IT's PHYSICS

Friday, September 5, 2008

11:46 PM

APPLICATIONS OF CONVECTION and RADIATION

convection first:

1. electric kettles

- hot air rises cold air sinks?same principle here.
- the hot water will go up and the cold water will go down.
- allows the cold water to get boiled at the bottom.
- this is how ur water boils! (:

2. household water system

3. air conditioners
- the hot air will rise (thats why your air-cons are at the top) and get cooled
- the cooled air will then sink down and let the hotter air go up
- kind of like taking turns.
- soon they will all be the desired temperature.

4. refrigerators
- same as air conditioners (:

next radiation:

1. teapots
- shiny = can keep cool or hot
- black CANT keep cool or hot better.

2. the greenhouse
- the plants in the greenhouse emit infrared radiation too.
- this inturn causes the temperature to rise!

LOVE PHYSICS PEOPLE.

2:37 AM

EVAPORATION

liquid ----> gas = evaporation

BEFORE evaporation

AFTER evaporation

occurs at ANY temperature

thermal energy may be absorbed by the liquid from the surroundings

FACTORS.

1. temperature

increase in temperature of liquid = increase in rate of evaporation

2. humidity of surrouding air

increase humidity = decrease in rate of evaporation

3. surface area of the liquid

increase exposed surface area = increase in rate of evaporation

4. movement of air

increase in movement of air = increase in rate of evaporation

5. pressure

increase in pressure = decrease in rate of evaporation

6. boiling point of the liquid

increase in boiling point = decrease in rate of evaporation

LOVE PHYSICS PEOPLE.

1:12 AM

.:MELTING:.

solid ----> liquid = melting

pure substance = definite melting point

to melt,the intermolecular bonds have to break.
thermal energy breaks the bonds and allows the change in state from
solid ---> liquid.

.:SOLIDIFICATION:.

liquid -----> solid = solidification

pure substance = solidifies and freezes at a temperature equal to its melting point

LOVE PHYSICS PEOPLE.

Thursday, September 4, 2008

11:21 PM

GOOD conductors of heat and BAD conductors of heat

the GOOD ones

COOKING UTENSILS
- stainless steel,aluminium
- direct heating involved

SOLDERING
-mainly iron with a bit of copper
-copper is a better conductor than iron

the BAD ones

HANDLES OF APPLIANCES
- prevents us from getting burned!

TABLE MATS
- protects those nice wood table from getting damaged

SAWDUST
- used to readuce ice from melting.WHY?its an insulating property

WOODEN LADLES
- once again, it insulates.

WOOLLEN CLOTHES
- keeps people warm and toasty! (:

LOVE PHYSICS PEOPLE.

8:31 AM

WHAT IS INTERNAL ENERGY?

total energy of intermolecular bonds = internal energy
2 componente basically ONE: kinetic energy TWO: potential energy

KINETIC component

- vibration of the particles related to temperature

- higher the temperature = more vigorous the vibrations

example: microwave

POTENTIAL component

- stretching and compressing of the intermolecular bongs

- amount of potential energy = force between particles and spaces in between the particles

example: slinky

LOVE PHYSICS PEOPLE.

2:04 AM

the THREE states of MATTER

as you can see on the right

1. GAS ---------------------------->
* no fixed shape or volume
* low density
* compressible (can be packed in together)

2. LIQUID ------------------------>
* fixed volume but not fixed shape
* high density
* incompressible (cant be packed in together)

3. SOLID -------------------------->
* fised volume and shape
* normally hard and rigid
* high density
* aslo incompressible (also cant be packed in together)

nothing much for chapter 7 though.
mainly just this,the 3 states of matter.
easy enough?
so far...

LOVE PHYSICS PEOPLE.

2:01 AM

boiling

liquid ----> vapour = boiling
thermal energy pushes the atmosphere back,
thus allowing the molecules to get further apart,
spaced out.

condensation

vapour ---> liquid = condensation
thermal energy is given out during condensation.

LOVE PHYSICS PEOPLE.

12:24 AM

THERMAL ENERGY FLOWS FROM

HIGHER TEMPERATURE

to

to

to

to

to

to

to a region of
LOWER TEMPERATURE

SOOOO.....................
that means that thermal energy is transferred only when there is a difference in temperature.

if you dont believe me or the textbook,
then try this thing out.im quite sure you'll believe me after that,yeah?

alright,FIRST, you take 3 basins, fill them with water.
name them D, E and B.
then you fill basin D with cold water (not till your hand will freeze in kay?)
then you fill basin E with normal room temperature water (like tap water?)
then you fill basin B with hot water (dont get burned!it hurts like mad.)
ohkay then get a nice soft towel to dry your hands on later.
now you need a pair of hands.
it has to be from the same ONE person.
take one hand and place it in basin D, and the other in basin B.

your hand in D will feel cold.
WHY?
your hand is losing thermal energy to the water!

your hand in B will feel hot.
WHY?
your hand is gaining thermal energy to the water!

next, dry your hands on the towel,
then place BOTH hands in the basin E.
your hands will feel like it's in water.

oh yeah and neither hot nor cold.
WHY?
your hand is not gaining or losing thermal energy,
there is THERMAL EQUILIBRIUM
remember ohkay? (:

thermal energy is not transferred like
you put your hand there then WOOOMPH its transferred.
not all the time it's like that.
it happens at BBQs too.
so can you imagine?
if transferring thermal energy meant that you have to put your hand there,
then that means,
you must put your hand on the BBQ pit to feel the heat.
C-R-A-Z-Y right?
HAHAHAHAHAHAHA
you will get burned!
like i did...WAIT i didnt try it..i got burned another time.
I STILL HAVE THE MARK!
it's not nice..really..really..

well yeah, thermal energy is transferred by THREE ways.

ONE: CONDUCTION
TWO: CONVECTION
THREE: RADIATION
(chim?)

let's quote the textbook:
"conduction is the process of thermal energy transfer without any flow of the material medium."

ohkay so how does conduction work?
- the free electrons in metals move randomly around
- when there is thermal energy at one end of some metal rod
- it will cause the atoms at the end with the heat to vibrate vibrate and vibrate
- they will in turn cause all the others around them to vibrate vibrate and vibrate too!

and since (as we learned in chapter 7)
the atoms in a solid are closely packed together,
then it will be much easier for them to vibrate and collide with each other and thus
allow the heat to travel much much much faster.
agree?

let's quote the textbook:
"convection is the transfer of thermal energy by means of currents in a fluid (iquids or gases.)"

and so how does convection work?
ever heard of hot air rises and cold air sinks?
well basically,its kind of the same idea here.

let's imagine bowling water.
as the water at the bottom boils,
the water expands.
as it expands,it becomes less dense.
the water will then rise as the water around it is denser.
thus we can see that:
hot air = rise
cold air = sink
BUT!!reminder:
convection currents can occur ONLY in FLUIDS, which is liquids and gases.
the structures of solids does not allow the essential bulk movement in convection.
thus,there is no convection currents in solids.

let's quote the textbook:
"radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium."

radiation is actually different from both convection and conduction.
it does NOT require a medium to transfer energy!

the thermal energy from infrared waves are called radiant heat.
so infrared radiation is absorbed by everything!
this causes the temperature of the object/surface of itself to rise up!

factors that affect the rate of infrared radiation:

1. colour and texture.
- black absorbs better than white
- dull surfaces give off infrared radiation

2. surface temperature
- the closer the temperature of the surface/object to the surroundings,
the higher the rate of infrared radiation.

3. surface area
- larger surface area = higher rate of infrared radiation

LOVE PHYSICS PEOPLE.