Thermal physics Flashcards
What are 2 distinguishing properties of solids?
- Have a definite shape
2. Have a definite volume
What are 2 distinguishing properties of liquids?
- Have no definite shape
2. Have a definite volume
What are 2 distinguishing properties of gases?
- Have no definite shape
2. 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
2. 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
2. 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)
2. 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 to 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
2. 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?
The energy required to break bonds between molecules and change the state of a substance
What is specific 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
2. 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
What is the effect of surface colour on emission?
- White is a poor emitter of thermal radiation
2. Black is a good emitter of thermal radiation
What is the effect of texture on emission?
- Dull surfaces are good emitters of thermal radiation
2. Shiny surfaces are poor emitters of thermal radiation
What is the effect of surface colour on absorption?
- Black is a good absorber of thermal radiation
2. White is a poor absorber of thermal radiation
What is the effect of texture on absorption?
- Shiny surfaces are poor absorbers of thermal radiation
2. 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
2. White surfaces are good reflectors of thermal radiation
What is the effect of texture on reflection?
- Dull surfaces are poor reflectors of thermal capacity
2. 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
