2. Thermal Physics Flashcards
What are 2 distinguishing properties of solids?
- Have a definite shape
- Have a definite volume
What are 2 distinguishing properties of liquids?
- Have no definite shape
- Have a definite volume
What are 2 distinguishing properties of gases?
- Have no definite shape
- Have no definite volume
Describe the molecular structure of solids
Strong forces of attraction between particles
Regular pattern
Very close together
Vibrate about fixed positions
Describe the molecular structure of liquids
Weaker forces of attraction between particles
No regular pattern
Close together
Slide past each other
Describe the molecular structure of gases
Very weak forces of attraction between particles
No pattern
Widely separated
Move randomly at high speeds
What is the effect of an increase in temperature of a gas?
The hotter the gas, the faster the molecules move
What causes pressure on the walls of a container?
- Collision of molecules with the walls of a container
- Collision applies a force across the surface area of the walls
Explain pressure
Change of momentum of the particles striking the walls creating a force
What is Brownian motion?
Random molecular bombardment of small particles
What causes Brownian motion?
Molecules in a gas/liquid colliding at high speeds with small particles
Describe evaporation
The escape of more-energetic molecules from the surface of a liquid
What is the effect of evaporation?
Cooling of the liquid
What are 3 factors that influence evaporation?
Surface area
Temperature
Draught
How does surface area affect evaporation?
A greater surface area results in more area for particles to escape the liquid
How does temperature affect evaporation?
High temperatures give particles more energy to escape from the surface of the liquid
How does draught affect evaporation?
If the air is less humid, fewer particles are condensing
Why does evaporation result in the cooling of a liquid?
- Average energy of remaining molecules decreases
- Temperature of the liquid also decreases
What happens to a body in contact with an evaporating liquid?
It cools
State Boyle’s Law
For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume
Describe thermal expansion of solids at constant pressure
Expand slightly
Why do solids expand slightly?
Due to strong bonds holding molecules together
Describe thermal expansion of liquids at constant pressure
Expand more than solids
Why do liquids expand more than solids?
Due to weaker bonds between molecules
Describe thermal expansion of gases at constant pressure
Expand significantly
Why gases expand significantly?
Very weak bonds between molecules
Give 2 examples of properties that change when a substance is heated
- Volume (and density)
- Electrical resistance
What is a fixed point?
A temperature at which some easily identifiable change occurs
What are the 2 fixed points used in a thermometer?
Lower point and upper point
What is usually used as the lower point?
0 °C
What is usually used as the upper point?
100 °C
What is the need for fixed points?
To know the temperature without having to measure it directly.
What does a liquid-in-glass thermometer consist of?
Thin glass capillary tube
A liquid that expands with temperature
A glass bulb
A scale
What is sensitivity?
The ability to detect small changes in temperature.
What 3 factors increase sensitivity?
Use a smaller glass bulb
Use a narrower tube
Use a thinner wall
How does using a smaller glass bulb increase sensitivity?
It contains less liquid so absorbs more heat in a less time
How does using a narrower tube increase sensitivity?
A small change in volume results in the liquid moving a larger distance along the tube
How does using a thinner wall increase sensitivity?
Heat is transferred into the bulb more easily
What is linearity?
When a given change in temperature causes the same change in length
What is range?
The difference between the lowest and highest temperatures a thermometer can measure
What are 2 factors that increase range?
- Use a wider tube
- Use a larger glass bulb
What are 3 characteristics that make a liquid suitable to be used in a thermometer?
Low specific heat capacity
Very low melting point and very high boiling point
Expand uniformly over a large temperature range
What is used to measure high temperatures?
Thermocouple thermometer
What is used to measure temperatures that vary rapidly?
Thermocouple thermometer
What does a thermocouple thermometer consist of?
Digital meter
Probes
Constantan wire
Copper wire
Cold junction
Describe the use of a thermocouple thermometer
Two different metals are joined to form two junctions
Temperature difference between two junctions causes a tiny voltage
This makes a current flow
Digital meter measures current and converts to temperature reading
What is the relationship between the temperature difference and current in a thermocouple thermometer?
The greater the temperature difference, the greater the current
What is internal energy?
Kinetic and potential energy of a substance
How does an increase of a substance’s temperature affect its internal energy?
The hotter the substance’s temperature, the more internal energy it has
What is the average temperature of a material related to?
Average kinetic energy of the molecules
What is thermal capacity?
The amount of energy required to raise the temperature of a material by 1 °C
What is the formula for thermal capacity?
Q = mc
Define specific heat capacity?
The amount of energy required to raise the temperature of 1kg of a material by 1 °C
Describe an experiment to measure the specific heat capacity of a substance
Drill 2 holes in the block
Measure the mass of the block
Place the heater in one of the blocks and a thermometer in the other
Use cotton wool to insulate the block
Note the initial temperature of the block and turn on the heater for x seconds
Note the final temperature
Calculate energy transferred using E = power ÷ time
Use the formula Q = mc∆T to find specific heat capacity
What is the formula for specific heat capacity?
c = Q ÷ m∆T
What is the melting point?
The temperature at which the substance changes from solid to liquid
What is the boiling point?
The temperature at which the substance changes from liquid to gas
Describe melting and boiling?
A substance’s temperature increases as it is heated
As it changes state its temperature remains constant
Energy is used to break bonds between the molecules
Describe condensation and solidification
A substance’s temperature decreases as it cools
As it changes state its temperature remains constant
Energy is used to form bonds between the molecules
What are 2 differences between boiling and evaporation?
Boiling occurs at a fixed temperature while evaporation occurs at any temperature
Boiling occurs throughout the whole liquid while evaporation occurs at the surface of a liquid
What is latent heat?
The energy required to change the state of a substance
What is latent heat of fusion?
The energy required to change a substance from solid to liquid
What is latent heat of vaporisation?
The energy required to change a substance from liquid to gas
In terms of molecules, what is latent heat?
Energy required to change the state of 1kg of the substance
Describe an experiment to measure specific latent heats for steam and for ice
Measure the mass of the vacuum flask and water
Place the heater in water and switch on the power supply
Start the stopwatch once water starts to boil
Take several measurements of potential difference and current and average
Switch off the power supply and stop the stopwatch after 300s
Measure the mass of the vacuum flask and water
Calculate decrease in mass
Calculate energy transferred using E = Pt
Substitute calculated values into the formula E = mL
What is the formula for specific latent heat?
E = mL
How is thermal energy transferred in solids?
Conduction
What are good conductors of heat?
Metals
What are poor conductors of heat?
Non-metals
What are poor conductors of heat referred to as?
Insulators
How does conduction occur?
- Delocalised electrons bump into positive ions
- This helps to transfer vibrations along the metal
Exam tip ; )
If a question on heat mentions metal, the answer will probably be to do with conduction.
How is thermal energy transferred through fluids?
Convection
Exam tip ; )
Fluids = liquids and gases
How does convection occur?
Molecules gain energy
Warmer, less dense molecules rise
Cooler, more dense molecules sink
Warmer, less dense molecules cool and so sink
Cooler, more dense molecules gain heat and so rise
What is the resulting process called?
Convection current
Exam tip ; )
If a question on heat mentions liquids or gases the answer will probably be about convection.
Heat does not rise (only hot gases or liquids rise).
How thermal energy transferred in a vacuum?
Radiation
What is thermal radiation?
Heat transfer by infrared
What is infrared a part of?
Electromagnetic spectrum
How thermal energy transferred in a vacuum?
Radiation
Exam tip ; )
If a question on heat mentions liquids or gases the answer will probably be about convection.
Heat does not rise (only hot gases or liquids rise).
What is the effect of surface colour on emission?
- White is a poor emitter of thermal radiation
- Black is a good emitter of thermal radiation
What is the effect of texture on emission?
- Dull surfaces are good emitters of thermal radiation
- Shiny surfaces are poor emitters of thermal radiation
How thermal energy transferred in a vacuum?
Radiation
What is the effect of surface colour on absorption?
- Black is a good absorber of thermal radiation
- White is a poor absorber of thermal radiation
What is the effect of texture on absorption?
- Shiny surfaces are poor absorbers of thermal radiation
- Dull surfaces are good absorbers of thermal radiation
What is the effect of surface colour on reflection?
- Black surfaces are poor reflectors of thermal radiation
- White surfaces are good reflectors of thermal radiation
What is the effect of texture on reflection?
- Dull surfaces are poor reflectors of thermal capacity
- Shiny surfaces are good reflectors of thermal capacity
Exam tip ; )
Shiny things do not reflect heat (they reflect thermal radiation).
Black things do not absorb heat (they absorb thermal radiation).
Describe experiments to show the properties of good and bad emitters
Use beakers and boiling water…
Describe experiments to show the properties
of good and bad absorbers
Use conical flasks and a bulb…
What are 3 factors of radiation emission?
Surface area of the body
Surface temperature of the body
Surface colour of the body
How does surface area affect radiation emission?
Greater surface area causes more radiation to be emitted
How does temperature affect radiation emission?
Higher temperature cause more radiation to be emitted
How does an insulator keep something warm?
Insulator contains trapped air, which is a poor conductor of heat
Trapping the air prevents heat transfer by convection
This reduces rate of heat loss from the object
What is mass?
Amount of matter
What is heat capacity?
Amount of space
What is density?
How compact the matter is - tells you how much matter is in a particular volume
What is the density equation?
Density kg/m3= mass(g/kg)/volume (cm3/m3)
p=m/v
How do you convert cm2 to m2?
Divide by 10,000
How do you find the density of solid objects?
Measure mass with balance
Measure length, width + height of object
Calculate volume using volume = length x width x height
Calculate density
Repeat for different objects
How do you find the density using water displacement with a Eureka can?
Measure mass using balance
Fill eureka can up to spout
Add object + measure water that leaves eureka can - this is equal to volume of object
Calculate density
Repeat for different objects
How do you find density with water displacement with a measuring cylinder?
Measure mass using a balance
Measure initial volume of water in measuring cylinder
Add object = measure new reading in measuring cylinder
Calculate volume of object by subtracting volume readings
Calculate density
Repeat for different objects
What are 4 common ways to reduce the accuracy of these experiments?
Not zeroing balance
Not taking measurement from measuring cylinder at eye level (parallax error)
Placing measuring cylinder on uneven surface before taking a measurement
Not filling eureka can up to the spout
Would a measuring cylinder or beaker lead to more precise measurements?
Measuring cylinder
Objects made out of the same material have the same…
Density
An object will float if its average density is…
Less than the liquid it is floating in
Why does a boat float?
Average density of boat and air inside is less than density of water - air is much less dense than water
What is pressure a measure of?
How spread out a force is
As you increase the size of the force you ..
Increase the size of the pressure
As you decrease the size of the area over which a force acts, you increase
The pressure`
Pressure equation
Pressure = force/area
Measured in pascals (Pa) or N/cm2
Area is measured in m2 (so don’t forget to convert)
What causes a balloon to be forced outward when it is filled with a gas?
more particles inside balloon
more collisions with the balloon
greater outward pressure
Does pressure increase in depth?
Yes
Why are Magdeburg hemispheres forced together when there is a vacuum inside them?
no outward pressure cos vacuum inside
inward pressure caused by atmosphere pushing them together
What is the pressure difference equation?
Pressure difference = density x g x depth
P = pgh
Explain how pressure is exerted by gases in containers
particles continually colliding w each other and the wall of container+ exert force on wall
pressure is the combined force of all the collisions
Explain why heating a material will change the energy stored in the material and raise its temperature?
particles increase in speed
kinetic energy increases
thermal store increases
What are the differences between evaporation and boiling?
Boiling occurs at boiling point, but evaporation occurs below boiling point
Evaporation only occurs at surface, whereas boiling occurs throughout the liquid
A source of energy is needed for boiling whereas energy is taken from the surroundings for evaporation
What happens during evaporation?
faster particles near surface able to escape liquid
avg kinetic energy of remaining particles is lower than before
lower avg kinetic energy corresponds to lower temp
thermal energy stored in surroundings is transferred to cooler liquid by heating
Does the temp remain constant during a change in state?
Yes
What is the change in thermal energy equation?
ΔE = mc ΔT
What is Boyle’s law?
For a gas with a constant mass and temp the pressure will increase when the volume decreases
Why does boyle’s law occur?
elastic collisions with the particles and container cause gas to exert an outward force on container
when gas expands at constant temp speed of particles is unchanged
force of each collision is unchanged
however particles collide less frequently, resulting in avg force and pressure to decrease
Why does boyle’s law occur?
elastic collisions with the particles and container cause gas to exert an outward force on container
when gas expands at constant emp speed of particles is unchanged
force of each collision is unchanged
however particles collide less frequently, resulting in avg force and pressure to decrease
What is the equation for volume and pressure of a fixed mass of gas at constant temp?
P1 x V1 = P2 x V2
P1 is initial pressure
P2 is final pressure
V1 is initial vol
V2 is final vol
What units do you use for pressure?
Pascals
Pa
What happens to the particles of a gas when temp increases?
as temp of gas increases, speed and avg kinetic energy of particles increases
What is absolute zero?
-273 degrees Celsius
Where particles stop moving
How to convert between kelvin and Celsius?
Temp (K) = temp (c) + 273
Temp (c) = temp (K) -273
-273 c = 0K
What is the relationship between kinetic energy and absolute temp of the gas? (Measured in kelvin)
Directly proportional
Eg as temp of gas doubles kinetic energy of particles would double
Why is pressure directly proportional to absolute temperature (kelvin) if volume remains fixed?
as temp increases speed and kinetic energy of particles increases
increases force of each collision and frequency of each collision
pressure that gas exerts increases
What is the relationship between pressure and volume?
As pressure increases volume decreases
What is the pressure law?
P1/T1 = P2/T2
T1 = initial absolute temp, T2 = final absolute temp both in kelvin
what is specific heat capacity?
heat energy required to raise the temperature of a 1kg mass of a substance by one degree
equation
change in thermal energy = mass x c x change in temperature
How to find specific heat capacity of a solid?
Find mass of copper block in kg using balance
Place thermometer into smaller bore of copper block and measure initial temp of sample
Place copper block into insulating holder and place onto heatproof mat
Lubricate electrical heater w petroleum jelly and insert into larger bore in metal block
5.Connect electrical heater to power pack, add ammeter and voltmeter to circuit
Switch on power supply, start stopwatch - record values of voltage and current for heating circuit
Wait for temp of copper block to rise by between 20 and 30c
Switch of power supply, record time
Wait until temp on thermometer stops rising and record final temp of copper block
How to find specific heat capacity of water
Water in beaker
Find mass
Measure initial temp
Place electrical heater into beaker of water
Add ammeter and voltmeter
Start stop clock, switch on power supply
Wait for temp to rise between 20 and 30 c
Switch off power supply and stop stop clock, record time
Wait until temp stops rising and record
What does thermal energy transfer from, to?
Thermal energy will transfer from object w higher temperature to object w lower temperature
How can thermal energy be transferred?
Conduction, convection and radiation
What is the rate of transfer larger for all types of transfer?
larger temperature difference
larger surface area
What is conduction?
transfer of thermal energy when particles collide
usually in solids
Why are metals particularly good conductors?
contain delocalised electrons
delocalised electrons can move quickly throughout material, colliding + passing on energy to ions
What is convection?
Convection is transfer of thermal energy in liquids + gases :
- hotter parts of gas/liquid expand and become less dense, then rise
- cooler parts of gas/liquid contracts and become more dense, sink
What is radiation?
thermal radiation = transfer of thermal energy by infrared waves
Which objects emit the least/most infrared radiation?
hotter objects w larger surface area emit more infrared radiation
dark + matt objects are good at absorbing + emitting infrared radiation but poor at reflecting it
light + shiny objects are good at reflecting but poor at absorbing + emitting
How can thermal energy transfer be reduced?
conduction -> vacuum, insulating material
convection -> vacuum, trap liquid/gas
radiation -> shiny material will reflect it
How does a radiator heat an entire room?
Radiator heats air by conduction - hot air expands, decreases in density + rises
Cooler parts of air sink and become heated by radiator forming convection current
How have polar bears evolved to reduce heat loss?
thick fur, traps air + prevents convection
air is poor conductor, so prevents conduction
Describe the arrangement and motion of particles in solid
close packed regular structure
fixed pattern, strong forces of attraction between molecules
high density
vibrate around fixed position
Describe arrangement and movement of particles in a liquid
close packed, irregular structure
not fixed, but still strong forces of attraction, so flow over each other
high density
move at random
Describe motion and arrangement of gas particles
very spread out
no fixed position, very weak forces of attraction
low density
move with rapid random motion
expands to fill space
What happens to particles in material when it is heated?
increases its thermal store
particles increase in kinetic energy, move faster
How to obtain a temp time graph to show constant temp during change of state
cool stearic acid
1. Record initial temp
2. Record temp every 30 secs using stop clock
3. Continue until stearic acid reaches 45c
4. Plot graph of temp against time
What is the role of free electrons?
> When a metal is heated, the FE in the heated part gain kinetic energy, and move faster.
> These FEs diffuse and collide with other FE and ions in the cooler parts of the metal.
> As a result, kinetic energy is transferred into these new FE’s.
What are the best conductors of energy?
Metals (contain free electrons)
What are the best insulators?
Wool and Fibreglass (insulating air pockets)
Convection
Circulation of a liquid or gas caused by increasing its thermal energy.
Convection only takes place in…
… fluids because of circulation within a fluid.
Why does convection occur when a substance is heated?
Heating a fluid makes it less dense, which will cause it to rise. This pushes cooler fluid to the bottom where it is also heated, etc etc.
Evaporation
When particles escape the surface of the fluid and entre the air.
How does evaporation cool a substance?
> Relatively weak forces of electrostatic attraction exist between the particles of a liquid.
> The faster particles with more kinetic energy break away from the particles at the surface and escape through evaporation.
> This leaves the liquid cooler because the average kinetic energy of each particle has decreased.
The higher the temperature of an object…
… the more infrared radiation it emits at a given time.
What is infrared radiation?
Energy transfer by electromagnetic waves.
Do all human bodies emit or absorb infrared radiation?
Both
What are the best surfaces for absorbing radiation?
Dark, Matt Surfaces
What are the best surfaces for reflecting infrared radiation?
Light, Shiny Surfaces.
Why do dark matt surface absorb Infrared radiation Better?
The surface has lots of cavities.
What are the best surfaces for emitting infrared radiation?
Dark, Matt Surfaces.
The bigger the temperature difference between an object and its surroundings…
… The faster the rate at which energy is transferred.
3 factors the rate of energy transfer relies on.
> Shape, size and material of an object.
> Nature of the surface that the object is in contact with.
> Temp. diff between an object and its surroundings.
Expansion by heating
THe expansion of a substance due to increasing its temperature.
Which state expands the most when heated?
Gas
What is the kinetic theory?
Each material is made of tiny particles that are in continuous motion and the motion increases as they gain the average kinetic energy which is proportional to the temperature of the matter
What is the arrangement of particles in solids?
Close together, regular pattern and equal distances between molecules
What is the arrangement of particles in liquids?
Close together random equal distances between the molecules
What is the arrangement of particles in gases?
Far apart random
What is the movement of solid particles?
Vibrate about a fixed position
What is the movement of liquid particles?
Vibrate but may change position with each other
What is the movement of gas particles?
Move quickly in any direction randomly
What is the forces between solid particles?
Held together by strong forces of attraction
What is the forces between liquid particles?
Attract each other but their attraction can’t hold them in a fixed positions
What are the forces between gas particles?
Virtually free of any attraction
Why do solids have a fixed shape and cannot flow?
Their particles cannot move from place to place
Why can’t solids be compressed or squashed?
The particles are close together and have no space to move into
Why can’t liquids be compressed?
Because the particles are close together
What is the kinetic model of gas?
When the particles of a gas collide with the walls of the container they exert a force. The pressure of a gas is the sum of all these forces divided by the area. The force is larger if the particles are moving faster or if there are more particles colliding within the walls. Thus the higher the temperature and the greater the number of collisions and the harder the collisions the larger the pressure
What is the kinetic model of gas in terms of momentum?
Collisions with walls causes rebounding that causes change in momentum. Since the force is the rate of change of momentum, there is a force exerted on the walls of the container therefore there is a pressure
What is evidence for moving particles?
Brownian motion
How do particles travel?
In zig zag pattern
Why do particles travel zig-zag?
Air molecules are moving faster and randomly so they bombard the smoke particles randomly from all directions causing the smoke particles to move randomly.
State the equation needed to convert Celsius into kelvin
C + 273
For the liquid in glass thermometer, the thermometric property is the volume of the liquid. Explain.
When a liquid is heated, it expands. The expansion is related to the temperature. This volume can be calibrated with the temperature.
For the bimetallic strip, the thermometric property is the length of solid. Explain
It consists of two strips of different metals- iron and brass. When the temperature rises each metal expands a different amount which causes the bar to bend. The greater expansion of brass compared that of iron makes the brass curve round on the outside of the iron. This causes a pointer to move up and down indicating a temperature change.
For the thermistor, its thermometric property is its electrical resistance. Explain
Electronic components that suffer a resistance change with change of temperature. For a thermistor the measurement of a specific temperature will result in a specific resistance across the thermistor.
For the thermocouple, its thermometric property is the electromotive force. Explain.
It consists of two wires, each of a different metal. Both ends form junctions. One of those junctions is kept at a constant temperature- 0 C and as the temperature of the other junction changes a small voltage is generated. The voltage is measured by a millivoltmeter and this can be calibrated to give a reading of the temperature.
Explain how you can calibrate a thermometer.
Place a thermometer in a mixture of ice water and mark the O degree mark. Then place the thermometer in boiling water and mark the thermometer. 100 EQUALLY spaced divisions can be placed between to represent 1 C marks
Define Sensitivity
The change in property per unit degree
What increases the sensitivity of a thermometer?
The narrower the tube
What is the most sensitive substance?
thermocouple then alcohol then mercury
Define Range
It is the difference between the maximum and minimum temperatures that the thermometer can read.
How can you increase the range?
Increasing the diameter of the capillary and decreasing the volume of the bulb
Which substances have the biggest range?
Thermocouple, alcohol freezes at -114 C whereas mercury at -39 C, and alcohol boils at 78 C whereas mercury boils at 357 C
Define linearity
When the liquid expands by the same amount for every degree Celsius rise in temperature
Which substance is NOT linear?
Thermocouple
Which substances are linear?
Mercury and alcohol
Define responsiveness
How long it takes for the thermometer to react to a change in temperature
How long it takes for the thermometer to react to a change in temperature
Responsiveness
Making the glass bulb thinner or making the bulb smaller
Which substance is the most responsive and why?
Thermocouple because the probe is too small and it has small heat capacity
How does heat flow?
From where it is hot to where it is not until thermal equilibrium is reached
What happens when a matter is heated?
The volume will increase slightly
Explain why a material expands when heated
When any matter is heated its particles speed up so their vibrations take up more space so they expand slightly in all directions
How do you increase thermal expansion?
The length is increased
The material has a higher expansion
Temperature difference
What are bimetallic strips used for?
Thermostats
State the relationship between pressure and temperature and explain.
Increasing the temperature increases the pressure. Molecules move faster and there is an increase in collisions and the average force per collision increases. Since the area of the flask does not change and the force increases pressure increases.
State the relationship between volume and temperature and explain.
Increasing the volume will increase the temperature . This is because the average velocity of the gas particles increase which increases rate of collisions and the average force.
State the relationship between pressure and volume and explain.
Volume is inversely proportional to pressure. A decrease in volume increases the number of gas particles. This results in an increase in the number of gas particles close to the cylinder walls and therefore an increase in the number of collisions with the wall. Increased force per area therefore greater pressure.
State Boyle’s Law
p1 * V1 = p2 * v2
How does conduction occur in metals?
Free electrons move at high speed at hot end and have high KE. Collide with slow moving electrons and KE is transferred during collisions.
How does conduction occur in non metals?
Molecules vibrate slowly at hot end and have more KE and attractive forces between molecules cause them to vibrate too. KE passes from hot to cold. This process is slow and that is why non- metals are good insulators of heat.
What is radiation?
The transfer of heat by electromagnetic waves
What affects the rate of absorption/emitter of radiation?
Dull and rough black surfaces are good absorbers and emitters of radiation.
Shiny and smooth white surfaces are poor absorbers and poor emitters of radiation
Everyday examples of radiation?
Cooling fins at the back of fridge are painted black
Solar panels are painted black
Glass wall in thermos flask are silvered
What is conduction?
The transfer of heat from two objects in contact and there is no difference between them.
What is convection?
The transfer of heat by movement of heated liquids or gases
How does convection take place?
Heated gas expands and the volume increases but the mass is UNCHANGED it becomes less dense rises upwards and the cold gas sinks and a convection current in set up. Heat is transferred from hot region to cold region.
Why is there a vacuum in a thermos flask?
As to prevent convection from taking place
How does thermal energy rate transfer increase?
Temp difference across the ends increased
The cross-sectional area is increases
Length of bar is reduced
Using better conductor
State insulating materials used in the house to reduce amount of thermal energy lost
Plastic foam around hot water tank
Glass/wool insulations in the loft
Wall cavity is filled with wool/plastic foam
Double glazed windows- two sheets of glass with air in between
Explain how the thermos flask is made to avoid heat loss
Vacuum- conduction and convection prevented
Foam support- Traps air which is a poor conductor of heat
Plastic stopper- poor conductor of heat, convection and evaporation can only occur when it is removed
What is evaporation?
The process of converting liquid into vapours
How does evaporation happen?
The energetic molecules at the surface leave the surface because they are less bonded so the average energy decreases and therefore the temperature decreases
Characteristics of evaporation
Happen at any temperature
Happens at the liquid surface only
What is boiling?
Process of converting liquid into vapours at boiling point
How does boiling happen?
As a result of increasing the temperature of liquid the molecules start moving faster and gain enough energy to break intermolecular bonds and escape from liquid. Average energy stays THE SAME and so does the temperature.
Characteristics of boiling
Happens only at boiling point of liquid
Happens anywhere in liquid
How to increase the rate of evaporation?
Increase temperature
Increase SA
Reduce humidity
Increase wind
Define specific heat capacity
The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1kg of the substance by 1 C
Define specific heat capacity
The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1kg of the substance by 1 C
Give the equation for heat capacity
C * m
the specific heat times the mass
Give the equation for the specific heat capacity
E = mass(kg) * specific heat capacity * change in temperature
E=mcΔt
What is fusion?
Substance changes from solid to a liquid
What happens when a substance changes its state?
THE TEMPERATURE REMAINS CONSTANT
Define specific latent heat of fusion
The energy that is needed to change the state of a 1kg from solid to liquid at its melting point
Define specific latent heat of vaporization
The energy that is needed to change the state of a 1kg from liquid to vapour at its boiling point
The energy that is needed to change the state of a 1kg from liquid to vapour at its boiling point
E= mass * specific latent heat
E=mL
When we have two equations to do in one question what do we do first?
E= mL then E= m*c *Δt
Specific latent heat of fusion = ?
Specific latent heat = total heat supplied / mass
L = Q / m
L = specific latent heat (J/kg or J/g)
Q = total heat energy supplied (J)
m = mass (kg or g)
Boyle’s law = ?
Pressure * volume = constant
Pressure and volume are inversely proportional
P1V1 = P2V2
v = velocity (m/s)
P = pressure
Specific heat capacity equation
Q = m * C * delta T
(need to rearrange it)
C = specific heat capacity (J/kg/degree C)
Q = heat energy supplied (J)
m = mass (kg)
Delta T = change in temperature (degree C)
Specific latent heat of vaporisation
Lv = Q / m
Pressure = ?
Pressure = force / area
P = F / A
P = pressure (Pa)
F = force (N)
A = area (m^2)
Kinetic molecular model of a:
1-Solid
2-Liquid
3-Gas
1-Fixed shape and volume, with high density and won’t flow.
2-Takes the shape of bottom of the container it’s in, has a fixed volume, medium density and it flows.
3-Takes up the shape and volume of the container it’s in, has a low density and it flows.
Describe/explain Brownian Motion and give an example
Brownian motion is the random jerky motion of particles viewed through a microscope.
It’s caused by rapid collisions between moving invisible particles(molecules) and larger other particles. They collide then bounce off each other.
E.g. dust particles in the air or pollen grains in water.
Describe the movement of a particle in Brownian motion
Moves in a total long way (jerky moments) but doesn’t move far from the point in the center.
The space and attraction between molecules in:
1-Solids
2-Liquids
3-Gases
1-High forces and small spaces. Fixed shape.
2-Medium forces and small spaces. Random motion.
3-Very low forces and large spaces. Random motion of molecules.
Describe a short experiment to demonstrate evaporation
Put some ethanol onto the back of your hand. It feels a little cold and the ethanol evaporates off your hand. If you give it a blow it increases the rate at which the reaction occurs.
Evaporation increases when? (3)
Its warmer
Wind is supplied
A greater surface area
Boyle’s law
Pressure and volume are inversely proportional.
pV = constant
P1V1 = P2V2
Temperature remains constant.
What happens to the molecules when they expand due to heat and what is this process called?
Thermal expansion
- The molecules move/(vibrate if it’s a solid) faster so the gaps between the molecules increase.
When thermal expansion occurs.
Order the 3 states into the order of how much they expand. (Lowest to highest)
Solid
Liquid
Gas
Uses of thermal expansion? (2)
Thermometers
- Thermostats
What is a thermostat? Describe the experiment.
Take a bimetallic strip and hold it above a flame from a bunsen burner. The strip will bend away from the flame. This is because the molecules at the bottom of the bimetallic strip (that’s closest to the flame) expands.
Disadvantages of thermal expansion? (3)
Bridges (don’t want them to expand)
Power lines (can break)
Freezing pipes (can burst)
What are fixed points?
- Fixed points of water?
The freezing and boiling point of a substance
- 0 degree C and 100 degree C
Define sensitivity
The expansion (or change) per degree celcius
Define thermal expansion
The same expansion (or change) per degree celcius
What does the range of a thermometer depend on?
The boiling point and freezing points of the liquid or other properties as well.
What apparatus is used to measure temperature difference between 2 different metals?
Thermocouple
Describe the wires of a thermocouple and give examples of each
There are 3 wires.
Wires 1 and 2 are made of the same material e.g. copper
Wire 3 is different to wires 1 and 2 e.g. constantan
You can’t have 2 wires made of very conductive metals or both that are made of very conductive metals.
You would have one of each.
What must the junction have on a thermocouple?
2 different wires meeting at the junction
How do the wires cause a voltage in a thermocouple?
Examples of meters that are used to measure the voltage produced. (4)
The two different wires joined together, at the same temperature will cause a voltage. This is read on a meter such as:
Galvanometer
Millivoltmeter
Milliammeter
Digital voltmeter or ammeter
In a thermocouple what is commonly done with the cold junction?
It’s put into ice. This is so we can compare the temperature to a known value (0 degree C)
Advantages of a thermocouple? (6)
Rapid response to change in temperature
Small thermal capacity - doesn’t affect the measurement
Can be used as a data logger for continuous measurement
Can measure a large range
Has a small area
Can be used for remote readings if connected via the internet to a PC and the internet
What is the internal energy?
The internal energy is the sum of all the KE and PE energies of its atoms/ molecules
How do you increase the internal energy? (2)
Supply more thermal energy
- Doing work on the system e.g. compressing it
Define heat capacity
Heat capacity is the amount of energy required to raise the temperature of a body by 1K or 1 degree C
Define specific heat capacity
Specific heat capacity is the amount of energy required to raise 1Kg (unit mass) of a substance by 1K or 1 degree C
Specific heat capacity = ?
Include units and symbols
specific heat capacity = energy supplied / (mass * delta feta(or temperature))
C = Q / (m*delta T)
Specific heat capacity = C (J/kg/degree C)
Energy supplied = Q (J)
Change in temperature = delta T(or feta) (degree C)
1-What is latent heat of fusion?
2-What is latent heat of vaporisation?
1-Solid to a liquid vice-versa
2-Liquid to gas vice-versa
Define specific latent heat
Specific latent heat is the energy needed per Kg of a substance to change the state
Specific latent heat of fusion = ?
Include units and symbols
Specific latent heat of fusion = energy supplied / mass of substance melted
L = Q / M
L = specific latent heat (J/Kg)
Q = energy supplied (J)
M = mass (kg)
Remember to use the E = Pt or E = VIt if Q is not directly given to you.
If you use g for mass then change the unit for L to J/g
Specific latent heat of vaporisation = ?
Include units and symbols
Specific latent heat of vaporisation = energy supplied / mass of substance vaporised
L = Q / M
L = specific latent heat of vaporisation (J/kg)
Q = energy supplied (J)
M = mass (kg)
Remember to use the E = Pt or E = VIt if Q is not directly given to you.
If you use g for mass then change the unit for L to J/g
Rules for latent heat? (2)
Energy is supplied to melt or boil a substance
- Energy is given out on freezing or condensation
4 methods of heat transfer and what they can happen in
Conduction = mainly solids
Convection = fluids
Radiation = doesn’t require a medium so it can happen in a vacuum
Evaporation
Thermal radiation is the same as what?
Infrared radiation which is an EM wave
1-What objects are better are absorbing and emitting radiation?
2-What objects are worse at absorbing and emitting radiation? Why?
1-Dull and dark objects
2-Shiny and light objects. Shiny objects reflect light.
Ways to insulate a house? (5)
Double glazing
Cavity wall insulation
Loft insulation
Insulate hot water tank - so you don’t waste heat
Turn down the radiators
Give an example of Brownian motion.
Smoke particles in airPollen in waterDust particles in air
Explain how Brownian motion supports the idea that matter is made from tiny particles in continuous motion.
In Brownian motion particles can be seen moving in a random way. This is because larger visible particles are bumping into very tiny particles.
How do particles in a solid move?
Particles vibrate about a fixed position.
How do particles in a liquid move?
Particles can move and slide past each other.
How do particles in a gas move?
Particles are able to move freely at different speeds in a random motion.
How are particles in a solid arranged?
Particles are closely packed and arranged in a regular pattern.
How are particles in a liquid arranged?
Particles are arranged in an irregular way but with little space between particles.
How are particles in a gas arranged?
Particles are randomly postioned and far apart from each other.
A gas is heated from 27ºC to 127°C. The initial pressure was 150000 Pa. Calculate the final pressure.
First we must convert into Kelvin
27oC = 27 + 273 = 300 K
127oC = 127 + 273 = 400 K
P1 / T1 = P2 / T2
150000 / 300 = P2 / 400
(150000 x 400) / 300 = P2
60000000 / 300 = P2
200000 Pa = P2
A gas is reduced in pressure from 1000000 Pa to 200000 Pa. If the final temperature is 0oC what was the initial temperature in celsius?
First we must convert into Kelvin
0oC = 0 + 273 = 273 K
P1 / T1 = P2 / T2
1000000 / T1 = 200000 / 273
1000000 x 273 = 200000 x T1
T1 = 1000000 x 273 / 200000
T1 = 273000000 / 200000
T1 = 1365 K
= 1365 - 273
= 1092oC
A gas is heated from 57oC to 327oC. The initial pressure was 100000 Pa. Calculate the final pressure.
57oC = 57 + 273 = 330 K
327oC = 327 + 273 = 600 K
P1 / T1 = P2 / T2
100000 / 330 = P2 / 600
(100000 x 600) / 330 = P2
60000000 / 330 = P2
181818.18 Pa = P2
A gas is reduced in pressure from 500000 Pa to 100000 Pa. If the final temperature is 27oC what was the initial temperature in celsius?
27oC = 27+ 273 = 300 K
P1 / T1 = P2 / T2
500000 / T1 = 100000 / 300
500000 x 300 = 100000 x T1
T1 = (500000 x 300) / 100000
T1 = 165000000 / 100000
T1 = 1500 K
= 1500 - 273
= 1227oC
A gas initially occupying a volume of 0.25 m3 at a pressure of 100000 Pa is compressed until its pressure is 5 times greater. Calculate the new volume.
p1V1 = p2V2 100000 x 0.25 = 500000 x V2 25000 = 500000 x V2 25000/500000 = V2 0.05 m3 = V2
A gas initially occupying a volume of 4 m3 at a pressure of 100000 Pa is forced into a container that has a volume of 2 litres. Calculate the pressure of the gas in the container.
Convert the 2 L to m3 2 L = 0.002 m3 p1V1 = p2V2 100000 x 4 = p2 x 0.002 400000 = p2 x 0.002 400000/0.002 = p2 200000000 Pa = p2
A gas initially occupying a volume of 0.2 m3 at a pressure of 150000 Pa is forced into a container that has a volume of 6 litres. Calculate the pressure of the gas in the container.
Convert the 6 L to m3 1 L = 0.006 m3 p1V1 = p2V2 150000 x 0.2 = p2 x 0.006 30000 = p2 x 0.006 30000 / 0.006 = p2 5000000 Pa = p2
A gas initially occupying a volume of 0.4 m3 at a pressure of 60000 Pa is compressed until its pressure is 4 times greater. Calculate the new volume.
p1V1 = p2V2 60000 x 0.4 = 240000 x V2 24000 = 240000 x V2 24000/240000 = V2 0.1 m3 = V2
State Boyles Law
As the volume of gas is halved the pressure doubles
What will happen to the volume of a gas as the temperature doubles?
the volume of the gas will double i.e.
A block of copper has a volume of 10cm3 and a mass of 90g. Calculate its density.
ρ = m/Vρ = 90/10ρ = 0.09kg / 0.00001m3ρ = 9000 kg/m3
Aluminium has a density of 2700 kg/m3. Calculate the volume of a 4kg block.
ρ = m/VV = m/ρV = 4/2700V = 0.00148 m3
What is 1m³ in cm³?
10^6 cm3.
What is 1 litre in cm³?
1000 cm³
What is 1 litre in m³?
0.001 m³ (10^-3)
density, mass and volume
density = mass/volume
A correct unit for density is
g/cm³ or kg/m³
What measuring instrument would you use to measure volume?
measuring cylinder
what measuring instrument would you use to measure mass?
electronic balance or electronic scales
How should you use an electronic balance to make as accurate measurement as possible?
Set it to zero first
Place on a flat level surface
How could a student make the measurement form a measuring cylinder as accurate as possible?
Place eyes level with the water.
Place on a flat surface
Use a clean cylinder
A student wants to measures the density of water.
State two factors that she should keep the same throughout her experiment.
Explain why.
The temperature of water should be kept constant as hot water is less dense.
The water should be of the same type, for example water with salt in it could be more dense.
How do Inuits stop themselves sinking into the snow?
The weight is spread over a larger area.
This means that there is less force per unit area acting on the surface and therefore the surface can support the object better.
Why shouldn’t you walk on wooden floors in pointy high heels?
The weight is spread over a small area. This means that there is a lot of force per unit area acting on the surface and therefore the floor will get damaged.
What is the formula for pressure?
pressure = force /area
P - pressure in pascals (Pa)
F - force in newtons (N)
A - area in m2
A 70kg man has shoes with an area of 200 cm2. Calculate the pressure exerted on the floor by the man.
35000 Pa
The 70kg man puts on stilts (they have a mass of 5 kg). The area is now reduced to an area of 20 cm2. Calculate the new pressure exerted on the floor by the man.
375000 Pa
A canister of gas has a pressure of 200000 Pa. The internal surface area is 200 cm2. Calculate the force exerted on the interior surfaces of the canister by the gas.
4000N
What is the formula for pressure at a depth?
pressure = density x g x depth
P = ρgh
A chamber is submerged so that its top is 3m below the surface of a freshwater lake. The canister is 4m in height. Calculate the difference in pressure between the top and bottom of the canister:
For the top: P = 1000 x 10 x 3= 30000Pa
For the bottom:P = 1000 x 10 x 7= 70000Pa
The difference is therefore 70000 - 30000 = 40000 Pa
A water tower is filled to a depth of 6m. Calculate the pressure of the water at the bottom of the container.
P = ρgh
P = 1000 x 10 x 6
= 60000Pa
The density of water is 1000
What are the units for density?
g/cm3 or kg/m3
What are the units for mass?
kg/g
What is the density of water?
1g/cm3 or 1000kg/m3
How do objects float?
If their density is less than the fluid they are in
How do you find the density of solids?
Measure the length, width and height to calculate the volume
Find the mass using a top pan balance
Density is mass/volume
How do you find the density of an irregular solid?
Calculate the volume by determining the displacement of water.
Find the mass using a top pan balance
Density is mass/volume
How to find density of a liquid?
Put liquid into measuring cylinder to find mass and volume.
Mass is the mass with liquid - mass of cylinder on its own
Density is mass/volume
How to find the density of a gas?
Fill flask with gas and using a top pan balance find mass of flask and gas. Remove gas with a vacuum and now reweigh the flask.
The change in mass is the mass of gas.
Place the flask upside down in water; water will rush up the tube to replace the removed gas. Volume of water is the volume of gas.
Density = mass/volume
What is the formula for pressure?
Pressure = force/area
What are the units for pressure?
N/m2 or N/cm2
What is another name for N/m2?
Pascals
Why must a knife be sharp to cut well?
Sharp knife has a smaller SA in contact with what it is cutting
Pressure = F/A
So if A is smaller the pressure will increase if the F is constant
Force exerted on the knife is concentrated over a smaller area and a larger pressure is exerted
Why do Inuit wear snow shoes?
The force exerted is the person’s weight
Pressure = F/A
The snowshoes increase the area in contact with the snow so the force is more spread out.
An increase in A with F constant leads to a decrease in P
What is special about the P in fluids?
It acts in all directions
What is the pressure at depth h in a fluid of density p given by?
P = hpg
What is g?
10N/kg
What is one bar?
100,000 Pa
What happens to the Magdeburg hemispheres when the air is pumped out?
Originally there is equal pressure on the inside and outside of the hemispheres; the air particles collide with the inside and outside surfaces of the spheres and so exert a significant yet equal force on either side.
When the air is removed, there is a large force on the outside pushing the spheres together as there are no air particles exerting a pressure on the inside of the spheres.
What is 1m3?
1,000,000 cm3
What means that the pressure in liquids doesn’t act in all directions?
If the liquid is moving
What happens to pressure with depth?
it increases
Properties of particles in a solid?
tightly packed
held in a fixed pattern y strong forces between them
vibrate around fixed positions
Properties of particles in a liquid?
tightly packed
not held in fixed positions
bound by strong forces between particles
move at random; Brownian motion
Properties of particles in a gas?
very spread out
move with a rapid, random Brownian motion
no fixed positions
forces between them are very weak
Why do most solids have high densities?
The particles they are made from are very closely packed in a regular arrangement.
Strong forces between the particles which give solid objects their definite shape and sometimes; much strength.
Do solid particles move?
YES! The vibrate about their fixed positions
How do gases exert a pressure?
Air particles move in Brownian motion
Random motion
Collide with wall; change direction
Change momentum
Exert a force on the wall
Pressure = F/A
As more particles collide with the wall the F increases and so pressure also increases
What is Boyle’s law?
p1V1 = p2V2
What is constant in Boyle’s law?
temperature
What is the Boyle relationship?
Pressure is inversely proportional to volume. If the pressure doubles, the volume halves.
Why does the relationship work for Boyle?
If p is inversely proportional to 1/V
then 1 = k
Therefore P x V must be k (a constant)
Why can gases be compressed?
The particles are very spread out
Why does and decrease in volume increase the pressure? PARTICLE THEORY
Temp constant - av.speed of particles is the same
If the same no of particles are squeezed into a smaller space they will collide with the walls of the container more often.
Each particle exerts a tiny force on the wall with which it collides.
More collisions/second means a greater av.F on the wall.
If P = F/A and force increases and area decreases then P increases!
What is the Temp/Press Law?
p1/T1 = p2/T2
What is the relationship between P and T?
Pressure is directly proportional to Temperature
Why is P dir.prop to T?
Volume is constant to the space they occupy is the same.
When heated, particles move randomly but with a higher av.speed. This means that they collide more frequently with the container and harder too.
They exert more Force.
If P = F/A and F increases then P also increases.
What is the relationship between Ke and temperature?
The temperature of a gas in Kelvin id proportional to the av.Ek of its particles.
Why is there no pressure at absolute zero?
The particles have no kinetic energy and so cannot collide and exert a pressure
What is absolute zero?
Temperature below which it would be impossible to cool the gas further
-273 degrees Celsius (0K)
Experiment for Boyle’s Law?
Put air in a glass tube. Add some oil.
A pump is used to increase the pressure of the oil and so reduce the volume of air.
Experiment for the Pressure Law?
Gas is enclosed in a flask with a pressure sensor so that V = constant.
Heated by placing it in a water bath and a thermometer will record the temperature of the water.
How is temperature kept constant for Boyle’s Law?
Wait a few seconds between readings
How is mass constant for Boyle’s Law?
The gas is permanently trapped between the oil and the tap of the tube so its mass is constant.
What happens to the motion of the particles in a solid when they absorb thermal energy?
Thermal energy is transferred to kinetic energy so this increase in the av.Ek of the particles causes the particles to vibrate more vigourously.
In a cooling curve, why does the rate of change of temperature decrease?
The temperature was getting closer to the temperature of the iced water and so its cooling effect was reduced.
Why doesn’t temp change when a substance is melting?
The material absorbs Therm.Energy but the Temp doesn’t rise b/c the TE used in the solid to break its bonds and melt it and is not being used to increase the temp of the substance.
What is evaporation?
vaporisation (liquid turning to gas) that occurs at the surface
What is boiling?
liquid turning to gas that occurs through the whole of the liquid
When can particles near the surface of a liquid escape and become gas particles?
If they are travelling in the right direction to escape the liquid
If they are travelling fast enough (have enough KE) to overcome the attractive forces of other particles in the liquid
Near enough to the surface
What is atmospheric pressure on Earth?
100 kPa
What is the motion of particles in a liquid?
move in random directions at low speeds
What is the motion of particles in a gas?
move in random directions at high speeds
Describe boiling
Heating a liquid gives it heat energy so particles move faster. When all the particles gain sufficient energy t overcome their attraction to each other, bubbles of gas form in the liquid
Describe melting
Heating a solid makes the particles vibrate faster until they vibrate fast enough to overcome the forces of attraction between them and they can move
What is evaporation?
when particles escape from a liquid and become gas particles
Which particles are most likely to evaporate from a liquid?
the fastest particles (most KE)
What happens when the fastest particles evaporate?
The av.speed and KE of the remaining particles decreases. This means the temperature also falls - liquid cools.
What’s particles in a gas?
Gases consist of very small particles constantly moving in random directions.
Particles hardly take up any space - most of gas is empty space
What is Brownian motion?
The random motion of particles in all directions caused by collisions with particles surrounding them.
How do molecules in a gas move?
Randomly; Brownian motion; exerting force (and therefore pressure) on the insides of a container
What is absolute zero?
0K; -273°C; The temperature at which no heat energy is transfered; all atoms stop moving
What does an increase in temperature do to the speed of gas molecules?
It makes them move faster; increases speed
What is the relationship between pressure and temperature (in K) for a gas in a sealed container?
P1/T1 = P2/T2 ; pressure is directly proportional to temperature
What is the relationship between pressure and volume?
P1V1 = P2V2 ; pressure and volume are inversely proportional to each other
What are three factors that affect the pressure of a gas?
number of particles, volume, speed of particles
What is temperature?
Proportional to the average kinetic energy of all particles
What is heat?
total kinetic energy in all particles in a substance
What happens to the pressure when the volume of the container is made smaller?
Speed and number of particles remains the same. Smaller volume means greater chance of collisions with the wall, meaning a greater force exerted on the container walls and as of the equation P=F/A the pressure increases
What is the equation for change in thermal energy?
shc x density x temp
