Electric Circuits

A circuit is made up of wires and components with a power source where current can flow and the components will do work.

A simple circuit is where a bulb is connected to a power source so the bulb can give off light.  This is called a simple series circuit.

An electric current will not flow if we do not have a power source (a cell, battery or power pack). It also won't flow if the circuit is not complete. One end of the power source must be joined to the other end by the wires and components of the circuit.

The simplest complete circuit is a piece of wire from one end of a battery to the other. An electric current can flow in the wire from one end of the battery to the other, but nothing useful happens. The wire just gets hot and the battery goes flat.

To do something useful with the electric current, we need to put an electrical component into the circuit, such as a lamp or motor that can use the current to make something happen.

complete and incomplete series circuits

                                   The bulb will only light if there is a battery and a complete circuit

We usually add in a switch to the circuit, so that we can break the circuit and stop the electric current when we want to.

Series Circuits

In a television series, you get several episodes, one after the other. A series circuit is similar. You get several components one after the other.

If you follow the circuit diagram from one side of the cell to the other, you should pass through all the different components, one after the other, without any branches.

 

If you put more lamps into a series circuit, the lamps will be dimmer than before.

This bulb will be brighter than these bulbs because current is shared between components in a series circuit.  

In a series circuit, if a lamp breaks or a component is disconnected, the circuit is broken and all the components stop working.

 

Series circuits are useful if you want a warning that one of the components in the circuit has failed. They also use less wiring than parallel circuits.

An incomplete circuit

Electric Current

When electric charges move in a wire, we say that an electric current flows in the wire. It's like the way a current of water flows in a river.

Current is the movement of electrons around a circuit.  According to convention electrons flow from the negative side of the battery to the positive side of the battery.  (this makes sense really because electrons are negative and will be attracted to the positive side of the battery)  but it is wrong! 

Electrons flow from the positive side of the battery to the negative side. 

For an electric current to flow, we need two things:

  • something to make the electricity flow, such as a battery or power pack

  • a complete path for the current to flow in.

An electric current will not flow if we do not have a power source (a cell, battery or power pack). It also won't flow if the circuit is not complete. One end of the power source must be joined to the other end by the wires and components of the circuit.

The simplest complete circuit is a piece of wire from one end of a battery to the other. An electric current can flow in the wire from one end of the battery to the other, but nothing useful happens. The wire just gets hot and the battery goes flat.

To do something useful with the electric current, we need to put an electrical component into the circuit, such as a lamp or motor that can use the current to make something happen.

conventional current

Current is a measure of how much electric charge flows through a circuit. The more charge that flows, the bigger the current.

Current is measured in units called amps. The symbol for amps is A.

Measuring current

A device called an ammeter is used to measure current. Some types of ammeter have a pointer on a dial, but most have a digital readout. To measure the current flowing through a component in a circuit, you must connect the ammeter in series with it.

Current in Series Circuits

The current is the same everywhere in a series circuit. It does not matter where you put the ammeter, it will give you the same reading. This is because there is only one path for the current to flow.

The current is the same everywhere in a series circuit.

Current is not used up

You might think that the current gets less as it flows through one component after another. But it is not like this. The current is not used up by the components in a circuit. This means that the current is the same everywhere in a series circuit, even if it has lots of lamps or other components.

There is a simple rule with current: Whatever goes in must come out. 

 It's like a vindaloo....

Adding more Cells

The current in a series circuit depends upon the number of cells. The more cells you add, the greater the current.

The current is 'pushed' around the circuit by the power from the batteries.

So;

the more cells=more power=the faster the current!

Adding more bulbs or other resistors will have the oppositie effect.

Adding resistance slows down the current.  This is because the current needs to travel through each component which in turn resists the current.  Which is a good job really because otherwise bulbs wouldnt give out light and buzzers wouldn't buzz and my coffee machine wouldn't make my coffee hot....

It's like you running a 100 m on flat ground.  you could probably do it in about 15 s. now imaging you are running a 100 m hurdles... what is going to happen?  The time it takes you will decrease because now you have to run, jump, land, start running again... well that's what its like for the current when you add resistors/componenets. 

Now imagine if you were running 100 m hurdles with a lion chasing you.... you would speed up... well thats what happens when you add more batteries.. the current will speed up because it now has a bigger push.

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