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Thermal physics Flashcards

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1
Q

What are the main features of solids?

A
  • Fixed volume
  • Fixed shape
  • Strong attraction between particles
  • Particles can’t move freely but can vibrate (low energy)
  • Regular arrangement
  • High density
  • Can’t be compressed
  • Can’t diffuse
  • The particles are packed very closely together in a fixed and regular pattern
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2
Q

What are the main features of liquids?

A
  • Fixed volume
  • Acquires shape of container
  • Forces of attraction between particles are less strong than between a solid
  • Particles can freely move and slide over each other (greater energy)
  • Irregular arrangement
  • Medium density
  • Can’t be compressed
  • Slowly diffuse
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3
Q

What are the main features of gases?

A
  • Not fixed volume
  • Not fixed shape
  • No or very weak forces of attraction between particles
  • Particles move randomly and are spread out (highest energy)
  • Irregular arrangement
  • Low density
  • Can be compressed
  • Diffuse quickly
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4
Q

What is melting and how does it happen?

A
  • When a solid changes into a liquid
  • Happens at a specific temperature called the melting point
  • Requires heat energy which transforms to kinetic energy. Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solidexpandsuntil the structure breaks and the solid melts
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5
Q

What is boiling and how does it happen?

A
  • When a liquid changes into a gas
  • Happens at a specific temperature called the boiling point
  • Requires heat which causes bubbles of gas to formbelowthe surface of a liquid, allowing for liquid particles to escape from the surface and within the liquid
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6
Q

What is freezing?

A
  • When a liquid changes into a solid
  • This is the reverse of melting and occurs at exactly thesametemperatureas melting, hence the melting point and freezing point of a pure substance are the same.
  • Requires a decrease in temperature or loss of thermal energy
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7
Q

What is evaporation?

A
  • When a liquid changes into a gas and it occurs over a range of temperatures
  • Evaporation occurs only at thesurfaceof liquids where high energy particles can escape from the liquid’s surface atlowtemperatures, below the b.p. of the liquid. (It’s a more superficial process than boiling)
  • The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate
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8
Q

What is condensation and how does it happen?

A
  • When a gas changes into a liquid and it occurs over a range of temperatures (requires a decrease in temperature or loss of thermal energy)
  • When a gas is cooled its particles lose energy and when they bump into each other they lack the energy to bounce away again, instead they group together to form a liquid
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9
Q

What is temperature?

A

The average kinetic energy of the particles in a substance

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10
Q

What is the relationship between the movement of particles and temperature?

A
  • Particles gain kinetic energy and move faster as their temperature increases. The same way particles lose energy and move slower as temperature decreases
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11
Q

What is pressure?

A

The force exerted by gas molecules colliding with surfaces of objects or other particles

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12
Q

Pressure in a gas

A
  • Molecules in a gas are in constantrandommotion at high speeds
  • Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide: With the walls of its container and with other molecules
  • Pressure in a gas is caused by the collisions with the surface (walls) of the container
  • The pressure is defined as theforce per unit area. P=F/A (Unit: Pa)
  • As the gas particles move about randomly they collide with the walls of their containers
  • These collisions produce anet forceat right angles to the wall of the gas container (or any surface). Therefore, a gas athighpressure hasmore frequent collisionswith the container walls and a greater force. Hence the higher the pressure, the higher theforceexerted per unit area
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13
Q

What do the forces and distance between particles affect?

A

the properties of solids, liquids and gases

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14
Q

What is Brownian motion?

A

Brownian motionis the random movement of particles in a liquid or a gas produced by large numbers ofcollisionswith smaller particles which are often too small to see (the random motion of microscopic particles in a suspension is evidence for the kinetic particle model of matter)

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15
Q

How is Brownian motion observed?

A
  • When small particles (such as pollen or smoke) are suspended in a liquid or gas, they can be observed through a microscope moving around in a random, erratic fashion
  • When observing Brownian Motion, even with a microscope, only themicroscopic particlescan be seen
    • The pollen or smoke particles are seen to move
    • Smalleratoms and molecules, of water or air, are still too small to be seen
  • Theselight, fast-moving atoms and moleculescollide with the larger microscopic particles. The collisions give the particles a little nudge, causing them to change their speed and directions randomly, each time they are struck by a molecule
  • Microscopic particles may be moved by collisions with light fast-moving molecules and correctly use the terms atoms or molecules as distinct from microscopic particles
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16
Q

How do you convert kelvin to celsius and vice versa?

A

T (in K) = θ (in °C) + 273 T (in °C) = θ (in K) - 273

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17
Q

What is the absolute zero?

A
  • Thekelvin temperature scalebegins atabsolute zero
    • 0K is equal to -273 °C
    • An increase of 1 K is the same change as an increase of 1 °C
  • It is not possible to have a temperaturelowerthan 0 K
  • This means a temperature in kelvin willneverhave a negative value
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18
Q

What is the effect on pressure when volume is changed?

A
  • Pressure and volume are inversely proportional. (P∝1/V) meaning the pressure increases when the volume decreases.
  • This is because when the gas is compressed, the particles will hit the wall and each other more frequently which increases the pressure
  • pV = constant
  • It can also be written as: P1V1=P2V2, P1= initial pressure (Pa), P2= final pressure (Pa), V1= initial volume (m^3), V2= final volume (m^3)
  • Graph: The curve ‘tends towards zero’ (meaning the curve gets closer and closer but never touches the axis, or zero is an inversely proportional curve)
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19
Q

What is the effect on pressure when temperature is changed?

A
  • Pressure is directly proportional to temperature (P∝T)
  • When you heat a container containing gas, the gas particles will gain kinetic energy and move faster. When the particles move faster, they will hit the walls of the container more often and more force will be exerted on the walls per unit area which increases the pressure.
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20
Q

What is thermal expansion?

A

Thermal expansion is a process where a body expands in either area, volume or shape when heated. When heat energy is supplied to an object, the particles have more kinetic energy and start moving faster. This causes the particles to move further apart and as a result it increases in area, volume or shape. Different states of matter expand different amounts depending on how strong the forces are that hold the particles together. (Molecules don’t expand, the space between them does)

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21
Q

How do solids expand?

A

Expand slightly because the low energy molecules can’t overcome the intermolecular forces of attraction holding them together

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22
Q

How do liquids expand?

A

Expand more than solids because the molecules have enough energy to partially overcome the intermolecular forces of attraction holding them together

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23
Q

How do gases expand?

A

Expand significantly because the high energy molecules have enough energy to completely overcome the intermolecular forces of attraction holding them together

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24
Q

What are the uses of thermal expansion?

A
  • We often use hot water to warm up the lid of a jar. This expands the lid (metals expand more than glass), making it easier to remove.
  • Liquid in thermometers expand and contract as the temperature changes. The volume of the liquid at a given temperature is how we read the temperature off of a thermometer.
  • Overhead cables have to be slack so that on cold days when they contract, they won’t snap or detach.
  • Bimetallic strips in thermostats
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25
Q

How do thermostats use thermal expansion?

A

Thermostats have a bimetallic strip. This is a strip in which there are two metals, with different coefficients of linear expansion, placed side by side. Therefore, when the strips warm up, one of the metals linearly expand more than the other, causing the bimetallic strip to bend. When it becomes hot enough, the strip bends enough to close the circuit, and the air conditioner turns on, cooling down the room. Once the room has reached the desired temperature, the strip slowly unbends, opening the circuit and turning off the air conditioner. The same mechanism can be used for heaters – when it is warm, the strip bends away from the circuit, and is it grows colder, the strip straightens out until it closes the circuit and the heater can turn on again.

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26
Q

What are the consequences of thermal expansion?

A
  • The expansion of solid materials can cause them to buckle if they get too hot. This expansion can be significant enough to causestructural damage to buildings, bridges, and other structures.
  • This could include:
    • Metal railway tracks
    • Road surfaces
    • Bridges
  • Things that are prone to buckling in this way have gaps built in, this creates space for the expansion to happen without causing damage
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27
Q

What is internal energy?

A

Internal energy is the total energy stored inside a system by the particles that make up the system due to their motion and positions

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28
Q

How does heating a substance affect its thermal energy?

A

A rise in the temperature of an object increases its internal energy. Heating a system changes a substance’s internal energy by increasing thekinetic energy of its particles.
- Thetemperatureof the material, therefore, is related to theaverage kinetic energyof the molecules
- This increase in kinetic energy (and therefore internal energy) can: Cause thetemperatureof the system to increase
Or, produce achange of state(solid to liquid or liquid to gas)

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29
Q

What is the specific heat capacity?

A

The specific heat capacity is the energy required per unit mass per unit temperature increase (1 kg of the substance by 1°C)

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30
Q

What is the equation of heat capcity?

A

The equation is c = ∆E/m∆θ (change in thermal energy/mass x change in temp)
Unit: joulesperkilogramperdegree Celsius : J/kg°C

31
Q

What does it mean if a substance has a high heat capacity and low heat capacity?

A

If a substance has ahighspecific heat capacity, it heats up and cools down slowly (ie. it takes more energy to change its temperature) ex water
If a substance has alowspecific heat capacity, it heats up and cools down quickly (ie. it takes less energy to change its temperature) ex copper

32
Q

What is the experiment to measure the heat capacity of substances?

A
  1. Start by assembling the apparatus, placing the heater into the top of the block
  2. Measure the initial temperature of the object from the thermometer
  3. Turn on the power supply and start the stopwatch
  4. Whilst the power supply is on, the heater will heat up the block. Take several periodic measurements, eg. every 1 minute of the voltage and current from the voltmeter and ammeter respectively, calculating an average for each at the end of the experiment up to 10 minutes
  5. Switch off the power supply, stop the stopwatch and leave the apparatus for about a minute. The temperature will still rise before it cools
  6. Monitor the thermometer and record the final temperature reached for the block
  7. c = ∆E/m∆θ
    m= mass of object is given
    Δθ=final temperature -initial temperature

ΔE=IVt(f)–IVt(i); I-current, V-voltage, t(f)-final time in seconds, t(i)- inital time in seconds

33
Q

What are the errors in the experiment to see heat capacity of substances?

A

Systematic Errors:

  • Make sure the voltmeter and ammeter are initially set to zero, to avoidzero error

Random Errors:

  • Not all the heat energy supplied from the heater will be transferred to the block, some will go into the surroundings or heat up the thermometer
    • This means the measured value of the specific heat capacity is likely to be higher than what it actually is
    • To reduce this effect, make sure the block is fully insulated
  • Ajoulemetercould be used to calculate energy directly
    • This would eliminate errors from the voltmeter, ammeter and the stopwatch
  • Make sure the temperature value is read at eye level from the thermometer, to avoidparallax error
34
Q

When does pure water melt and boil?

A

Pure water at standard atmospheric pressure melts at 0°C and boils at 100°C

35
Q

What happens during boiling?

A
  • When a liquid is heated by adding thermal energy the temperature of the liquid rises until the liquid boils
  • At theboilingpoint, even if more thermal energy is added, the liquid does not get any hotter. This means that the internal energy is not rising. The additional thermal energy goes into overcoming the intermolecular forces between the molecules of the liquid
  • As the forces are overcome, the liquid becomes gas
  • This isevaporationor vaporisation; the liquid is now agas
36
Q

What happens during melting?

A
  • When a solid is heated by adding thermal energy it melts.
  • At themeltingpoint, even if more thermal energy is added, the solid does not get warmer. This means that the internal energy is not rising. The additional thermal energy goes into overcoming the intermolecular forces between the molecules of the solid
  • As the forces are overcome, the solid becomes liquid
37
Q

What happens during condensation?

A
  • The particleslose kinetic energyand move more slowly
    • They no longer have enough energy to overcome theintermolecular forcesof attraction between molecules
    • The particles getcloser together
    • They only have enough energy toflowover one another
  • The gas has condensed into a liquid withno change of temperature
38
Q

What happens during solidification?

A
  • The particleslose kinetic energyand move more slowly
    • They no longer have enough energy to overcome theintermolecular forcesof attraction between molecules
    • The particles getcloser together
    • They only have enough energy tovibrateabout theirfixed position
  • The liquid has solidified into a solid with no change of temperature
39
Q

What happens during evaporation?

A
  • Evaporationis a change in state of a liquid to a gas
  • Evaporation occurs whenmore energetic moleculesmoving near the surface of the liquid have enough energy to escape
    • The average energy of the liquid isreduced, therefore liquids arecooled downby evaporation
  • The process of evaporation can be used to cool things down:
    • If an object is in contact with an evaporating liquid, as the liquid cools the solid will cool as well. This process is used in refrigerators and air conditioning units
  • It happens;
    • At any temperature
    • Only from the surface of a liquid
40
Q

What are the three factors that affect evaporation?

A

Temperature, surface area and air movement

41
Q

How does temperature affect evaporation rate?

A

Higher temperature leads to a higher rate of evaporation. This is because increased temperature increases thekinetic energyof the molecules in the liquid. Molecules with more energy are more likely toovercome the intermolecular forcesholding them in the liquid state and escape the surface

42
Q

How does surface area affect evaporation rate?

A

Larger surface area leads to a higher rate of evaporation. This is because evaporation only occurs at the surface and a larger surface means more evaporation.

43
Q

How does air movement affect evaporation rate?

A

Increasing air movement, increases the rate of evaporation. This is because it clears the evaporated molecules from the air and allows more water molecules to escape

44
Q

What are the similarities between boiling and evaporation?

A
  • Liquid to gas
  • Both require heat
45
Q

What are the differences between boiling and evaporation?

A
  • Evaporation can happen at any temperature from the melting point to the boiling point while boiling only happens at the boiling point
  • Evaporation is on the surface while boiling happens in the entire body of the liquid
46
Q

What is conduction?

A

Conduction is when two solids of different temperatures come in contact with one another, thermal energy is transferred from the hotter object to the cooler object

47
Q

What are good conductors?

A
  • solids which easily transfer heat (The best ones are metals)
    • For example; a metal pan or a ceramic tea cup
48
Q

What are bad thermal conductors?

A
  • Bad thermal conductors (also called insulators) are solids which do not transfer heat well
    • For example; a woolen blanket or layers of cardboard or paper
49
Q

How does thermal conduction happen in solids?

A

Thermal conduction happens in solids when a substance is heated and the atoms at the hotter end of the solid will vibrate more than the atoms at the cooler end. As they do so theybump into each other, transferring energy from atom to atom, these collisions transfer internal energy untilthermal equilibriumis achieved throughout the substance. This occurs inall solids, metals and non-metals alike

50
Q

Why does thermal conduction not happen in liquids and gases?

A

Thermal conduction in gases and liquids is bad because for thermal conduction to occur the particles need to be close together so that when they vibrate the vibrations are passed along. This does not happen easily in fluids because in liquids particles are close, but slide past each other and in gases particles are widely spread apart and will not ‘nudge’ each other. Both types offluid, liquids and gases, are poor conductors of heat.

51
Q

There are many solids that conduct thermal energy better than thermal insulators
but do so less well than good thermal conductors

A
52
Q

What are experiments that show conduction?

A

One experiment is the wax method

  • Take rods of different materials with the same dimensions (same length and cross-sectional area), and attach a drawing pin to the end of each using the same mass of wax.
  • Using a container with four rubber-lined holes in the side. Insert the rods through the holes, pushing them in the same amount. Fill the container with boiling water and start the stopwatch. Time how long it takes the drawing pin to fall off each rod and compare the times. The rod that allowed the pin to fall off fastest is the best conductor.

Comparing conduction in wood and paper

  • A cylindrical rod made of half wood and half metal is wrapped tightly in paper
  • Using a gentle flame, and holding the rod clear of the top of the flame, gently heat the paper at the join of the wood and metal
    • Turn the rod so that the paper is well-heated all around the circumference of the rod
    • Stop when the paper is clearly discoloured
  • Remove the rod from the flame, gently unwrap the paper and observe the burn pattern
    • A distinct pattern is seen; Where the paper touched the metal surface it is undamaged, Where the paper touched the wood surface it is charred
      (Metal is a good conductor of heat, so where the paper touched the metal, heat was transferred from the paper into the metal and along the length of the metal
      which prevented the paper getting hot. Wood is a good insulator, meaning it is a poor conductor of heat, where the paper touched the wood, heat was not transferred from the paper, this meant that the paper did get hot enough to start to burn)
53
Q

When does convection happen?

A
  • Convection is an important method of thermal energy transfer in liquids and gases
  • It can’t happen in solids only in fluids (liquids and gases)
  • Both temperature rising and falling can create convection currents
  • Heat does not rise- it is the hot gases or liquids which rise due to the change in density when they were heated.
54
Q

What is convection?

A

Convection occurs when particles with more heat energy move and take the place of particles with less heat energy.

55
Q

How does convection happen when a liquid/gas is heated?

A

The molecules push each other apart,making the liquid/gas expand. This makes the hot liquid/gasless densethan the surroundings. Thehot 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 aconvection current

56
Q

How does convection happen when a liquid/gas is cooled?

A

The molecules move together,making the liquid/gas contract. This makes the cold liquid/gasmore densethan the surroundings. Thecold liquid/gas falls, so that warmer liquid or gas can move into the space created. The warmer liquid or gas gets cooled and also contracts and falls down. The resulting motion is called aconvection current

57
Q

Experiment to show convection?

A
  • A simple demonstration of convection in liquids involves taking a beaker of water and placing a few crystals of potassium permanganate in it, to one side, as shown in the diagram above
  • When the water is heated at that side, the potassium permanganate will dissolve in the heated water and rise along with the warmed water, revealing the convection current
58
Q

What is thermal radiation?

A

Thermal radiation is infrared radiation and all objects emit this radiation. The hotter an object is, the more thermal radiation it emits. Thermal energy transfer by thermal radiation does not require a medium (only way that heat can travel through a vacuum, why heat reaches us from the Sun)

59
Q

What is thermal equilibrium?

A

For an object to be at a constant temperature it needs to transfer energy away from the object at the same rate that it receives energy

60
Q

Why does an object cool down?

A

If the rate at which an object receives energy is less than the rate at which it transfers energy away then the object will cool down

61
Q

Why does an object warm up?

A

If the rate at which an object transfers energy away is less than the rate at which it receives energy then the object will heat up

62
Q

What affects thermal radiation?

A
  • Thesurface colourof the object (black=more radiation)
  • Thetextureof the surface (shiny surfaces=less radiation)
  • Thesurface areaof the object (greater surface area=more area for radiation to be emitted from)
63
Q

How do black objects absorb and emit thermal radiation?

A

Black objects are very good at absorbing thermal radiation, for example black clothes make you feel hotter in sunny weather. Black objects are also very good at emitting thermal radiation, which is the reason that chargers for laptops, and radiators in cars are coloured black - it helps them to cool down

64
Q

How do shiny objects absorb and emit thermal radiation?

A

Shiny objects reflect thermal radiation and so absorb very little. They also emit very little, though, and so take longer to cool down

65
Q

How do dull objects absorb and emit thermal radiation?

A

Dull objects are reasonable absorbers and reasonable emitters

66
Q

How do white objects absorb and emit thermal radiation?

A

White objects are poor absorbers and poor emitters.

67
Q

How does radiation contribute to the Greenhouse effect?

A
  • The temperature of the Earth is affected by factors controlling the balance betweenincomingradiation and radiationemitted
  • The Earth receives the majority of its heat in the form of thermal radiation from the Sun. At the same time, the Earth emits its own thermal radiation, with a slightly longer wavelength than the thermal radiation it receives (the surface temperature of the Earth is significantly smaller than the surface temperature of the Sun)
  • Some gases in the atmosphere, such as water vapour, methane, and carbon dioxide (greenhouse gases) absorb and reflect back longer-wavelength infrared radiation from the Earth and prevent it from escaping into space. These gasesabsorbthe radiation and thenemitit back to thesurface
  • The rate of absorption and emission of radiation on Earth contributes to theGreenhouse Effect (This is the natural process that warms the Earth’s surface from the Sun
  • The Sun’s thermal radiation reaches the Earth’s atmosphere where: Some radiation isreflectedback to space, any radiation not reflected isabsorbedand re-radiated by greenhouse gases, the absorbed radiation then warms the atmosphere and the surface of the Earth
68
Q

Experiment to test thermal radiation

A
  1. Set up the four identical flasks painted black, grey, white and silver
  2. Fill the flasks with hot water, ensuring the measurements start from the same initial temperature
  3. Note the starting temperature, then measure the temperatures at regular intervals e.g. every 30 seconds for 10 minutes
69
Q

Errors in thermal radiation experiment

A

Systematic Errors:

  • Make sure the starting temperature of the water is the same for each material since this will cool very quickly
    • It is best to do this experiment in pairs to coordinate starting the stopwatch and immersing the thermometer
  • Use a data logger connected to a digital thermometer to get more accurate readings

Random Errors:

  • Make sure the hole for the thermometer isn’t too big, otherwise the heat will escape through the hole
  • Take repeated readings for each coloured flask
  • Read the values on the thermometer at eye level, to avoid parallax error
70
Q

Everyday applications of thermal energy transfer

A
  • A heater warms up the air adjacent to it by conduction. This hot air becomes less dense, causing it to rise. It is replaced by the cold air in the room, which is then heated by the heater, and this rises, and so on. This is convection.
  • When you cook food on a stove, heat is transferred from the hot burner to the pot or pan through conduction. Similarly, when you touch a hot object, such as a stove, heat is transferred to your hand through conduction
71
Q

How are the three types of thermal energy transfer shown in a fire burning wood or coal

A

Conduction: When the fire is lit, the part of the wood or coal that is in direct contact with the flame heats up. This heat energy is transferred from one particle of the wood or coal to another, moving through the solid via conduction.

Convection: As the wood or coal burns and releases heat, the air surrounding the fire becomes warmer and expands. This warm air is less dense than the cooler air above it, causing it to rise. As the warm air rises, cooler air moves in to replace it, coming into contact with the fire, warming up, and rising in turn. This creates a convection current that helps to spread the heat from the fire throughout the room.

Radiation: The fire emits infrared radiation, which travels out in all directions from the fire. This radiation can travel through the air, allowing it to heat up objects that are not in direct contact with the fire.

72
Q

How are the three types of thermal energy transfer shown in a car radiator?

A

Conduction: The engine of the car generates heat during operation. This heat is conducted through the engine block and to the coolant fluid that circulates around the engine. The heated coolant then flows into the radiator.

Convection: Inside the radiator, the hot coolant flows through thin tubes which maximize surface area for heat transfer. As the coolant flows, it transfers heat to the cooler air flowing through the radiator fins. This heated air becomes less dense and rises, and cooler air moves in to replace it, creating a convection current that cools the coolant.

Radiation: The radiator is designed to maximize the surface area available for heat dissipation. The heat from the coolant is not only transferred to the air by convection but is also radiated from the surface of the radiator to the surrounding environment.

73
Q

How does an insulator keep something warm?

A

The insulator contains trapped air, which is a poor conductor of heat, trapping the air also prevents it from transferring heat by convection, this reduces the rate of heat loss from the object, meaning that it will stay warmer for longer

74
Q

Objects willalwayslose heat until they are in thermal equilibrium (same temperature) with their surroundings

A

IG Physics (6 decks)

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