30/09/2013
• Superior Mirage, Svalbard, Northern Norway,
June 2013
Physics in Everyday Life
Physics in the Home 1:
Washing-up, Temperature and Microwaves
Andrew Robinson
Photo: Pamela and Jamey Burr
The Washing Up and the Laundry
• We’ll start with why we use
detergent in the bowl, dish
washer and washing machine
when we wash things
• To show how this works, first
we need to look at the nature
of matter (the building
blocks)
Atom: Basic Building Block of
Chemistry
• The nucleus in
black is composed
of protons and
neutrons
• The electrons orbit
around the
nucleus
This is the Rutherford Model: the electrons do not actually follow
circular orbitals, but they do move around the nucleus, so we still call
it orbiting
Structure of the Atom
• The shapes of the orbitals are much more
complicated than the simple picture
These shapes
represent high
probabilities of find
the electron
somewhere within the
shape.
Because of the
Uncertainty Principle,
we can never
determine the exact
position of the
electron
Molecules
• Collection of atoms
• Bonded together
• The chemical bond is an exchange or sharing
of electrons
Water molecule H2O
Oxygen gas O2
Nitrogen gas N2
Carbon Dioxide CO2
1
30/09/2013
Protons and Neutrons
• Protons and neutrons are about
the same mass, and they cluster
together in the nucleus.
• The proton has a property known
as “positive electric charge”
• The neutron has no charge
+
Electrons
• The electron has a much lower mass than the
proton ( 1/2000th as much) and orbits at a
distance from the nucleus.
• It is more mobile and can leave the atom, or
be shared by neighbouring atoms.
• This bonds the two atoms together with a
“chemical bond”
We often represent a proton with a + sign
Crystals
• Table Salt
• Sodium Chloride
• An electron is transferred
from the sodium
completely to the chlorine
• This is called an ionic
bond
• Ionic bonds tend to be
broken up in water – salt
dissolves
Amorphous Materials: Glass
• Glass has the
formula SiO2
(Silicon Dioxide)
• The crystal
structure is not
regular
Gypsum crystals in the Cueva de los Cristales (Cave
of the Crystals), Naica, Chihuahua, Mexico
Gypsum: Calcium sulphate dihydrate
Chemical formula CaSO4·2H2O.
Hydrocarbons
• Molecules made of hydrogen and carbon.
• The hydrogen and carbon share their
electrons in covalent bonds.
• Hydrocarbons with additional atoms like
oxygen or sulphur (sulfur!) in them are
generally known as “organic” molecules
2
30/09/2013
Octane
• Octane is an additive put in fuel to help give
more efficient combustion.
• It has a chemical formula of C4H8
• Four carbon and eight hydrogen atoms
• The carbons are arranged in a chain
• There are lots of ways of arranging 4 carbons
and 8 hydrogen atoms. Each of these isomers
is a different chemical, with different
properties
24 Isomers of
Octane!
n-Octane
n signifies
normal – the
linear chain
Also represented as a ball and stick
model, to give more information
about the structure
Weaker Bonds Between Atoms and
Molecules
• There are other bonds, which are much
weaker than chemical bonds.
• These are due to electric attraction and
repulsion between atoms. Chemists call them
Van der Waals forces
Water
• The water molecule has a
strong charge distribution
• It is a strongly polarised
molecule
δ+
δδ+
δ-
O
Slightly positive
δ+
δ+
C
Carbon Monoxide CO
C
δ-
O
Slightly negative
• Since water is highly polarised, it tries to
organise itself to maximize attraction between
molecules
Liquid Water –
random ordering (almost)
The space filled model
Surface Tension
• The strong electrical interactions between
water molecules gives a high surface tension
effect
Ice: Solid Water –
highly ordered crystals
3
30/09/2013
• At the surface, there is a strong sideways
interaction between the water molecules, and
this provides the surface tension force
Water Strider
• Supported by
surface tension
– think of the
surface of the
water as a
rubber sheet
Hydrophilic Molecules and Surfaces
Hydrophobic Molecules and Surfaces
• Water, as a polar solvent, dissolves other
things which are also polar, such as salt (NaCl),
or sugars.
• Molecules which dissolve well in water are
called hydrophilic
• Surfaces of objects which attract water are
called hydrophilic surfaces
• Molecules which do not dissolve well in water,
such as most oils (hydrocarbons) are called
hydrophobic
• Surfaces which repel water are called
hydrophobic
Olive oil
Oil and water do
not mix – the oil
is hydrophobic!
Red wine (with extra red dye!)
– Waterproof clothing
prevents the water from
spreading on the surface
(hydrophobic)
Detergent Molecules
• Soaps and detergents are designed so that the
molecules have hydrophobic parts and
hydrophilic parts
Washing up liquid (dish soap)
Maple syrup
Hydrophobic “tail”
Hydrophilic “head”
4
30/09/2013
• The detergent molecules tend to lower the
effect of the strong water to water
interactions (because they get in the way!)
• This lowers surface tension
• The detergent molecule can also remove
grease/oil
• The hydrophilic “heads” of the detergent pull
the grease off the surface
The hydrophobic “tails”
of the soap molecule
can dissolve in the
grease, but not in the
water
grease
• When the grease/oil comes off the surface, it
is surrounded by detergent molecules in a
large droplet called a micelle
Surface Tension and Water
• Water wants to arrange itself with a minimum
surface area
• This minimizes the number of molecules at
the surface and maximizes the number which
are completely surrounded by other water
molecules.
• The natural shape
for this is a sphere
http://en.wikipedia.org/wiki/Surface_tension (cutting a water drop video
Droplets Spreading
Water Spreads out over
whole surface
Total Wetting
Dew Drop
• Almost no
wetting
Partial Wetting
Hydrophilic Surface
Hydrophobic Surface
(Greasy or oily)
No Wetting
5
30/09/2013
• Water on glass – partial wetting
• This glass surface is mixed hydrophilic and
hydrophobic (so it’s dirty!)
What is Temperature?
• We use temperature in the home all the time
– Air conditioning
– Heating
– Cooking
– Air temperature outside
• But what is “temperature”?
Take a Break
• Bakewell Bridge, Derbyshire, England
Temperature and Energy
• Temperature is a number on a scale
• It is a measure of the average vibrational and
rotational energy of molecules or atoms
Vibrating water molecules
Video of vibrating solid
http://www.youtube.com/watch?v=ZARiF4-tM9I
Temperature Scales
The Celsius Scale
• The most fundamental scale is know as the
Kelvin Scale (K)
• Absolute zero (0 K) is the temperature at
which there is no energy of vibration, so all
molecular and atomic motion stops.
• This is important in the sciences, but does not
give a very useful scale for everyday use (since
absolute zero is -273oC (Celsius)
• Sometimes called the centigrade scale
(because there are 100 degrees C between the
two fixed standard points)
• The freezing point of water is defined as 0oC
• The boiling point of water is defined as 100oC
Anders Celsius
1701-1744
Swedish Astronomer and Scientist
He originally defined 100oC as freeing point and 0o as
boiling point, but everyone ignored this…
6
30/09/2013
The Fahrenheit Scale
• Fahrenheit invented the mercury in
glass thermometer and defined a
temperature scale with 180o between
the fixed points
• 32oF is the freezing point of water
• 212F is the boiling point of water
Thermometers
• Bimetal thermometer
• Metals tend to expand
when heated, so the strips
get longer at higher
temperature
• If one strip gets longer
than the other, then the
strip will curl
– In his original proposal, body temperature
was defined as 96oF
Daniel Gabriel Fahrenheit (1686 – 1736) DutchGerman-Polish (born in Danzig/Gdansk, lived in the
Netherlands)
• The thermometers use a
bimetallic spiral which
twists as the
temperature increases
• The bimetallic switch is
often used as a cut-off
when water gets hot in
an electric kettle
• Mercury thermometer ban in Europe 2013
Banned
Unknown
“Not a National priority”
• Mercury in glass thermometers
• These work because mercury, a liquid metal at
room temperature, expands as it gets hotter
General
purpose
thermometer,
typically -10oC
to +110oC
• Clinical
thermometers can
be shorter, because
they only have to
measure a limited
range of
temperatures
Microwave Ovens
• The microwave oven generates
microwave radiation
• Electromagnetic radiation with a
wavelength of around 10 cm.
• The radiation has to be exactly
the right wavelength, so that it
excites water molecules in the
food, and heats them up
http://www.sfu.ca/phys/346/121/resources/physics_of_microwave_ovens.pdf
7
30/09/2013
Standing Waves
• The microwaves bounce around inside the
oven, reflecting back on themselves.
• They form a standing wave inside the oven.
antinode
node
• Consequences of physics on microwave
cooking
What Does Microwave Energy do to
Water Molecules?
• Water in the food which is at the antinode
absorbs microwave radiation strongly.
• It vibrates more and so becomes hotter.
• It collides with molecules in the food itself and
makes them vibrate more too, so the whole
object gets hotter.
• The oven usually has a
turntable so that different
parts of the food goes
through the node
• The microwave oven can also be used as a
mailbox
1. It is difficult to heat up frozen objects, because ice
crystals do not absorb microwave radiation as much
as free water molecules
2. You can’t have metal objects in the microwave
because they will absorb the radiation AND disrupt
the standing wave patterns
3. A turntable is needed to spread the heat out
4. You are cooking by steaming, so getting food to
brown/crisp requires an infra red heater as well
Danby, North Yorkshire, UK. (Wikimedia)
8