CONDUCTION OF ELECTRICITY

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

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?

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?

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’

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

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-+

+

1.4 (c) recall that electrons can be shown to have a negative charge, and protons, a positive;

Electron diffraction

Electron microscope

Electron wavefunction visualization

3 quarks

Proton cancer therapy

Rutherford gold foilexperiment

Electron and proton 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.

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).

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

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.

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

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 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.

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

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.

link to earth socket

5 kV supply

internal 50M resistor

bare 4mm plug

044

coulomb meter

insulating handle

metal disk on 4mm plug

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

Demonstration 2: A filament lamp

Demonstration 3: A spark in air

Demonstration 4: Fluorescent tube

Demonstration 5: Electrolysing copper sulphate solution

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.

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

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 ΔQ is the charge that flows in a time Δt.

The coulomb is not a base unit

The base unit for Charge = As

t

QI

tIQ

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

dt

dQI

tIQ

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

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

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.

1. Electric currents are made of moving charged particles.

2. Ions in a current may move very slowly.

filter papersoaked

microscope slide blob

soldered

lead

Try this

Sheet of Derivation of I =nAve

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

constantan wire

copper wire

n copper = 8.0 x 10 28

n constantan = 3.4 x 10 28