipment, it’s easier to use circuit symbols.
The symbol for a battery, any type of battery, is a long line next to a short line. The longer line is the positive side of the battery. Two batteries joined together are drawn like this.
This kind of switch is shown like this.
The symbol for a light globe is either this or this. I’m going to be using the first one.
Wires are just shown as lines, but they’re always straight lines.
So this simple circuit can be drawn with circuit symbols. Battery, switch, light globe and wires. This “circuit diagram” is much easier to draw and to follow.
When the switch is pressed an electric current can flow though the circuit and the light globe turns on. Now there are many more circuit symbols.
An ammeter, which measures how much current is flowing, a voltmeter, which measures voltage, these kinds of so called SPDT switches, which have three connections, and all of these electrical and electronic components (diodes, LEDs, resistors, capacitors, fuses, motors, and speakers) have their own symbols, and there are many more. We’ll be learning about some of these things in this series.
Sometimes, in more complicated circuits, wires have to cross over each other. If the wires are not connected they’re drawn like this with a little kind of bridge symbol. If they are all connected a little dot is often used.
Getting back to this simple circuit, notice how the wires coming out of each component come out in the one line. When you draw circuit diagrams don’t do this, where the wires change direction at the component; it makes it harder to see. Always use simple clear rectangular circuit diagrams. Also if you connect more than one wire to a battery, draw it something like this. A circuit diagram has to clearly show where the electric current will flow.
Now this is a simple circuit, where one switch switches on one light globe as is this one, although the source of electricity for this circuit is a long way away. However, many electric circuits have two or more light globes connected to a single switch. In this bathroom, this switch which you can see in the mirror, switches on the one light globe, this switch switches on the fan, there it goes there, but this switch switches on two of the heat lamps and this switch switches on the other two heat lamps.
The switch on this powerboard can switch on both of these lights at the same time. How is this wizardry achieved? Let’s take a look.
Part D: Series and Parallel Connections
There are actually two ways that two or more light globes can be connected to a source of electricity. Light globes can be connected in series with each other or they can be connected in parallel with each other.
Let’s start with this simple electric circuit. To add another light globe, I can simply disconnect this wire, connect it here and then use this wire to connect the two light globes together. Now when I press the switch, the electric current can flow through the switch, through the first light globe, through the second light globe, and then back to the battery.
I can draw a circuit diagram to represent this circuit. The light globes are said to be connected in series. The switch is also connected in series with the light globes. The word series tells us that one component follows the other.
Now though connecting light globes in series works, no household lights or lights in our cars or anyone else are ever connected to each other in series. Why is that?
Well, if two light globes are connected in series, and the switch is pressed, the electric current has to be able to run in a continuous stream first through one and then through the other.
However, light globes and other devices can break! This bathroom heat lamp, which is like a light globe, is broken.
You can see that the filament wire has burned out right here, so electricity can’t flow through it. If a broken light globe is placed in series with a working light globe, the circuit is not complete, so not even the good light globe will light up. Let me simulate the situation with real light globes.
If I remove the first light globe here, then the second light globe turns off as well because the circuit is no longer complete. If I replace the first light globe and then remove the second light globe, then the first light globe turns off as well, because, once again, the circuit is no longer complete.
So, if these three light globes were connected in series and one of them broke, then none of them would turn on. And, frustratingly, it wouldn’t necessarily be obvious which of them broke, so you would have to replace each one in turn until you found the faulty one. Now in this situation it would be pretty easy to find the faulty one.
In a car though, removing lights is a little more tricky, and usually involves screws and clips which are sometimes difficult to get to, so you wouldn’t want to waste time trying to find the broken light by trial and error.
Another reason that light globes are not connected in series has to do with power output.
These three light globes all produce the same amount of light when they’re connected individually to the 6 volt battery. When I connect two of them in series, they are both much dimmer and in fact the overall brightness is less than a single light globe on its own. This is because each light globe has to share the voltage of the battery and so they only get 3 volts each. If I connect three light globes in series, they produce even less light. This is obviously not ideal.
When you buy, say, a 4W light globe, you want it to produce the amount of light that you need, and you want it to produce the same amount of light even if it’s to be used in a room where the one switch switches on more than one light globe.
So if two household lights were connected in series in one room and three were connected in series in another room, then they would have different brightnesses in the two rooms and you can’t have that.
So, light globes are not connected in series, but in parallel.
Here once again, I have a single light globe. I can now take a second light globe and connect it with these two wires here, and these two globes are said to be connected in parallel.
It’s called a parallel connection because in a circuit diagram the light globes and the wires that connect them are parallel to each other. So why are light globes always connected in parallel? Well there are two main reasons.
Firstly, whatever brightness a light globe has when it’s connected on its own is exactly the same as when it’s connected in parallel with other light globes. The brightness of each light globe is not affected by the other globes in the circuit. The brightness of each globe just depends on the power supply and on what power the light globe’s been designed to produce for that power supply.
Secondly, if one light globe breaks, which I’ll simulate by removing it, the other light globe still works. The electric current can still flow through this wire, through the switch, through these two wires, through this light globe, and then back through these two wires to the battery. When I replace the first light globe and remove the second, the first light globe still works, because, once again, it has its own independent connection to the battery.
On the circuit diagram we can see that when the switch is pressed, the current can flow out of the battery, through the switch, and then through both light globes, because the current splits where the two wires are connected. If one light globe is removed or if it breaks, the current can still flow through the other one. And of course, it’s immediately obvious which light globe has broken, so you just replace it.
By the way, this junction doesn’t have to be here. It can also be here and this circuit operates in exactly the same way.
I can also move this junction along the wire closer to the battery and, once again, the circuit will operate in exactly the same way.
So there are a number of ways of constructing a circuit so that it operates the way you want it to. Likewise the circuit diagram can also be drawn like this. Both circuit diagrams are depicting the same circuit where the switch is controlling the operation of two light globes that are connected in parallel.
Now not only are light globes connected in parallel with each other, every individual circuit within a house is connected in parallel with every other circuit.
So there might be a light globe in a bathroom and another one in a bedroom but both of these circuit sections are connected in parallel with each other.
You can have one light on, or the other light on, or both of them on. Parallel connections allow circuits to operate independently of each other.
You might also have two lights in your kitchen and three in your lounge room. Each set of lights can be switched on and off independently of one another, because they all have their own individual connection to the source of electricity. Here I’ve shown a battery, but for most houses, the electricity comes from outside the house so technically I should really draw this symbol, which is the symbol of the (AC) power supply that power stations supply us with.
And it’s not just the fixed lighting. The same goes for the power sockets as well. This drill and this fan, which are connected to the same powerboard, are in parallel with each other. I can turn them on and off independently of one another.
So, electric current flows from one terminal of a source of electricity, through an appliance like a light globe or an electric motor, and then back out through another wire to the other terminal of the source of electricity. Things can be connected in series like the switch and the light globe in this circuit, or in parallel, like these two light globes in this circuit.
In more complicated electronic circuits, like those in a computer, circuit components can be found in multiple levels of overlapping series and parallel connections. However, everything we’ve learned about series and parallel connections still holds true.
In this simple electronic circuit with a resistor and 3 LEDs, the LEDs are connected in parallel with each other and all three are connected in series with the resistor. The current flows out of the battery, flows through the resistor and then splits up evenly to flow through the LEDs, before joining up again. If one of the LEDs is removed the other ones still work, because the current can still flow in an unbroken path from one terminal of the battery to the other. So, as I said, things can be connected in series or in parallel.
However, some things use more electricity than other things. This 2000 Watt heater uses far more electricity than this 4 Watt light globe, like 500 times the amount of electricity. How do we measure electricity? Well, that’s what we’re going to look at in our next episode. See you then.