Parallel circuits
Ask an older person in your family about Christmas tree lights and you may get a surprising answer.

"Oh, they were dreadful !" one person might say.

"I spent so much time sorting them out," another might say.

You may wonder what they were talking about because Christmas tree lights are supposed to be bright, colourful and a charming addition to any Christmas tree. The older people were not referring to this aspect of Christmas tree lights but what happened if one went out.

In the past when Christmas tree lights were set up they would shine brightly for some time then the chances were that a bulb would go out. The bulb went out because its filament snapped and electricity could no longer pass through it. In Christmas tree lights in the past all the bulbs were connected in a line. This arrangement is called a series circuit. Electricity passed from the power source to the first bulb and then on to the next bulb and so on until the circuit was completed. If a bulb went out the flow of electricity was stopped and all the bulbs went out. The problem was you could not tell which bulb had blown because they all went out at the same time.

The only way to find out which bulb had blown was to test each bulb in the circuit. This was done by removing the first bulb, replacing it with a new one and switching on the circuit. If all the bulbs lit up you had found the blown bulb and could go back to enjoying other Christmas activities. Usually you were not so lucky. The bulbs remained out when you tested the first bulb. This meant that you then had to put the first bulb back in place, take out the second bulb, replace it with a new bulb and try lighting the circuit again. Usually this did not work either, so you may have had to test a dozen or even more bulbs before you found the one that had blown. All the time you were fiddling with the Christmas tree lights other people were having fun. This is the reason why older people have a dread of Christmas tree lights.

Most of your work on circuits involves making series circuits. You start with a power supply of one or more batteries. If you have two or more batteries you line them up with their positive terminals pointing in the same direction. We say that the batteries are arranged in series. Next you connect a wire from the positive terminal to the switch, then a wire from the switch to a component, such as a bulb. If you then use a wire to connect another bulb so both are in a line you have arranged the bulbs in series. Finally you connect a wire to run from the second bulb to the negative terminal of the battery. When you switch on both bulbs light up but if one blows they both go out just like the bulbs in older sets of Christmas tree lights.

There is a second way to link components together in a circuit. You can arrange them in parallel. When you do this you put the components side by side. If you made a parallel circuit with two bulbs in it you would have a row of batteries as in the series circuit but a wire would run from the positive terminal to each bulb and a wire would run from each bulb to the negative terminal. You would then have two loops to carry electricity and in each loop you might add a switch. When you switch on both loops both bulbs shine brightly and if one blows the other just keeps on shining as before. The reason for this is that electricity flows directly from each battery to each filament so if one filament is broken the other is not affected.

Today many sets of Christmas tree lights are arranged in parallel. If a bulb goes out the rest keep shining. The blown bulb is easily spotted because of the dark area around it on the tree. The bulb can then be removed and replaced in a few moments so you do not have to miss out on any Christmas fun.

Are parallel circuits used in the home?
Yes, they are. The lights in a room are arranged in a parallel circuit. Imagine a room with a central light fitting and two or more wall lights. If a bulb blows in the central fitting or one of the wall lights the other bulbs keep shining. If they were arranged in a series circuit, when a bulb blew the whole room would be cast into darkness. The sockets in a room are also arranged in a parallel circuit. If a piece of electrical equipment blows a fuse in one socket, electrical equipment working off other sockets just keeps going.

Are streetlights arranged in parallel?
Yes, they are. When one lamp goes out the others keep shining. If the lamps were arranged in series and a lamp failed to work the whole street would be cast into darkness.

Are the lamps on cars and other vehicles arranged in parallel?
Yes, they are. Imagine what would happen if they were arranged in series. A car is being driven along a country road at night. There are no streetlights and the headlights are shining into the distance so the driver can move along quite quickly. Suddenly the filament in a sidelight breaks and all the lights go off. The driver is left moving at speed in pitch darkness and has to brake hard and hope the car will stop safely before it hits something.

Do bulbs shine differently in a parallel circuit?
They shine more brightly than the bulbs in a series circuit. The bulbs in a series circuit share the voltage of the batteries. It is the voltage that gives them the power to shine. If there is one bulb on its own, it uses all the voltage of the batteries and shines brightly. If two bulbs are arranged in series they have to share the voltage and shine less brightly. If three bulbs are arranged in series they also have to share the voltage and shine even less brightly. In a parallel circuit each bulb receives the full voltage of the batteries so all of them shine as brightly as if in a circuit on their own.

Can other components be arranged in parallel besides bulbs?
Yes, they can. You can arrange a motor in parallel with a bulb or a motor in parallel with a buzzer or you can arrange a motor, buzzer and bulb in parallel.

Can batteries be arranged in parallel?
Yes, they can, but for most circuit work they are always arranged in series. When you add batteries together in series the voltages add up and give extra push to the electrons. For example, when you connect two 1.5V batteries in series they give the circuit a voltage of three volts. When you add batteries in parallel the voltages do not add up. For example, when two 1.5V batteries are arranged in parallel they give a voltage of 1.5V to the circuit. So there is no advantage in arranging the batteries in parallel if you want to increase the voltage.