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KS3 3.2.2 Current

In this section we learn more about the way current flows in series and parallel circuits, then we will learn more about charges and how they create electric fields.

Current in Series circuits

In the previous section, 3.2.1, we listed two important facts about series circuits but the first, concerning current, we said we would leave until this section.

Here is the fact again:

For components connected in series:
1) the same current flows through each component or through each part of the series circuit.

So each Ammeter will indicate the same value.

As we explained in section 3.2.1, this is because all the components in a series circuit are connected like a series of TV programmes; you have to watch each one in order to watch the next; if one slows you down then your whole progress through the series slows down.

We also said that the charges flowing through the circuit are like cars bumper to bumper, so if one is slowed or stopped then they are all slowed or stopped since they all pass round and round the circuit through each component and through the voltage supply (the battery); like this:

So, in the first diagram, if Ammeter 1 reads 1.5A, then so will Ammeter 2. Easy, isn't it?

Its not possible to produce a "difficult" question concerning current in a series circuit.
But let's have a go:

Consider the following circuit:

If ammeter 1 reads a current of 2.3A, what will be the reading on ammeter 2?

Well, its just a series circuit, so the current is the same at all points in the circuit, so if it is 2.3A at one point then it is 2.3A at any other point.
So, the answer is 2.3A

Current in Parallel circuits

Here is a typical "parallel" circuit, seen in the previous section 3.2.1:

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 parallel paths.

We dealt with the first statement in the previous section, so we will consider the second statement.

Consider the following circuit which shows currents I1 and I2, each of 0.5A, flowing down the parallel paths through the lamps.
The question is: what is the current at the junction marked by the red dot and by IT ? (where IT stands for "total" current)

Well the second statement says "the total current (IT) is the sum of the currents through the parallel paths."

So IT must be equal to 0.5 A plus 0.5 A;
IT = 1.0 A

Using the letters we can write an equation:

So in the above example you can see that I1 = 0.5 A and I2 = 0.5 A
and IT = I1 + I2
so IT = 0.5 + 0.5
therefore IT = 1.0 A

The equation just puts into symbols what we wrote above in words; you could learn either.

Notice - on the other side of the lamps there is another junction, as shown below:

But the exact same statement and equation apply,
IT = 0.5 + 0.5
IT = 1.0 A

Quick Question:
A current flows from a battery and splits at a junction into two parallel paths; 1.2 A flows down one path, whilst 1.6 A flows down the other path. What current has flowed from the battery? (remember to leave a space before your unit)

Current: Flow of electric charge, in amperes (A).
In series: If components in a circuit are on the same loop.
In parallel: If some components are on separate loops.
Electric Field: The area where other objects feel an electrostatic force.

Static Electricity (or Electrostatics)

The study of static electricity is known as Electrostatics.

So, they are both about electric charges, but one is about moving charges whilst the other is about STATIC or stationary charges.

Static electricity will build up on objects whenever the charges cant flow or get away.
So, static electricity effects most often involve insulators, eg rubbing a balloon (insulator) on a jumper (insulator), making the balloon stick to a wall (another insulator!).
(Static electricity effects can involve conductors but only if the conductor is isolated from all other conductors, eg a metal sphere dangling from a piece of string, or mounted on a piece of plastic.)

What causes Static Electricity effects?

Static electricity effects such as dust sticking to a TV screen as soon as you polish it, or a balloon to stick to a wall, or "lightning", are all due to.......friction.

Friction causes charging

If you take a polythene rod and rub it with a woollen duster/cloth you will charge the rod - a fact!
But how can you be sure of this?
Take another polythene rod, rub it with a woollen duster then suspend the two rubbed rods near each other.
You will find that the rods move away from each other; they repel each other.

The only reason they do this is because they have both been charged identically.
The rubbing with the cloth has charged them; friction has charged them.

Since this repelling force is due to electrostatic charging, we call it an electrostatic force.

So, Friction Causes Charging

Since we are describing a simple experiment, let's add another step.
If you change one of the polythene rods for a cellulose acetate rod, rub it with the woollen duster and place it near to the first rubbed poly rod, what happens?

They attract each other; another electrostatic force.

Ok, friction causes charging, but why? And why do different materials charge differently?

Why does friction cause charging?

It's all to do with the atom!

I hope you know that everything is made up of atoms and that atoms consist of a nucleus containing protons and neutrons, surrounded by orbiting electrons.
We often draw an atom like this:

The Electrons are negatively charged
The Protons are positively charged
(The Neutrons are not charged)

The Protons and Neutrons are held tightly within the Nucleus
The Electrons are tiny compared to the Protons & Neutrons and are not held tightly to the nucleus.
Most atoms, like the one shown here, have an equal number of positive and negative charges, making them "not charged" or neutral.

When a Polythene rod is rubbed with a woollen duster, friction occurs and this literally rubs the loosely held electrons from some of the atoms within the duster onto the Polythene rod, so the Polythene rod gains electrons.
Since the Polythene rod now has extra electrons it has become negatively charged.
And, don't forget, since the duster has lost those electrons, it has become positively charged.

When a Cellulose Acetate rod is rubbed with a woollen duster, friction occurs, rubbing loosely held electrons from the rod onto the duster.
So, this time, the rod loses electrons.
Since the Acetate rod now has less electrons than protons it has become positively charged.
And, don't forget, since the duster has gained those electrons, it has become negatively charged.

Notice that charging is ALWAYS due to the transfer of electrons from one material to another.
It is NEVER due to the movement of protons or neutrons; they are held firmly within their nucleus and do not move.
You can't always predict which way the electrons will transfer during friction but you now know which way they move for Polythene and for Acetate, so learn these movements.

The Law of Electrostatics

Now that we know about the two types of charge, we can look back to the 2 diagrams of the rods dangling from the retort stands and write a conclusion which is also a Law:

"Like charged objects repel each other,
Unlike charged objects attract each other."

So, we now know that the 2 rubbed poly rods are both negatively charged, so they repelled each other;
and we now know that the rubbed acetate rod is positively charged so when brought near to a poly rod, they attracted each other.


Friction causes charging.
It causes electrons to transfer from one object onto another.
The object that gains electrons becomes Negatively charged.
The object that loses electrons becomes Positively charged.
Unlike charged objects attract.
Like charged objects repel.

Have a go at these questions by filling in the blanks.

Electric Fields

Around every charged object, such as the rods mentioned above, there is an Electric Field.
We can't see this field with our eyes but its presence can be detected, for example, by bringing another charged object into the field at which point we detect a force, or we bring our charged object up to certain uncharged objects like a piece of paper and again we detect a force.

This Electric Field is another example of a "force field".
You might have met the idea of a "force field" when you were studying Gravity. Back then you learnt that there was a Gravitational Field around a mass such as a planet, and that other masses that came into the field experienced a force which we call "weight". This was an example of a force which "acted at a distance"; no contact was needed.

So, the Electric Field, like gravity, produces a force which "acts at a distance"; no contact is needed. We call it an Electrostatic force.

Describing a simple Electric Field

One of the simplest Electric Fields is that around an isolated charged sphere, and the simplest way to describe it is to draw it!

The lines are called field lines and, as you can see, they radiate outwards from the positively charged spherical object. (NB. They would radiate inwards if the sphere was negatively charged.)

If another charged object comes within this field, it experiences a force. If it is a positively charged object it feels a force in the direction of the arrows (ie a repelling force in our example); if it is a negatively charged object it feels a force in the opposite direction to the arrows (ie an attractive force in our example). This agrees with the Law of Electrostatics, doesn't it?

The force felt is greatest where the field lines are closest; so an object closer to the sphere feels a stronger force, which is what you would expect, isn't it.
And the force is felt even when there is no contact between the Sphere and the other charges, which is what we said earlier.

You could draw your own diagram for the field pattern around a Negatively Charged Sphere; and you could show the size and direction of the forces felt by other positive and negative objects that enter the field at different distances from your sphere.

Now, to finish off this section, have a go at the following questions.


Negatively charged: An object that has gained electrons as a result of the charging process.
Positively charged: An object that has lost electrons as a result of the charging process.
Electrons: Tiny particles which are part of atoms and carry a negative charge.
Charged up: When materials are rubbed together, electrons move from one surface to the other.
Electrostatic force: Non-contact force between two charged objects.