Download Protons for Breakfast

PROTONS FOR
BREAKFAST
AUTUMN 2012
WEEK 2
LIGHT
1
This is what you said on the feedback forms in Week 1
You said …
Fascinating – but NOT
enough time both to follow
and understand, and to
retain/absorb properly as
well!
So many questions!
Do we know why certain
particles(?) are positive
and others negative.
Very
interesting. Is
light a wave or
particle. How
does heat travel
through
vacuum?
Michael said…
Yes indeed. But we will repeat things we said last
week this week. And next week! Hopefully after
three weeks things will begin to gel.
Yes – it seems impossible to find a place to start
asking questions– like trying to take the first bit
from a big apple!
To the best of my understanding we have absolutely
no idea what electric charge is. Given that exists –
we thing the positive and negative represent a kind
of symmetry. There you go – a profound mystery in
Week 1 – and one that lives in front of us all our
lives.
Glad you found it interesting.
We will talk about light this week, but very roughly light is
emitted by individual atoms and absorbed by individual atoms.
Since atoms are in just one place light appears to be ‘a
particle’ – i.e. something which exists in a particular place. But
when light travels, it travels as a wave.
‘Heat’ is the energy of jiggling of atoms. We will see in Week
3 that jiggling atoms emit electromagnetic waves. These
waves can then travel through the field which still exists even
in emptiness of space – the ‘not stuff’ – and then be absorbed
by other atoms. Thus some of the energy of jiggling of atoms
is transferred from atoms in one place to atoms in another
place.
Enjoyed thoroughly.
I am glad you enjoyed it. It makes me feel happy to
have caused a little pleasure in the world.
A very good lecture! Looking forward
Thank you. This week will be hard – but
to next week’s session!
worth it!
What is the random space between
The space is called the ‘vacuum field’
stuff called if there’s no atoms
and even though it is not made of
nearby?
particles it still exists. Whatever is
there is still able to propagate electric
waves.
2
You said …
At last someone has
explained the
difference (or lack of
it!) between electricity
and magnetism. I
would love an
explanation of how an
electromagnet works,
i.e. a substance that is
only a magnet when it
has a wire wrapped
round it that has
electricity running
through it….? Looking
forward to next week!
Michael said…
Yes. What we call ‘magnetism’ is really our view of what
is happening when electrical particles are moving.
Electromagnets are complicated but let me offer you
the beginning of an explanation.
 An electromagnet is made of a piece of iron with
wire wrapped around it.
 When an electric current flows through the wire it
makes a weak magnetic field typically around 0.001
to 0.010 tesla. For reference the Earth’s field is
typically less than 0.0001 tesla at the Earth’s
surface.
 When the iron is exposed to this weak field it
becomes magnetised. All materials become a little
bit magnetic when exposed to a magnetic field but
iron is exceptional. If the magnet is constructed
well, fields more intense than 1 tesla can be created.
 What is special about iron? As in atoms of all
substances, electrons in atoms of iron arrange their
orbits so as to minimise their mutual electric
repulsion. This causes them to orbit the iron nucleus
in one direction. This is quite common. But in iron,
electrons in neighbouring atoms also orbit in the
same sense. Again this is because of the electrical
repulsion between electrons. The result is that
microscopically each atom acts like a little loop of
electric current – a tiny magnet – and that all the
neighbouring atoms of are lined up with the loops in
the same direction.
 Although all the atoms are aligned with their
neighbours, normally one region of iron – called a
domain – balances out the magnetic field of other
regions. This is like the way small bar magnets
spontaneously align themselves so as to cancel out
each others magnetic effect.
 If the iron is prepared correctly, even the weak
field of the wire is able to make some domains grow
and others shrink – amplifying the effect field of
wire.
I told you it was complicated!
3
You said …
Wow, I never
thought I was
eating protons,
electrons and
neutrons! Now
there’s a fact!
But why do some
atoms have a
positive charge
and some have a
negative charge?
Michael said…
It is indeed a fact and one that struck me as so profound
that I named the course after it!
Atoms are normally electrically neutral. The protons in the
nucleus attract just the right number of electrons to create
an electrically neutral object. But atoms differ in how strong
the electrical force is on their outer ‘valence’ electrons.
So when two different types of atoms come close one type
of atom can ‘steal’ an electron from another atom. This
creates an atom with an extra electron – called a negative
ion, and an atom lacking an electron, called a positive ion.
This happens all the time. Sometimes we call these
exchanges a chemical reaction and sometimes we call them
physics – like when the rubber in the balloons stole electrons
from the wool in my shirt.
Are you saying there is no ‘magnetic
Nearly. Magnetic forces obviously
force’ that it’s only another way of
exist – how else would fridge magnets
looking at electric forces? So is there
work! But what we call ‘magnetic
no such thing as magnetism?
effects’ arise from the motion of
electric charges.
So in a fundamental sense, magnetic
effects can be understood as being
basically electrical.
How does a radio
We have a really nice animation of this week. But briefly
station send out
electrons in the antenna of the transmitter jiggle up and
the radio signal?
down and send a wave through the ‘not stuff’ and a little
while later electrons in the antenna of a radio jiggle up and
down in response.
Really like the links to real life and
I am glad you liked it. But science is
useful. Also like the use of video and
about ‘real life ‘ so I can’t think what I
sound as a different way of
could have done that would not have
communicating an idea.
touched our real lives.
Great. This week I will overwhelm you!
Well done. I’m interested and not
overwhelmed and keen to come back
But next week – Week 3 – I make up
next week.
for it!
I’m interested about the
Yes. Colour is such a profound experience that
different colours of light
understanding what is happening when we
for next week.
experience different colours can be
philosophically disturbing! Watch out!
4
You said …
Michael said…
I definitely learnt something!
I am glad you learned something and enjoyed
Very interesting and fun.
yourself. But I wonder what you learned… !?
 What are we
Great Questions – well enunciated. Unfortunately quite
measuring when tricky to answer!
we talk about
 Technically a volt is ‘an amount of energy per unit
volts?
charge’. If you know what the units mean, one volt is one
 Why does the
joule of energy per coulomb of charge. So a high voltage
large number of
means we are giving each electrical particle – an
volts generated
electron in this case - lots of energy.
by the van de
 The Van der Graaf (VdG) generator only separates
Graaf machine
electric charge slowly. The electric current – the
not cause
amount of electric charge separated per second – is
damage when
probably only a millionth of an ampere (amp) or so.
someone is
 But the dome can only contain a small amount of charge
holding on, but
and so the voltage builds up quickly on the dome.
will hurt if that  This is analogous to the way the level of water rises
person lets go
quickly when water drips into a narrow tube However f
before it is
the same flow drips into a broad tube the level only
discharged?
rises slowly.
 And how do
 When you hold onto the dome – you too become filled
amps fit into all
with electricity. However it takes a lot of electricity to
this?
‘fill you up’ – the voltage only builds up slowly.
Technically we say the person has a much larger
capacitance than the dome. So when the VdG pours
charge onto the dome and the person, the voltage rises
much more slowly than the rate at which the voltage
rises for the dome alone – it often took minutes before
the person became charged up.
 Since charges of the same sign repel – if charges flow
on your hair – the hair tries to get as far away from
your body as it can – hence the amazing hair ‘sculptures’.
 If the person lets go before the whole dome is
discharged, then when they separate - all of a sudden
the dome charges up very quickly and a small amount of
electric charge on the dome will jump the gap.
 An ampere is a measure of the amount of electric
charge which flows per second – one ampere (or amp for
short) is the flow of 1 coulomb of charge per second.
5
You said …
Michael said…
Spent the last term in
Yes. My own son has been learning about atoms and
school learning about atoms I can’t quite understand how his teachers make it
and ‘stuff’ like that and I’ve as dull as they do! Atoms – so far beyond our
learnt more here! Thank
direct personal experience – are one of humanity’s
you!!
greatest discoveries. You’re welcome!
Can you change colour Mmmm. The answer is basically, ‘No’, but this is a very
of electrons?
interesting question. We will look at what we mean by
colour this week and perhaps then the simple answer ‘No’
will make a little more sense.
Fascinating!!
Indeed.
How does a radio pick We will look at that this week with a nice animation. I
up signals?
think I gave a preview answer a little way above.
Looking forward Excellent. I am too – but it has been a crazily busy week. The
to next Tuesday! morning after the last course I travelled to Holland to give a
training course on how to measure temperature at eth
European Space Agency’s technology research centre. I didn’t
even get to use my one weak joke. I meant to say “Hey, this is
not Rocket Scien… Oh… yes it is!” But I forgot.
Very informational, can’t
Yes, this week will be full of even more information!
wait for next week.
Yes, very good. Can’t wait till next week.
Well here we are already!
Great, really enjoyed it.
I am glad.
What makes “hard” Great question. Something is hard if when the atoms are
substances hard and pushed they don’t get out of the way.
soft substances
An example of a hard material would be diamond. When
soft?
one pushes the surface atoms in a diamond crystal the
atoms try to move. But the electrical energy increases
very quickly as the atoms are displaced and so it takes
lots of energy to move the atoms even a tiny distance. So
pushing hard doesn’t change the shape.
An example of a soft material would be rubber. In
rubber, atoms are arranged in long strings that are
tangled together. However the strings can slide past each
other. When you press the surface of rubber, the
electrical interactions cause the atoms to change shape.
Does that make sense?
This was very interesting and I would
You can! And if you are reading this –
love to come back again! 
you have!
Interesting, interactive, fun! 
Excellent.
6
You said …
Thank you for such
a wonderful first
session, it was
definitely a lot
better than
science class! Why
are nuclei so very
heavy if they are
so tiny?
Michael said…
I am glad you enjoyed it and what a splendid question.
As I understand it nobody is able to explain why the
fundamental particles have the exact masses that they do.
Electrons are – to the best of our knowledge –truly
fundamental. By this I mean that we have never seen
evidence that they have any kind of internal structure –
they behave like fuzzy balls of electric charge no matter
how hard they are smashed together. Nobody has ever
explained why electrons have the mass that they do – it is a
mystery.
Protons and neutrons are made of component particles we
call quarks. We infer the existence of quarks because when
we smash protons into other protons they sometimes go
straight through each other – and other times they hit
sharp point-like scattering centres. We explain the mass of
protons and neutrons as arising from the masses of their
component quarks. But then we can’t explain why the quarks
have the masses that they do!
I’ve learnt some Greek words – electron and atom.
Wow! Did we really cover
How small the earth is compared to the sun. The
all of that! Not bad for
importance of electricity, atoms, protons and fields. Week 1!
The use of humour, audience participation, film and
music in teaching.
I enjoyed the interactive
Excellent. I don’t really have the hair for it but I
stuff! I liked my hair
have seen photos of some splendid examples. I
standing on end. Why
think a mix of listening and looking an interacting
doesn’t your QR code
is a nice way to learn – and not to drop off! My
reader work from the back guess is that the resolution of your camera-phone
seats?
is not sufficient to read all the detail in the code
at that distance. Try sitting nearer the front… if
you dare!
7
You said …
I enjoyed the
practical things and
having my hair stand
on end!
Question: How do
you know all the info
about the universe?
Thank you!
Michael said…
Excellent – another person who likes to be electrified.
Answer. Well I personally know ‘info about the universe’
because I have read about it – and then tried to write
about – and writing makes one face one’s own ignorance.
People in general know stuff by inferring it from their
observations. This involves looking in detail at – for
example: the motions of the planets; the colour of the
stars; the radiation which arrives at Earth from the
universe; the types of atoms we find on Earth and those
we find on the Sun. By putting lots and lots of information
together and seeking simple explanations people have very
slowly – over thousands of years – figured things out. You
are alive at a time of extraordinary knowledge.
Marvellous! Really enjoyable and looking I am glad you enjoyed it. 
forward to the next session!
I like the stuff and not stuff analogy.
Good. It is actually quite a good
analogy. When physicists describe the
difference they use very complicated
words!
How do you not get confused when you I sympathise. But we will practice with
simplify numbers, e.g. 100 = 102 with
these ideas many times in the coming
2
4
10 10  2 = 20 or e.g. 10000 = 10 10  weeks.
4 = 40. It mind boggles me!
What does an
What does an ioniser do? It makes ions . The ioniser
ioniser do? Why consists of very sharp pins and an electrical circuit which
would that be
pushes electrons towards the sharp end of the pins.
promoted as
being ‘healthy’
As the charge accumulates it attracts molecules to it – just
or helpful?
like our charged balloons did. However as soon as the molecule
touches the electron and sticks to it – it is repelled from the
pin. So negative ions stream away from the pins into your
room. The ions attract tiny particles of dust and cause them
to agglomerate into larger particles which settle out onto
your furniture and floor. After a few days the amount of dust
in the air is generally reduced. Aside from helping to reduce
airborne particulates, I don’t know of any healthful
applications of ionisers.
8
You said …
Michael said…
Ever since I
Before I answer this , please let me say well done for holding on
was a child
to your view of what makes sense. The motto of the Royal
Society – a top-notch science club – is “Take nobody’s word for
there is one
‘fact’ that has it!” .
never sat
However in this case it turns out to be true. People were aware
easily with
of the correlation between tides and the phases of the moon for
me. We are
centuries before Isaac Newton explained how such an effect
told that the could occur through the force of gravity. But things are not as
moon is
simple as is normally told. There are two components to what
responsible
causes the tides. The first is to do with gravity and the second
for causing
can be summarised as the ‘bath tub’ effect.
Gravity The gravitational effect of the moon at the Earth’s
tides – this
seems highly
surface is tiny – only a few millionths of the effect of the
implausible to Earth’s gravity. So the weight a human being ‘weighing’ 80.0000
me: how can
kg would change to 79.9998 kg when the moon was overhead
the moon so
‘lifting’ them up. Tides are affected by an even smaller force –
far away
the difference between the effect of the moon on different
cause great
sides of planet Earth. The effect is tiny. But it is regular.
Bathtub Next time you are in a bathtub try moving your hand
tracts of
heavy water
forwards and backwards so as to make the water ‘slosh’ from
move so
the front end of the bath to the back. If you move your hands
much? The
at just the right time you will slowly be able to get many
tidal range in kilograms of water ‘sloshing’ backwards and forwards with very
some parts of little application of force.
The tides are indeed caused a tiny gravitational effect, but
the world is
huge e.g.
over many rotations of the Earth the effect builds up until it
Channel
causes the seas and oceans to ‘slosh’ about in their ocean basins
Islands.
giving rise to the tides. The local shape of the sea floor (called
Surely the
‘bathymetry’ in a reference to the Greek word for depth and not
moon can’t
baths!) can cause some places (such as the Severn estuary) to
really cause
have exceptionally large tides
that?
Just brilliant!
Thank you 
Comment: Interesting and
We aim to educate and entertain - just like the
informative evening.
BBC – except that we don’t all resign every week 
Entertaining presentations. Great practical
I am glad you enjoyed it.
demonstrations to support talk.
9
You said …
You said rubbing
your balloon on
wool gives the
balloon negative
charge. Do any
materials leave a
positive charge?
Michael said…
Yes. When two materials touch.
Electrons in the valence
electrons of atoms from one
material (let’s call it Material A)
may prefer to move to the other
material (let’s call it Material B)
and so make B slightly negative.
Alternatively electrons could
move the other way. The details
are complicated but the effect is
roughly encapsulated in what is
called the tribo-electric series.
This list starts with the
materials that most easily loose
electrons (becoming positive)
and ends up with those that most
like to gain them (becoming
negative) . In this list taken from
the authoritative Wikipedia, wool
is above rubber so I think
electrons will leave the wool and
make the balloon negative. If we
had rubbed the balloon with
some of the materials below
rubber in this list then the
balloon would have become
positively charged.
Tribo-electric Series
Most Electron hating
(least Electrophilic)
Human skin
Acrylic, Lucite
Leather
Glass
Quartz
Human hair
Nylon
Wool <<<<<<<<
Lead
Silk
Aluminium
Paper (Small positive charge)
Cotton (No charge)
Steel (No charge)
Wood (Small negative charge)
Amber
Polystyrene
Rubber balloon <<<<<
Resins
Hard rubber
Nickel, Copper
Gold, Platinum
Acetate, Rayon
Synthetic rubber
Polyester
Styrene (Styrofoam)
Orlon
Plastic wrap
Polyethylene (like Scotch tape)
Vinyl (PVC)
Silicon
Teflon
Silicone rubber
Most Electron loving (Most
Electrophilic)
10