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)