Charge and Current

The title of this section is "Voltage and resistance" but we can't explain "voltage" or "resistance" without first introducing charge (and then current).

What is charge?

Charge is what flows in an electric circuit (consisting of conductors) when a battery or some similar energy source is connected across the circuit.

Scientists realised this soon after the first battery was invented by Alessandro Volta way back in the 1800's.

When they connected a battery across various circuits they saw that things happened eg. the conductors in the circuits got warm.

With their limited knowledge at the time they suggested that the battery was pushing some invisible things through the circuit.
The scientists suggested that as these things flowed through the circuit they passed their energy on to the conductors, causing them to warm up.

They called these things, charges.

But back then, they didn't really know what these charges were.

Today, we do! Charges are electrons.

So, a flow of charge is a flow of electrons.

And we call a flow of electrons an electric current.

Current is measured in "amperes" (A).

The flow of electrons (a current) in conductors

All electrical circuits consist of at least some conductors connected to either side of an energy source such as a battery.

The conductors (usually wires) are made from metal eg copper, because metal atoms have an abundance of free electrons which can be made to flow.
This is why we say that metals are good conductors of electricity compared with materials such as plastics which do not have atoms with free electrons; this makes them good insulators of electricity.

So, a piece of metal wire is ideal for carrying a flow of charge (current); it is literally full of electrons. We can think of them, as in the following diagram, like cars, bumper to bumper.

The electrons in the wire above are not moving or flowing, so there isn't a flow of charge (current).

What else is needed in order to make the charges flow?

There needs to be an energy source or Voltage across the wire.

Voltage

In the following diagram a battery is being used to provide the Voltage and it is connected across the piece of wire.

The battery seems to push on just one electron, but since the electrons are "bumper to bumper" in the wire, like the cars, they ALL move through the wire as shown by the arrows in the diagram.

The best way to think of the Voltage source is as a pump.

Just as a water pump takes in water at one end and pushes it out at the other end, so our Voltage source takes in the electrons/charge at one end and pushes them out at the other end, causing all of the electrons in the circuit to flow at the same time!

As long as the Voltage source continues doing its job, the electrons keep flowing and we have a steady flow of charge/electrons all the way around the circuit, meaning that the current will be the same at all points around the circuit.
It is important to try to understand this.

So, what is Voltage?
Voltage is a measure of energy in an electrical circuit; its the electrical push from the battery.
Being more precise, voltage is a measure of the amount of energy given to each unit of charge pushed out by the battery.
The larger the voltage, the greater the amount of energy carried by the charges flowing through the circuit. This is why large voltages can be much more dangerous than small voltages.

Voltage is measured in "volts".
Note: the symbol for the word "voltage" is V and the symbol for the unit "volts" is also V.
eg to write "the voltage = 4 volts" using symbols, we would use:
V = 4 V

Drawing circuits

We don't want to have to draw circuits like we have drawn the one above. We need a simpler system.
Instead we use symbols, for the battery, for the wires and for other components.

Drawing the above diagram using standard circuit diagram symbols, we get this:

It is much simpler but it shows the same things; the V next to the upper symbol represents the battery Voltage and the arrow at the bottom next to the letter I represents the current flow.

One final comment on circuit diagrams.
When you draw one:

• always use straight lines for the connecting wires between components, and


• always use 90 degree bends.

In other words just like the diagram above; never like this......

Other symbols such as those for lamps, voltmeters (for measuring voltage) and resistors will be shown in the following sections on this page.

Resistance and the size of current

All components, such as lamps, and even a piece of wire, resist the flow of an electric current.
A component with a high resistance lets only a small current flow, compared to a component with a low resistance.
Resistance is measured in "ohms" and is given the symbol Ω (the Greek letter "omega").

The size of the current in any circuit depends on just two things:
1. The voltage of the energy source (battery usually) and
2. The resistance of the circuit.

To calculate the current in a circuit we use these two quantities in the following formula:

Some components are just known simply as Resistors because that is all they do - resist the flow of current.

In the next example you are asked to calculate the size of the resistance.
To do so you need to rearrange the above formula:

NOTES:
1. Components with resistance reduce the current flowing in a circuit in much the same way as someone standing on a rubber water pipe will reduce the flow of water. And like in the water pipe where the effect is noticeable all through the pipe not just at the point where the foot is applied, so the electric current is reduced everywhere in the circuit.

2. When charges are pushed through a resistance by a voltage some of the electrical movement energy causes the resistor to rise in temperature; this is just like friction which occurs if we push an object through a tight passage. So some of the electrical energy is shifted to the surroundings. Components such as electric fires deliberately make use of this effect to heat a room.

So, to summarise:

• Charges are electrons.
• Metal wires are used in electrical circuits because they contain an abundance of free electrons, making them good conductors.
• To make the electrons flow through a circuit, as a current, an energy source (a Voltage/Potential Difference) is needed.
• The voltage(p.d) acts like a pump, taking in electrons and pushing them out, making them flow all the way round the circuit as an electric current.
• The voltage is an electrical push; a measure of the amount of energy given to each unit of charge.
• Resistance is a property of all components, some resist current greatly, some less.
• The size of the current flowing in a circuit depends on the total resistance of the circuit and the voltage across it.

Series and Parallel Circuits

There are just two types of circuit, series and parallel.

Series Circuits

The following lamps are connected "in series".

We describe these 3 lamps as being connected "in series" because:
the same current has to flow through each lamp in the circuit and
there is only one path for it to take.

The use of the word "series" here is the same as its use in "TV Series"; in other words - if you set about watching a TV Series you know that you have to watch Ep 1 then Ep 2, then 3 until you reach the end. The same is true with the charge flowing in a "series circuit"; it has to flow through each component in sequence/ in series until it arrives back at the p.d supply.

Now that you know what we mean by "series circuit", what is special about them?

For components connected in series:
1) the same current flows through each component (which we have already said).
2) the voltage(p.d) of the power supply is shared between the components.

NB We will leave 1) until the next section when we will discuss Current in more detail.

So, let's look at point 2).

Consider a 5V battery connected across 2 components, a lamp and a resistor, in series:

OK, what we want is to know the voltage(p.d) across the lamp and across the resistor.
We can easily measure voltage(p.d) by connecting a voltmeter across a component.
Let's do this for the lamp:

You can see the symbol for the voltmeter.
Ok, let's say that the voltmeter reads a voltage of 3V across the lamp.

Now what we want is to know the voltage across the resistor. Of course, we could use the voltmeter; just move its connections so that it is across the resistor and get our desired reading, but is their a way we can get our answer without using the voltmeter again?

Yes, this is where we can make use of point 2)
Look at it again:
2) the voltage(p.d) of the power supply is shared between the components.

So, the 5V of the power supply is shared between the lamp and the resistor. We already know from the voltmeter reading that the voltage across the lamp is 3V.

Therefore there must be 2V across the resistor.

We can express point 2) as a simple formula:

Parallel Circuits

The following 2 lamps are connected "in parallel".

The word "parallel" is used because, as you can see, the lines of the paths through which the current flows are parallel to each other.

For components connected in parallel:
1) the voltage(p.d) across each component is the same.
2) the total current through the whole circuit is the sum of the currents through the separate components.

We will leave point 2) until the next section which deals with Current in more detail.

So, let's look at point 1).

Its a very simple statement but it can be quite tricky to understand, but let's have a go at explaining it:

If you built the circuit above and connected a voltmeter across the top lamp, as shown below, but then slid the left side voltmeter connector further to the left and the right side voltmeter connector further to the right (as indicated by the yellow arrows) and kept sliding them, you would find that you were effectively measuring the p.d across the power source, which in this case is 5V.

If you then connected your voltmeter across the lower lamp, as shown below, and once again slid your probes around, you would, once again, find that you were measuring the same p.d, across the power source.

So, when components are connected in parallel, the p.d across each is the same! Simple as that.

Quick Question Point 1:
Two lamps are connected in parallel; the p.d across one of them is measured and found to be 4V. What is the p.d across the other? Check

Measuring Resistance

All components "resist" the flow of electric charge to some extent, so they all have some value of "resistance".
Components made from materials such as metals will be good conductors and have a low value of resistance.
Components made from materials such as plastics will be poor conductors OR good insulators and have a high value of resistance.

In this section we are going to learn how we can measure the resistance of any component.

We start by recalling the formula:

This tells us that to find the resistance of a component we need to know the voltage across it (V) and the current flowing through it (I).
Once we know these two values we use the formula to calculate the resistance. Easy!
We did an example like this above (and you have already answered some questions asking you to calculate R).

But sometimes when we are given an unknown component it is wise to take a number of pairs of voltage and current readings so we can be sure that the resistance remains constant for all values of V and I
To do this we make use of a simple test circuit such as the following:

You should already recognise the voltmeter at the bottom; the meter on the left is an ammeter which is used to measure current in amps (hence the name "ammeter"). The symbol on the right is actually a variable resistor; this is used to vary the current in the circuit in order to get the number of pairs of current and voltage readings that we desire.
Also notice how the voltmeter measures the voltage (or p.d) across the component whilst the ammeter measures the current flowing through it.

Here is an example of a number of pairs of readings that we might expect from a typical component:

If we add a 3rd row to the table to show the calculation of Resistance, R, for each pair of values, then for this component we get:

So this particular component is one whose resistance remains constant no matter how the current through the component, and hence the voltage across it, changes.

Another way to make use of the table of data values is to use them to plot a simple graph of Current against P.d.

The straight line tells us that the Resistance of the component is CONSTANT.

If the graph had shown a curve then we would know that the resistance was not constant whilst current and voltage changed. This can happen eg in the case of a filament wire used to make an old style light bulb; its resistance increases as the current through it increases and the bulb gets hotter.

Keywords

Now that we have reached the end of this section we can focus on 4 keywords highlighted in the KS3 specification. You have already met each one, but it is important to learn them.

• Potential difference (voltage): The amount of energy shifted from the battery to the moving charge, or from the charge to circuit components, in volts (V).
• Resistance: A property of a component, making it difficult for charge to pass through, in ohms (Ω).
• Electrical conductor: A material that allows current to flow through it easily, and has a low resistance.
• Electrical insulator: A material that does not allow current to flow easily, and has a high resistance.