Monday learning objectives explain that electric current is a net flow of charged particles; (c)...

Monday learning objectives• explain that electric current is a net flow of

• charged particles;

• (c) explain what is meant by conventional current and electron flow;

• (d) select and use the equation ΔQ = IΔt;

• (e) define the coulomb;

• (g) recall and use the elementary charge

e = 1.6 x 10-19 C;

What is electrical current?• Current is the rate of flow of charge or a net

flow of charged particles;• ΔQ = I• Δt• Or rearranged to ΔQ = IΔt - how it is given

on your data sheet• I is current (measured in Amperes)• Q is charge (measured in Coulombs)• t is charge (measured in seconds);

What causes electrical current?

• In most circuits, it is moving electrons. This is because most circuitry is made using metals and the metals have free electrons which carry the charge.

• In some circuits, such as those for electroplating, it can be moving ions. (We will look at those on Thursday)

Direction of electrical current.

+ -

Conventional current is always shown as going from plus to minus.

Electron flow direction

Electrons move in the opposite direction to the conventional direction of electric current.

+ -

Conventional current is always shown as going from plus to minus.

The earliest guesses, hundreds of years ago, were wrong. There is no point in changing.

+ -

Conventional current is always shown as going from plus to minus.

Why?

• The electron wasn’t discovered until 1897 by J J Thomson. The description of “conventional current” was completely established.

• Confusingly we also need to understand both conventional current and the electron flow (from negative to positive; i.e. in the opposite direction to conventional current)

The quantity of of electrical charge was first measured by

Coulomb• His full name was Charles Augustin de

Coulomb. He lived from1736 until 1806.

• The unit of electrical charge is called the ‘Coulomb’ in his honour.

• The are 8.5 x 1018 electrons in one Coulomb of negative charge.

The quantity of of electrical charge was first measured by

Coulomb• The are 8.5 x 1018 electrons in one Coulomb

of negative charge.

• This is Eight million, five hundred thousand million, million electrons.

• Or: 8,500, 000, 000, 000, 000, 000 million electrons in one negative Coulomb of charge.

The quantity of of electrical charge was first measured by

Coulomb• 8, 500, 000, 000, 000, 000, 000 million

electrons are in 1 Coulomb of negative electrical charge.

• If electrons were the size of an ordinary pea, this number of electrons would fill about a quarter of a million large football stadiums to overflowing.

The Coulomb definition

• Coulomb – The charge flowing past a point in 1s when current is 1A

The electron

• Electrons are all the same so they have the same charge. The charge on an electron is

• e = 1.6 x 10-19 C

Example

• What is the current if 5.0x1014 electrons pass per second?

• 5x1014 x 1.6 x10-19

• = 8 x10-5C

• How long would it take for 1 Coulomb to pass?

• 1.0 = 8 x10-5 x t so

• t = 12500s (3.47 hours!!!)

Question 4 hints

• Hint 1 – speed = dist/time

• Hint 2 – time = 8.3x10-9s

Electric current moves very slowly.

• The mean speed of the electric current in a metal is often less than 1 mm per second.

• One electron that leaves the negative terminal of the battery can take a few minute to reach the positive terminal of the battery.

• It collides with many millions of other electrons during its travel around the circuit.

What actually goes on in a metal?

• Look at figure 1

• Each copper atom has one delocalised electron. In physics you may see them referred to as conduction electrons (because they’re the ones that do the conducting!)

• Look at figure 2a

• The electron is moving around randomly from one metal ion to the next. It is totally random so on average there is no direction of motion. Some electrons will move to the left some to the right. No net movement

• Look at figure 2b

• The is now a current. The electrons are still moving from atom to atom but on average there is an overall movement to the left. So the current will travel to the left

Wednesday’s Learning Objectives

• (f) describe how an ammeter may be used to

• measure the current in a circuit;

• (h) describe Kirchhoff’s first law and appreciate that this is a consequence of conservation of charge;

• (b) explain that electric current in a metal is due to the movement of electrons, whereas in an electrolyte the current is due to the

• movement of ions;

Ammeters

• At GCSE you saw that an ammeter measures current (in Amperes).

RACE

• Hint; Think how an ammeter needs to be connected

How does an ammeter work?

• To understand this you need a bit of science that isn’t on your syllabus

• Ampere's LawThe magnetic field in space around an electric current is proportional to the electric current which serves as its source,

So

• A simple ammeter uses small coils that rotate in a magnetic field.

• Digital ammeters use a current sensor and a digital display

Important

• Your ammeter must be placed in series and it must have negligible (effectively zero) resistance. This is because if the ammeter is measuring current it shouldn’t affect the current (by reducing it) that you’re trying to observe.

Why in Series?

• Either all the current or a known fraction of that current must pass through the ammeter for it to measure it.

Multimeters

• Here you can change what fraction of the current flows through it

Mondays LOs

• (i)state what is meant by the term mean drift velocity of charge carriers;

• (j) select and use the equation I = Anev;

• (k) describe the difference between conductors, semiconductors and insulators in terms of the number density n.

Drift velocity

• As you saw on Thursday when an electrical current is applied to a metal the electrons will move (on average) slowly in one direction. We call this drift.

• We can work out how quickly this is happening. We call this drift velocity.

Why do they move slowly?

• As the electrons travel through a material they collide with an atom.

• the density of the material will dictate how likely that is to happen (the denser the material the more likely a collision is)

Number Density

• This is the number of charge carriers per m3. It can be worked out using the density and the atomic mass. Sometimes in exams it is called charge carrier density.

• If you have twice as many charge carriers in a given space then there will be twice as much current

Effect of number density

• If you have twice as many charge carriers in a given space then there will be twice as much current

• The bigger the number density is the better a material will be at conducting.

• Insulators will have almost no conduction electrons so there number density will be low.

Drift Velocity Equation

Where does it come from? Not on exam

If we look at a section of wire

• We can work out its volume. This is it’s drift velocity (to give us the length travelled in 1 second) multiplied by the X-sectional area.

• So the number of electrons in this volume will be equal to the number density multiplied by the volume. (i.e. nvA)

We already worked out this

• We’ve already seen that doubling the number density doubles the current.

• This also tells us that doubling the cross-sectional area doubles the current (because there is twice as much space for them to move in so twice as many can drift.

Also

• If we move the charge carriers through at twice the speed then 2 times as much current will pass through in the same time.

Finally

• If each charge carrier carried twice as much charge then the current would double too.

We’ve looked at conductors and insulators

• There are also semi conductors which would have a very low n.

• By adding a small impurity (known as doping) to them we can increase n.

• Therefore the conduction electrons can travel faster (don’t worry about why this is)