P1: Heat Transfer Flashcards
If two objects of opposite temperature are put in contact with each other. What happens?
Heat energy will flow from the hotter to the colder object until they are in thermal equilibrium
What is heat?
Energy that flows for on a hot region to cold region due to temperature differences between the two region.
What are the processes where heat energy flows?
Conduction
Convection
Radiation
What is conduction?
How heat energy flows from a hot to cold region of a substance without the molecules of the substance moving from the hot to cold region
What is conduction by molecules?
Molecules vibrate more when they are heated
Intermolecular forces allow the molecules to pass their vibrations from one to another
The stronger the forces the faster the vibrations are passed
Solids are the best conductors and gases the worst
A poor conductor is called an insulator
A vacuum does not allow conduction
What is conduction through metals?
Metals have an additional method of conduction.
Metals have electrons that can move from one atom to another
These free electrons can pass energy through the metal very quickly
Most of the best conductors are metals
How can heat transfer by conduction be increased?
Better conducting substance
Decreasing thickness of substance
Increasing surface area of substance
Increasing temperature difference across the substance
What is specific heat capacity?
The specific heat capacity of a substance is the amount of energy that is needed to raise the temperature of 1kg of the substance by 1°
What is the specific heat capacity of water?
4200 J/Kg°C
What is the unit of specific heat capacity?
J/Kg°C
When there is a phase change what is happening to the molecules as heat is added?
It gives it a chance to stay vibrating at the same speed which means they can spread out and form the next state. Energy is being used to break bonds
What is the process when a solid is going to a gas?
Vaporisation/Boiling
What is the process when a solid changes directly into a vapour?
Sublimation
Where does convection occur?
Liquid and gas
What is convection?
When heat energy flows from a hot to cold region of a substance by molecules of the substance moving from the hot to cold region
What happens when molecules are heated?
They move more quickly and occupy more space
What are denser hotter or colder fluids?
Hotter fluids are less dense than colder fluids
Which forms on top? Hotter or colder fluids?
Hotter
What is a convection current?
The path taken by rising hot fluids and sinking cold fluids
What is thermal radiation?
How heat energy moves from a hot to a cold region by means of electromagnetic waves
What is thermal radiation also called?
Infra red or IR radiation
Radiation travels far more quickly than the other forms of heat transfer. True or false?
True
What is the only type of heat transfer thy can travel through a vacuum?
Radiation
What allows radiation through best?
Gases are better than liquids, and liquids are better than solids.
How can we detect thermal radiation?
Skin
Thermopile
Special cameras
What objects give off thermal radiation?
All objects above absolute zero (-273°)
The hotter an object gets, the more radiation it lets off. True or false?
True
What are the best and worst emitters and absorbers of radiation?
Black rough surface
Black shiny surface
White rough surface
White shiny surface
What are some uses of a silvered surfaces?
Kettle
Firefighter
Marathon runner
What is evaporation?
The change of state from liquid to a gas, below the boiling temperature of the liquid
Where is the heat energy taken from?
Surroundings
Why does evaporation occur?
Because some molecules have more kinetic energy than others as if they are near the surface they can escape.
How is the rate of evaporation increased?
Temperature is higher
Surface are is larger
Windy
What is condensation?
Opposite of evaporation, a gas turns into a liquid. Heat energy is given to the surroundings. Hence condensation tends to occur on cold surfaces.
What is the most effective and least effective heat lost preventions?
Draught excluder Loft insulation Cavity wall Carpets Double glazing
What is payback time?
Pay back time is the time t takes you to get your money back. E.g. If loft insulation costs £400, but saves you £200 a year, the pay back time is 2 years
What is a U-Value?
Determines the quality of an insulator. The lower the U-Value the better the insulator.
What does a U-Value measure?
Rate at which heat energy is lost per metre squared for each degree temperature difference.
How does a flask work?
It keeps hot things hot and cold things cold. It does this by stopping or reducing the transfer of heat by conduction, convections or radiation
