Chapter 5: Thermal Effects Flashcards
Char of solid
Does not flow
Fixed shape and volume
Cannot be compressed
Vibrates
Particles arranged in a lattice formation
Internal energy formula
Kinetic energy + potential energy
How is thermal energy transferred (in terms of bodies and energy)
From the hotter body to the colder one, till they are the same temperature.
Kelvin to Celsius
K = C + 273
Relationship between temp and average KE per particle
Objects at the same temp have the same KE per particle. The higher the temp, the higher the avg KE per particle
Internal energy def
Total KE and PE of all atoms and molecules in a material
What is temperature (and in terms of particle theory)
The degree of hotness or coldness of an object
the average kinetic energy of particles within a substance
Thermal expansion property of liquid
More volume of liquid present = more expansion
what does thermal expansion depend on for solids
Depends on the material
3 Modes of heat transfer
Conduction
Convection
Radiation
What is conduction
The transfer of thermal energy from the hotter side to the cooler side by the vibrations between particles in a material
what is convection
Convection is the main way that heat travels through liquids and gases, doesn’t occur in solids
how does convection occur
-The molecules push each other apart, making the liquid/gas expand
-This makes the hot liquid/gas less dense than the surroundings
-The hot liquid/gas rises, and the cooler (surrounding) liquid/gas moves in to take its place
-Eventually the hot liquid/gas cools, contracts and sinks back down again
-The resulting motion is called a convection current
what is radiation
a special property
Radiation is the flow of heat from one place to another by means of electromagnetic waves (infrared radiation - all objects emit this)
It does not require a medium.
how does radiation occur
what doe very hot objectives also give
Thermal radiation is mainly infra-red waves, but very hot objects also give out light waves.
what is the best and worst heat emitter
best: matt black
worst: silver
what is the best and worst reflector
best: silver
worst: matte black
what is the best and worst absorber
best: matte black
worst: silver
what is the specific heat capacity of a substance
Specific Heat capacity (c) is the amount of energy
required to raise the temperature of 1 kg of a certain
substance by 1 degree celsius
what is the formula for specific heat capacity
c = Q/MΔt
what is thermal capacity
Thermal Capacity (Q) is the amount of energy required to raise the temperature of an object by 1 degree celsius
what is the formula for thermal capacity
q = mc
What are the 2 latent heats
Latent heat of fusion
latent heat of vaporization
What is latent heat of fusion
what happens to temp when it’s at the melting point
Energy required to separate particles so that they can form the liquid.
temp doesn’t change while its at the melting point until state is fully converted
Formula for latent heat
Q = ML
Q = heat required to change state
M = mass of material
L = latent heat
What is latent heat of vaporization
Energy absorbed during change of state from liquid to gas. Most of it is needed to separate particles so that they can form a gas.
What is specific latent heat of fusion
Energy required to change 1 kg of a substance from solid to liquid
What is specific latent heat of vaporization
Energy required to change 1 kg of a substance from liquid to gas
definition of evaporation
change from liquid to gas BELOW THE BOILING POINT
factors that affect rate of evaporation
larger surface area
higher temp
more airflow over liquid to help diffusion
where does evaporation occur
surface of liquid
where does boiling occur
throughout the liquid
why does evaporation cause a cooling effect?
the more rapidly moving particles/particles with higher kinetic energy. have enough energy to overcome attractive forces and escape the surface of the liquid as a gas.
temperature, as we know is the average kinetic energy. so if the particles with high kinetic energy have escaped, the average kinetic energy will obviously decrease.
Explain, in terms of energy, the process which takes place as a solid at its melting point
changes into a liquid at the same temperature.
heat/thermal energy is required to break bonds between molecules/overcome attractive forces and turn from solid to liquid.
Describe the motion of the molecules of the gas.
molecules move in random directions, at high speeds and molecules collide with eachother
which state expands the most when heated up
gas
why do substances expand
particles gain more kinetic energy when heated and start vibrating more rapidly, hence they gain separation from neighbouring particles
uses of thermal expansion of liquid
thermometers, alcohol or mercury in the thermometer will expand when placed in hot climates and shrink back down in colder climates
formula for energy
E = mass (m) x specific heat cap (C) x change in temp
E = MCΔT
what is thermal capacity dependent upon
mass (more mass will have higher thermal capacity)
material
a rise in the temperature of an object results in what
rise in temp –> increase in thermal energy –transfer–> increase kinetic energy therefore increased internal energy (ie = ke + pe)
experiment to find out the specific heat capacity of a liquid
0.5kg of liquid is used in a container that has insulation
thermometer used to measure the temp of the liquid
electrical heater with known power is placed in liquid
initial temp reading is taken
heater is switched on and timer is started simultaneously
timer is stopped when temp increases by 10 deg cel
assume the heater is 100% efficient
use C = E/M x Δt (E = power x time)
what makes a material a good conductor
if a material has delocalised electrons that are free to move around it makes them a better conductor as they will vibrate and hit the ions of the material
metal conduction experiment
heat a copper bar on one end, on the other place wax blobs with drawing pins being hung on the wax, the heat transferred through the metal will cause the wax to start melting and will eventually drop the pin
Describe thermal expansion of solids in terms of particles
When a solid is heated, particles speed up (thermal energy transfers to kinetic energy).
They start to vibrate more and thus vibrations take more space - so the solid expands slightly in all directions
opposite for when object is cooled
Give real world applications of thermal expansion
Electricity cables
Thermometers
Bimetal strips
bridges
train tracks
Describe use of expansion in electricity cables
often hung with some slack. If its hot - they expand even more. if its cold - they contract and won’t snap.
Describe the use of thermal expansion in thermometers
Mercury expands and contracts to give an indication of the temperature
Describe the use of thermal expansion in bimetal strips
strips of 2 metals are bonded together. When heated, one metal expands more than the other metal which causes the strip to bend. Used in thermostats (which are used to keep a constant temp)
In thermostats
what do modern devices use
they use thermistors instead of bimetal strips to keep the temp constant
Why do gasses expand so much in comparison to solids
Strength of attraction between particles.
In solids, the inter molecular force of attraction is extremely strong - which leads to little expansion since the particles are held so tightly together.
In gasses, they are very weak which leads to lots of expansion.
Factors that help increase the amount of thermal energy transferred in conduction
Temp diff across bar/solid is increased
Cross section area of solid is increased
Length of bar is reduced
How do non-metals conduct heat (particle theory)
When a material is heated, particles move faster and push on neighbouring particles as well, hence speeding those up. (transfer of KE)
How do metals conduct heat
why does this also mean they are good conductors of electricity
they also use the same ‘system’ as non metals except they also have
free DELOCALISED electrons that speed up when the metal is heated. They move randomly throughout the metal and collide with atoms and make them vibrate faster
this also means they are good conductors of electricity since electricity is the flow of electrons and they have free electrons to conduct electricity
What happens to particles at absolute zero (-273 deg Celsius)
the particles have least kinetic energy
Define brownian motion
random collisions between the microscopic particles that are in suspension (eg dust) and the particles of the gas or liquid
When is an object at constant temp
When energy transferred away from the object is at the same rate that it energy is received
What happens if the rate at which an object receives energy > the rate at which energy is transferred away from the object
vice versa
object heats up
vice versa: object cools down
How is the temperature on earth affected
why do we have global warming
by factors controlling the balance
between incoming radiation and radiation emitted from the Earth’s surface
a big example of a factor is the CO2 and H2O in the atmosphere which “insulate” the earth and reduces the rate of radiation. Increasing CO2 means reduced radiation from earth which causes global warming
At home
where is the heater located
where the the AC located
why?
heater - located near the floor since the cooler and denser air is near the floor. It takes this air in and heats it to warm air
ac - located near the ceiling since the hotter less dense air is towards the ceiling. Takes this hot air in and cools it and sends in down
does convection occur if the source of heat is above the container
No
you are heating the less dense fluid so then there is no displacement or convection current
Thermal processes in burning wood or coal
Radiation - from the burning of the coal or wood itself
Convection - a convection current is formed and the room is heated by hot air which circulates
How does a car radiator work
Unwanted thermal energy is carried away by a liquid coolant to an array of pipes.
here the heat is conducted to metal fins which is carried away by air flowing across the fins
Is radiation used to transfer heat in car radiators
only very little energy is transferred by radiation
most of it is conduction
Experiment to compare emitters
A metal cube is filled with boiling water
One surface of the cube is painted white, the other is painted black
A thermal radiation detector is placed at the same distance from either surface
meter readings are compraed
Experiment to compare absorbers
Metal plates are placed at the same distance from a radiant heater
One plate is painted white
one plate is painted matt black
rise in temp of plates are compared to compare the absorbers
Without of a supply of thermal energy what happens to
boiling
evaporation
boiling - stops
evaporation - continues
Formula for thermal capacity
in terms of energy
Q = c * delta t
q = energy
c = specific heat capacity
t = temp
If you had a diagram
with objects beside a source of heat
why wouldnt the objects be heated by convection
The heated air (convection currents) move upwards not sideways
