Section 3 - Heating Processes Flashcards
Describe solids in terms of particles.
- Strong forces of attraction
- Particles close together
- Fixed, regular arrangement -> Particles vibrate on the spot
- Particles have little energy
Describe liquids in terms of particles.
- Medium forces of attraction
- Particles close together
- Particles can move past each other -> Random directions at low speeds
- Particles have some energy
Describe gases in terms of particles.
- Weak forces of attraction
- Particles far apart
- Particles move freely -> Random directions at high speeds
- Particles have much energy
What happens when you heat a substance?
The kinetic energy of the particles increases, so they move faster. This eventually causes solids to melt and liquids to boil.
What must be done in order for a substance to melt or boil?
The particles must gain enough kinetic energy to escape the bonds holding them together.
Why are there flat parts on the heating graph of a substance when it is being heated?
When a substance is melting or boiling, all of the energy being input is used to break bonds, so the temperature does not rise.
Why are there flat parts on the heating graph of a substance when it is being cooled?
When a substance is condensing or freezing, bonds are formed, which releases energy, so the temperature does not go down.
What is specific heat capacity?
The amount of energy required to raise the temperature of 1 kg of a substance by 1*C.
What is the specific heat capacity of water?
4200J/kg*C
What is the unit for specific heat capacity?
J/kg*C
What is the equation for specific heat capacity?
Energy transferred (J) = Mass (kg) x S.H.C. (J/kgC) x Temp. Change (C)
E = m x c x 0
What is specific latent heat?
The amount of energy needed to change 1kg of a substance from one state to another (without changing its temperature).
Is specific latent heat the same for boiling and melting?
No
What is the specific latent heat of vaporisation?
The amount of energy needed to change 1kg of a substance from liquid to gas (without changing its temperature).
What is the equation for the specific latent heat of vaporisation?
Energy (J) = Mass (kg) x S.L.H. of Vaporisation (J/kg)
E = m x Lv
What is the specific latent heat of fusion?
The amount of energy needed to change 1kg of a substance from solid to liquid (without changing its temperature).
What is the equation for the specific latent heat of fusion?
Energy (J) = Mass (kg) x S.L.H. of Fusion (J/kg)
E = m x LF
What is the unit for specific latent heat?
J/kg
Which way does heat energy tend to flow from and to?
From hotter objects to cooler surroundings
What are the three methods of heat transfer?
- Radiation
- Conduction
- Convection
What is the main form of heat transfer in solids?
Conduction
What is the main form of heat transfer in liquids and gases?
Convection
How does temperature difference between a body and its surroundings affect the rate of energy transfer?
The greater the difference, the faster heat energy is transferred.
What is heat radiation?
The emission of infrared radiation by all objects.
What happens in terms of energy transfer when an object is cooler than its surroundings?
It absorbs more infrared radiation than it emits (so it warms up).
What happens in terms of energy transfer when an object is hotter than its surroundings?
It emits more infrared radiation than it absorbs (so it cools down).
Do all objects emit and absorb infrared radiation?
Yes
How does an object’s temperature affect the amount of infrared radiation?
The hotter the object, the more radiation it radiates in a given time.
Explain which surfaces are good emitters and absorbers of infrared radiation.
- Dark, matt surfaces - Good absorbers and good emitters
* Light, shiny surfaces - Poor absorbers and poor emitters
How do solar hot water panels work?
- Contain water pipes under a black surface
- Radiation from Sun is absorbed by the black surface to heat water in the pipes
- This water can be used for washing or pumped to radiators
Describe conduction of heat.
- When heated, particles vibrate faster
- In a solid, they are packed closely together, so they can collide and pass on this kinetic energy
- This process continues until the heat is spread all the way through the solid
Why do metals conduct heat better than non-metals?
- Metals have some free electrons, while non-metals don’t
- When heated, the electrons move faster and collide with other particles in the metal
- Since electrons move freely, this is much faster than transferring energy by passing it between neighbouring atoms
Why does conduction happens more in solids than liquids and gases?
In solids, the particles are held tightly together, whereas in liquids and gases they are not, so they cannot bump into each other as easily.
Why is air good for preventing conduction?
It is a mixture of gases, so the particles are far apart and do not collide easily.
Describe convection.
- When you heat up a fluid, the particles move faster and move further apart from each other
- So the fluid expands and becomes less dense
- The warmer, less dense fluid rises above colder, denser surroundings
- Cooler fluid moves to take the place of the warm rising fluid -> This creates convection currents
Why can’t convection happen in solids?
The particles can’t move - they just vibrate on the spot.
What type of energy transfer do radiators mostly rely on?
Convection
How can convection be reduced?
The fluid has to be stopped moving - for example: clothes, blankets and cavity wall insulation stop convection.
How does condensation happen?
- When a gas cools, the particles slow down and lose kinetic energy.
- This allows attractive forces to pull them closer together.
- When the temperature gets cold enough, the gas particles get close enough that condensation can happen.
What can be seen coming out of a kettle?
First of all, near the spout, there is a small region of invisible water vapour. This soon condenses to form condensed water particles, which can be seen as steam.
What is condensation?
When a gas turns to a liquid.
What is evaporation?
When a liquid turns to gas.
Where in the liquid can evaporation happen?
Near the surface.
What factors are needed for a particle to evaporate from a liquid?
- Travelling in the right direction to escape liquid
- Travelling fast enough to overcome attractive forces
- Near the surface
What happens during evaporation?
- Fastest particles are most likely to evaporate
- This decreases average speed of the remaining particles and the kinetic energy is decreased
- This means the temperature of the liquid falls
When is evaporation useful?
When you sweat, the evaporation cools you down.
What factors increase the rate of evaporation?
- Higher temperature
- Lower density
- Larger surface area
- Airflow over surface
- Lower humidity
Why does lower density increase the rate of evaporation?
The forces between particles are weaker, so more particles have enough energy to escape the liquid.
What factors affect the rate of heat energy transfer?
- Surface area to volume ratio
- Temperature of object
- Temperature of surroundings
- Surroundings being an insulator or conductor
- Type of material
What is a heat sink?
A device designed to transfer heat away from an object it is in contact with, such as a compuer component.
How do animals control heat transfer?
In cold: - Hairs stand up to trap layer of insulating air - Animals have small SA:V ratio In hot: - Blood diverted to skin - Animals have large SA:V ratio
What happens to most substances when heated?
Their particles gain kinetic energy, move further apart and the substance expands.
What is a use of substance expansion?
- Bi-metallic strips are made of two different strips of metals that expand at different rates.
- This means they bend one way when heated and another when cooled.
- The can be used in thermostats.
Energy can be transferred, stored and dissipated but not…
… created or destroyed.
What does dissipated mean?
When energy is spread out and lost.
When is a device useful?
When it can transfer energy from one form to another.
What is the equation for efficiency?
Efficiency = Useful energy out / Total energy in
OR
Effciency = Useful power out / Total power in
What are Sankey diagrams used for?
Showing how much of the total energy input is being usefully employed compared to how much is being wasted.
How can you tell how much energy is useful and wasted by looking at a Sankey diagram?
Look at the width of each arrow.
What are some methods for reducing heat loss in the home?
- Loft insulation
- Cavity wall insulation
- Hot water tank jacket
- Double glazing
- Draught-proofing
What is the difference between effective and cost-effective home insulation methods?
- Effective methods are the ones that give the biggest annual saving
- Cost-effective methods are the ones that have the shortest payback time
What is cavity wall insulation and how does it prevent heat loss?
- Foam squirted into the gap between bricks
- Reduces convection and radiation
- Air pockets reduce conduction
What is loft insulation and how does it prevent heat loss?
- Thick layer of fibreglass wool laid out across loft floor
- Reduces conduction and convection
What is draught-proofing and how does it prevent heat loss?
- Strips of foam and plastic around doors and windows reduces draughts of cold air
- Reduces convection
What is a hot water tank jacket and how does it prevent heat loss?
- Case attached around hot water tank
- Reduces conduction and radiation
What are thick curtains and how do they prevent heat loss?
- Big bits of cloth over the window create an air gap between room and window
- Reduce convection, conduction and radiation
What does a U-value show?
How effective an insulator is.
What does a high U-value show?
The insulator is ineffective.
What does a low U-value show?
The insulator is effective.
