3.1 Density
-Calculating volume
+​ Calculating the volume of a regular block of material -> measure the lengths of the
sides
+​ If the shape of the material is irregular -> work out the volume using the displacement
method (irregular = sides or angles of the shape are not equal)
+​ The object is placed into a measuring cylinder of water
+​ The increase in the volume of the water is the volume of the object
-Calculating density (density = mass/volume)
definition: density is mass per unit volume of a substance
-Comparing densities
+​ Gases have lower densities than liquids or solids because the particles in a gas are far
apart in a given volume, leading to a lower mass per unit volume
=> The materials with the highest densities are all solids
=> for the elements, the density usually increases as the atomic number increases
(mass is bigger)
-Floating and sinking
+​ People can tell whether something will float or sink in water by comparing its density
with that of water (if something is more dense than water, it will sink in water/ vice
versa)
-Liquids of different density
+​ When liquids are added together carefully, the less dense liquids will float on the
more dense liquids.
+​ The liquids that do not mix will form separate layers
-Gases of different density
+​ A balloon with helium is less dense than the air
+​ Solids and liquids are difficult to compress because their particles are already in
contact
+​ Gases are easy to compress because their particles are far apart
+​ When a gas is compressed, the same number of particles is in a smaller volume ->
density increases
+​ If a gas expands (increase in size), the same number of particles is in a larger volume
-> density decreases
+​ Heat causes gas to expand -> hot air inside balloon is less dense than the colder air
outside-> hot air balloon can float
3.2 Heat and temperature
HEAT
-Thermal energy can be transferred between objects
-Thermal energy can be stored in an object
-The thermal energy that is stored will eventually dissipate into the surroundings
-Thermal energy is measured in joules (J)
-When the thermal energy of an object increases, the particles in the object start to vibrate
faster -> the energy of the particles increases
-The water at the higher temperature contains particles that are moving faster-> total thermal
energy (heat) of the particles in the water is higher
-Larger volume of water in 1 glass-> more particles-> the total thermal energy (heat) of all
these particles is greater than in the water with fewer particles
-Temperature
+​ Temperature is not the same as heat
+​ Temperature gives information about 2 things [the direction that thermal energy will
be transferred, the average energy of the particles in an object(Eg temp of soup is
higher than temp of water-> 100 particles in the soup have more energy than 100
particles in water]
+​ The larger the temperature difference between 2 objects, the faster (the rate, the
speed) the thermal energy transfer
+​ Heat tells us about the total energy of the particles (total thermal energy)
-Heat and temperature in a sparkler (Why does 1 spark not cause serious burns?)
-The reason is:
+​ mass of spark is very small
+​ the temperature difference between the air and the spark is very large
+​ There are fewer particles in the spark than in the main part of the sparkler-> total
particle energy is much smaller ->total thermal energy or heat of the spark is very
small
+​ As the temperature difference between the spark and the air is large, thermal energy
will be transferred from the spark to the air quickly -> when the spark falls to the skin,
its temperature and heat have both decreased significantly
-How low can you go?
+​ As the temperature of an object decreases, the particles move more slowly
+​ If particles stop moving, this would be lowest possible temperature-> absolute zero =
-273C
3.3 Conservation of energy
-Energy is conserved = total quantity of energy stays the same
+​ There is useful energy and wasted energy
+​ There is energy input and energy output
+​ The total energy input = total energy output -> cannot be greater because energy
cannot be created
+​ a system: something that has been chosen to be studied, especially in terms of energy
changes
+​ Wasted energy is dissipated, not destroyed (dissipated means this energy spreads out
into the surroundings, moves to a colder place and become less useful)
+​ the law of conservation of energy states that energy cannot be created or destroyed,
only changed or transferred
3.4 Moving from hot to cold
-Feeling heat
+​ Thermal energy always moves from hotter places to colder places
+​ When thermal energy is removed from a hot object -> thermal energy has dissipated
-Feeling cold (Eg holding ice)
+​ Cold from the ice DOES NOT moves into the hand
+​ Thermal energy transfers away from the hand and into the ice
+​ Feel cold because thermal energy has been transferred away from the hands
-Dissipation
+​ When thermal energy moves from a hotter place to a colder place -> thermal energy
has dissipated from the hotter place
+​ The rate, or speed, of thermal energy transfer increases when the temperature
difference between the hot place and the cold place increases
3.5 Ways of transferring thermal energy
-Heat and particle movement
+​ In solids, the particles move by vibrating around fixed positions
+​ In liquids, the particles also vibrate, but the particles are not in fixed positions
+​ In gases, the particles are far apart and move in straight lines until they collide with
another particle or the walls of the container
-Conduction
+​ When particles are vibrating -> push against the particles beside them
+​ The faster and more vigorous the vibration of the particles, the greater the push-> the
particles beside vibrate more vigorously
+​ Conduction of thermal energy works best in solids where the particles are close
together and can only move by vibrating
+​ Metals are the best conductors of heat because of the way the particles are arranged
(The electrons that are free to move in a metal help to pass along the vibrations =
colliding with the metal atoms and transferring heat to them efficiently + metal atoms
are densely packed are are arranged in a very neat, orderly crystal pattern)
+​ Woods, plastics and fabrics such as wool, cotton are poor conductors
+​ The opposite of a conductor is an insulator
+​ Poor conductors of thermal energy are good thermal insulators
+​ Conduction does not work well in liquids because the particles move around more
when heated, rather than just vibrating
+​ Conduction does not work well in gases because the particles are far apart and the
collisions are not very frequent.
+​ Conduction cannot happen in a vacuum as there are no particles to vibrate in a
vacuum (chân không/không có gì trong đấy cả/ Ví dụ = outer space)
-Convection
+​ When liquids and gases are heated, the particles move faster and take up more space
+​ Taking up more space = the volume increases
+​ The number of particles does not change -> the mass of liquid or gas does not change
-> density will decrease
+​ E.g a gas or liquid that is heated at the bottom -> the particles at the bottom will start
to move faster and take up more space-> this part of the liquid or gas will become less
dense and start to float up through the colder, more dense parts
+​ Convection = the upward movement of warmer liquid or gas
+​ Colder, more dense parts of the liquid or gas then sink down -> colder parts are then
closer to the source of heat -> heated -> float again
+​ Convection current= the movement around the heated liquid or gas is called the
convection current
+​ The convection current causes all of the liquid or gas to eventually become heated
+​ Convection can happen in liquids and gases because the particles are free to move
+​ Convection cannot happen in a solid because the particles are not free to move; can
only vibrate about fixed positions
+​ Convection cannot happen in a vacuum as there are no particles to move
-Radiation
+​ Vacuum in space between Sun and Earth
+​ conduction and convection would not transfer thermal energy from the Sun to Earth
-> radiation
+​ Radiation is a type of wave that we cannot see (does not need particles)
+​ Thermal energy can be transferred by radiation very quickly
+​ All objects give out thermal energy through radiation.
+​ The hotter the object, the more radiation it will emit (=give out)
+​ Cooler objects absorb thermal energy by radiation from hotter ones
+​ Radiation can pass through transparent solids, liquids and gases
+​ The color and texture of an object affects its ability to emit or absorb thermal energy
by radiation
1.​ Best emitters and absorbers of radiation (dull,black, large surface area)
2.​ Worst emitters and absorbers of radiation (shiny, white and silver, small
surface area)/ shiny, white or silver surfaces reflect the radiation away
-Conduction, convection and radiation
+​ Most objects gain or lose thermal energy by a combination of conduction, convection
and radiation (room heater) đọc trong SGK?
+​ Some objects are designed to reduce the effects of conduction,convection or radiation
(vacuum flasks, double-glazed windows, car windows)
3.6 Cooling by evaporation
-Evaporation (surface phenomenon)
+​ definition: change of state from liquid to gas
+​ The particles in a liquid are moving around in random (= unpredictable) directions
+​ Some of these particles move faster than others
+​ Speed of the particles is also random
+​ The particles in the liquid that are moving faster have more energy (Some of the
particles have enough energy to escape from the surface of the liquid,not get pulled
back and become particles of a gas)
+​ Some of these are pulled back into the liquid
-Evaporation and cooling
+​ When the particles with the highest energy escape from the water -> lower the
average energy of the particles that remain -> temperature will decrease ->
evaporation causes cooling
+​ Eg: getting out of the swimming pool
-> when the water on the skin evaporates, the particles with the highest energy escape
-> water cool/ loses thermal energy
the thermal energy in the water droplets on the skins comes from the skin ->the
energy provided to change the state of the water from liquid to gas comes from the
skin
+​ Eg: when hot
produces sweat (sweat was produced in the skin-> same temp as skin)->sweat
evaporates -> temp drops -> thermal energy from the skin is transferred to the cooler
sweat-> lower more
+​ E.g some animals do not sweat very much -> cover themselves in water -> cooling
effect
+​ In very humid conditions, sweat may not be evaporate->body temp increase without
the cooling effect
+​ Eg of water coolers (sgk)/porous = water can slowly soak through the clay
-Evaporation and cooling of other liquids
+​ Different liquids have different forces holding the particles together
+​ Some liquids, such as perfumes, have weaker forces between the particles (evaporate
quickly so people can smell) ->equal volumes of perfume and water at the same temp
-> perfume evaporates first-> feel colder-> removes thermal energy faster
+​ Other liquids, such as liquid soap, have stronger forces between the particles->equal
volumes of liquid soap and water at the same temp-> liquid soap evaporates later->
removes thermal energy slower
The fan helps carry the sweat particles away