Transcript File

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.
+
-
Electron flow
direction
Conventional current is always shown as
going from plus to minus.
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
18
• The are 8.5 x 10 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 Law
The 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 crosssectional 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)