CONDUCTION OF ELECTRICITY

1
CONDUCTION OF ELECTRICITY
2

• 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

3
Class Experiment 1

• Charge the polythene rod with the duster and rub
  it onto the nanocoulombmeter.
• What happens? What charge does is gain?

• A coulombmeter stores the charge it measures
• Try using the acetate rod.
• What happens?
• Why there is a maximum charge that you can
  accumulate?

4
Class Experiment 2

• Use a free swinging charged rod and place in
  turn, a charged acetate and polythene rod next to
  it and observe attraction and repulsion.

• 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?

5

• 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

• Research
• The origin of the word electron
• Complete sheet Materials that cause static
  electricity

6
Class Experiment 3

• Rub a glass rod with silk and an amber rod with
  fur, use the nanocoulombmeter to detect any
  charge.

-
-

-

-

Thales of Miletus, William Gilbert
7
3 quarks
Electron wavefunction visualization

• 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
8
Electron and proton

• The proton (Greek p??t?? / 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.

• 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 electrum, which in turn came from the
  Greek word elektron (??e?t???) 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).

9

• 1.4 (d) explain frictional charging in terms of
  electrons removed from, or added to, surface
  atoms

10
Class Experiment 4 - Charging by induction.

• 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.

11
Important Concepts

• Electrostatic charge is defined as the absence or
  excess of electrons.
• Electrons are easily removed or added to an
  object by vigorously rubbing an object (rod) with
  another object (fur, silk, etc)
• There are two types of charge positive, which is
  the absence of electrons and negative which is
  the excess of electrons
• Charge is always conserved
• When two objects touch the electrostatic
  electrons transfer from one object to another
  until equilibrium is reached

• Charge by contact results in both objects having
  the same type of charge
• When a charged object is adjacent (but not
  touching) to an uncharged object the charges in
  the uncharged object redistribute
• There is no change in the net charge of the
  uncharged object
• An object charged by induction has the opposite
  charge as the charging object
• Initially the charge on the uncharged object
  polarizes and then a ground is provided to remove
  some of the charge
• The two objects never touch each other

12

• 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

13
Measuring Charge

• 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.

14
Class Experiment 5- Calculating the number of
electrons.

• 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.

15
Demonstration 1 'Spooning' charge

• 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

• Knowing the charge on an electron, calculate the
  number of electrons in a 'spoonful' of charge.

16

• 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 ?Q/?t
• 1.4 (i) recall that current is measured in ampère
  (A), where A Cs-1

17
Identifying charge carriers

• Demonstration 2 A filament lamp
• Demonstration 3 A spark in air
• Demonstration 4 Fluorescent tube
• Demonstration 5 Electrolysing copper sulphate
  solution

18
Class Experiment 6- Discharging a coulombmeter

• Charge a coulombmeter with a polythene rod to at
  least -1000nC. (Try by induction)
• Then discharge it by connecting a microammeter to
  it.

• Observe the microammeter as the coulombmeter
  discharges.

19
Class Experiment 7- Charging a coulombmeter with
a known current

• 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.

• Data should be measured for different currents
• Plot a graph of charge against time for the
  different currents

20

• 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)
• Where I current and ?Q is the charge that flows
  in a time ?t.

• The coulomb is not a base unit
• The base unit for Charge As

21

• 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.

• Charge can be found by working out the area of a
  current time graph

22
Demonstration 6- Shuttle ball

• 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.

• 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

23

• 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

24
Demonstration 7- Conduction by 'coloured' ions

• 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.
• In this demonstration you can see ions travelling
  as current flows. You should be struck by how
  slowly they go.

25

• 1. Electric currents are made of moving charged
  particles.
• 2. Ions in a current may move very slowly.

26
Try this

• Sheet of Derivation of I nAve

27
Calculate the drift velocity of electrons in a
circuit

• 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?

A
copper wire
constantan wire
n copper 8.0 x 10 28 n constantan 3.4 x 10 28