Download CONDUCTION OF ELECTRICITY

CONDUCTION OF
ELECTRICITY
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1.4 (a)Understand how
attraction and repulsion
between rubbed
insulators can be
explained in terms of
charges on the
surfaces of these
insulators, and that just
two sorts of charge are
involved;
Class Experiment 1
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Charge the polythene
rod with the duster and
rub it onto the
nanocoulombmeter.
What happens? What
charge does is gain?
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A coulombmeter
stores the charge it
measures
Try using the acetate
rod.
What happens?
Why there is a
maximum charge that
you can accumulate?
Class Experiment 2
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Use a free swinging
charged rod and place
in turn, a charged
acetate and polythene
rod next to it and
observe attraction and
repulsion.
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What happens?
What happens to the
force of
attraction/repulsion as
you bring the rod
closer?
What is the name of the
force acting on the
rods?
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1.4 (b) understand that
the name negative
charge was arbitrarily
given to the sort of
charge on an amber
rod rubbed with fur, and
positive to that on a
glass rod rubbed with
silk;
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Research
The origin of the word
‘electron’
Complete sheet –
‘Materials that cause
static electricity’
Class Experiment 3
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Rub a glass rod with
silk and an amber rod
with fur, use the
nanocoulombmeter to
detect any charge.
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Thales of Miletus, William Gilbert
3 quarks
Electron wavefunction visualization
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1.4 (c) recall that
electrons can be shown
to have a negative
charge, and protons, a
positive;
Rutherford gold foil
experiment
Electron microscope
Electron diffraction
Proton cancer therapy
Electron and proton
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The English name electron is a
combination of the word
electric and the suffix -on, with
the latter now used to
designate a subatomic particle.
Both electric and electricity
are derived from the Latin
ēlectrum, which in turn came
from the Greek word ēlektron
(ήλεκτρον) for amber; a
gemstone that is formed from
the hardened sap of trees (the
ancient Greeks noticed that
amber, when rubbed with fur,
attracted small objects).
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The proton (Greek πρῶτον /
proton "first") is a subatomic
particle with an electric charge
of one positive fundamental
unit
Ernest Rutherford is generally
credited with the discovery of
the proton.
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1.4 (d) explain frictional
charging in terms of
electrons removed
from, or added to,
surface atoms;
Class Experiment 4 - Charging by
induction.
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Using a gold foil
electroscope, charge it by
induction
Using two metal spheres
and a charged polythene
rod, charge by induction
Use a nanocoulombmeter
to measure the different
polarities and magnitudes of
charge.
Important Concepts
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Electrostatic charge is defined as the
absence or excess of electrons.
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Charge by contact results in both objects
having the same type of charge
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Electrons are easily removed or added to
an object by vigorously rubbing an object
(rod) with another object (fur, silk, etc)
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When a charged object is adjacent (but
not touching) to an uncharged object the
charges in the uncharged object
redistribute
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There are two types of charge: positive,
which is the absence of electrons and
negative which is the excess of electrons
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There is no change in the net charge of
the uncharged object
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An object charged by induction has the
opposite charge as the charging object
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Initially the charge on the uncharged
object polarizes and then a ground is
provided to remove some of the charge
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The two objects never touch each other
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Charge is always conserved
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When two objects touch the electrostatic
electrons transfer from one object to
another until equilibrium is reached
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1.4 (e) recall that the
unit of charge is the
coulomb (C), and that
an electron's charge, e,
is a very small fraction
of a coulomb;
Measuring Charge
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The charge on one electron = -1.6 x 10 -19 C
1 Coulomb is the charge carried by about
6.25 x 10 18 electrons
Coulombmeters measure charge and show whether
it is positive or negative
They measure in nanocoulombs (1 nC= 1 x 10 -9 C)
How many electrons in 1nC?
10 -9 C/(1.6 x10 -19C/electron)– 6 250 000 000
electrons!!
You can see that vast numbers of electrons move
around when you charge a plastic rod.
Class Experiment 5- Calculating
the number of electrons.
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Rub a polythene rod for
20 seconds and
measure the charge on
the rod.
Work out how many
electrons have moved
to produce the charge
measured.
Repeat rubbing for 40
and 60 seconds.
Demonstration 1 'Spooning' charge
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Electric charge can be picked
up and carried by a spoon, just
as if it were sugar or milk!
Fix a metal spoon to an
insulating handle, touch it onto
the terminal of a high voltage
supply, and carry the spoon
across to a nanocoulombmeter,
onto which the charge is
dumped.
Repeat the action
What do you notice?
Try the spoon upside down.
Does this make a difference?
Try a bigger spoon. What
happens?
Try a bigger potential difference
from the supply. What happens
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Knowing the charge on an
electron, calculate the number
of electrons in a 'spoonful' of
charge.
internal 50MW
resistor
5 kV
supply
link to
earth
socket
bare
4mm
plug
insulating
handle
metal disk
on 4mm
plug
04
4
coulomb
meter
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1.4 (f) recall that charge can
flow through certain
materials, called
conductors;
1.4 (g) understand that
electric current is rate of
flow of charge;
1.4 (h) recall and use the
equation I = ΔQ/Δt;
1.4 (i) recall that current is
measured in ampère (A),
where A = Cs-1;
Identifying charge carriers
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Demonstration 2: A
filament lamp
Demonstration 3: A
spark in air
Demonstration 4:
Fluorescent tube
Demonstration 5:
Electrolysing copper
sulphate solution
Class Experiment 6Discharging a coulombmeter
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Charge a
coulombmeter with a
polythene rod to at
least -1000nC. (Try by
induction)
Then discharge it by
connecting a
microammeter to it.
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Observe the
microammeter as the
coulombmeter
discharges.
Class Experiment 7- Charging a
coulombmeter with a known current
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Charging a capacitor. In order
to collect data to show the link
between charge and current, it
is possible to charge a
capacitor up using a cell.
The current is measured using
a nanoammeter and is
controlled using a resistor.
The capacitor is a
nanocoulombmeter and so
both the current and charge
can be measured; the charge
should be measured every 5
seconds.
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Data should be measured for
different currents
Plot a graph of charge against
time for the different currents
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The current is the rate of
charge or the quantity of
charge that flows per
second.
Current is measured in
amperes
1 ampere = 1 coulomb per
second ( 1 Cs-1)
Q
I
t
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Where I = current and ΔQ is
the charge that flows in a
time Δt.
Q  I  t
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The coulomb is not a base
unit
The base unit for Charge =
As
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Charge can be found by
working out the area of a
current time graph
The rate of charge transfer
may not be constant. It
could be continually
changing with time.
If so, the size of the current
at any time is the gradient
of the graph of charge
against time.
dQ
I
dt
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Charge can be found by
working out the area of a
current time graph
Q  I  t
Demonstration 6- Shuttle ball
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Connect a pair of metal plates
across a large potential difference.
Hang a conducting ball in the gap
and let it touch one plate.
The ball can deliver charge, the ball
shuttles to and fro between the
plates.
A sensitive current meter connected
between the plates shows that a
current is flowing. It is likely to be
only a few microamperes.
You can calculate the charge carried
by the ball if you know the current
and the time of travel of the ball
between the plates, because the
current is the rate at which the ball
carries charge across the gap.
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With a constant p.d., move the
plates to different distances apart
and measure the number of shuttles
per second (of the ball) and the
current.
Then fix the distance between the
electrode plates, vary the p.d. and
measure the number of shuttles per
second (of the ball) and the current.
On the lap tops, plot graphs of
current against number of shuttles
per second
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1.4 (j) understand and
describe the
mechanism of
conduction in metals as
the drift of free
electrons;
1.4 (k) derive and use
the equation I = nAve
for free electrons
Demonstration 7- Conduction
by 'coloured' ions
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When in the early 1800s
people first studied currents
from batteries (they called
them 'Voltaic piles'), few
thought that anything 'went
round' a circuit. It was Faraday
who invented the word 'ion'
now used to describe charged
particles, from a Greek word
meaning 'traveller', to help
insist that, yes, something
does travel.
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In this demonstration you can
see ions travelling as current
flows. You should be struck by
how slowly they go.
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lead
soldered
microscope slide
filter paper
soaked
blob
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1. Electric currents are
made of moving
charged particles.
2. Ions in a current
may move very slowly.
Try this
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Sheet of Derivation of I =nAve
Calculate the drift velocity of
electrons in a circuit
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A
copper wire
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constantan wire
n copper = 8.0 x 10 28
n constantan = 3.4 x 10 28
Using a micrometer and an
ammeter. Take the
measurement needed to
calculate the drift speed of
the electrons through the
copper wire and constantan
wire.
What do you think will
happen when the electrons
enter the more resistive
constantan wire?