Flashcards in Energy Transfer By Heating COPY Deck (49):
Energy is.............by conduction and convection
What are the best conductors of energy
Why are metals good conductors
Because metals contain lots of free electrons which move about at random inside the metal
Describe what happens when a metal rod is heated at one end
-The free electrons at the hot end gain kinetic energy and move faster
-These electrons diffuse (spread out) and collide with other free electrons and ions in the cooler parts of the metal
-As a result, they transfer kinetic energy to these electrons and ions
So energy is transferred from the hot end of the rod to the colder end
Where are the electrons held in a non metallic solid
In the atoms
How does energy transfer take place in non metallic solids
Energy transfer only takes place because the atoms vibrate and shake each other.
This is much less effective than energy transfer by free electrons
What materials are good insulators
Wool and fibreglass
Why are wool and fibre glass good insulators
Because they contain air trapped between the fibres. Trapped air is a good insulator
What do we use materials such as fibreglass for
Insulating lofts and hot water piping
When does convection occur
When particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy
What is transferred by convection
Heat energy is transferred from hot places to cooler places by convection.
Why do liquids and gases expand when they are heated
Because the particles in liquids and gases move faster when they are heated than when they are cold.
As a result, the particles take up more volume.
This is because the gap between particles widens, while the particles themselves stay the same size
Describe what happens during convection
The liquid or gas in hot areas is Less dense than the liquid or gas in cold areas, so it rises into the cold areas.
The denser cold liquid or gas falls into the warmer areas.
In this way, convection currents that transfer heat from place to place are set up
Energy may be ............ By evaporation and condensation
What happens during evaporation
The particles in a liquid have different energies.
Some will have enough energy to escape from the liquid and become a gas
Why does evaporation have a cooling effect
The remaining particles in the liquid have a lower average kinetic energy than before, so the liquid cools down as evaporation happens
Why does sweating cool you down
The sweat absorbs energy from your skin so that it can continue to evaporate
Factors affecting the rate of evaporation
-Increasing the surface area of a liquid
-Increasing the temperature of a liquid
-Creating a draught of air across the liquids surface,
Will all INCREASE the rate of evaporation
Give an example of condensation
In a steamy bathroom, a mirror is often covered by a film of water
What happens during condensation
There are lots of water particles in the air
Some of them hit the mirror (a cold surface), lose energy and stay there as the liquid
Factors that will increase the rate of condensation
Increase the surface area of cold surface
Reducing the surface temperature
What do solar panels do
Solar panels contain water which is heated by radiation from the Sun. This water may then be used to heat buildings or provide domestic hot water.
Are solar panels cost effective
Solar panels' payback time is high because they are expensive to buy and install.
However, they save money because no fuel is needed to heat the water
What is payback time
A means of evaluating the cost effectiveness of each method
What is the equation for payback time
Payback time (years) = cost of installation (£) / savings per year in fuel costs (£)
What do U values measure
How effective a material is as an insulator
The lower the U value, ......
The better the material is as an insulator
The bigger the temperature difference between an object and its surroundings,.....
The faster the rate at which energy is transferred by heating
Most substances........when heated
What is an example of when the expansion of substances when heated is a hazard
Expansion of roofs and bridges -
Bridges need expansion gaps as they would buckle without them.
What is an example of when the expansion of substances when heated is useful
The bi-metallic strip thermostat -
A bi-metallic switch consists of a strip of two different metals such as brass and steel bonded together.
When the temperature of the strip rises, the brass expans more than the steel, causing the strip to bend.
This can be used to switch on or off an electrical device (e.g. in a fire alarm - when heated, the strip bends towards the contact screw, and when it touches it, the circuit is completed and the bell rings)
The rate at which an object transfers energy by heating depends on:
It's surface area and volume
The material from which the object is made
The nature of the surface with which the object is in contact
What can and cannot be done to energy
Transferred usefully, stored or dissipated.
It cannot be created or destroyed
When energy is transferred,
Only part of it may be usefully transferred; the rest is 'wasted'
What can energy flow be represented using
What is wasted energy is eventually transferred to
It surroundings, which become warmer.
This energy also becomes increasingly spread out and so becomes less useful
The efficiency of a device can be calculated using...
useful energy (or power) out
Efficiency = -------------------------------------- ( x 100%)
total energy ( or power) in
All objects ............ And ........... Infrared radiation
All objects emit and absorb infrared radiation
The.........an object is the more infrared radiation it radiates at a given time
The HOTTER an object is the more infrared radiation it radiates at a given time
Dark matt surfaces are.....
Good absorbers and good emitters of IR
Light, shiny surfaces are....
Poor absorbers and poor emitters of IR.
Good reflectors of IR
What is the advantage of the design of cooling fins
A motor cylce is shaped with fins on its outside surface.
The fins increase the surface area of the engine in contact with air so the engine transfers energy to its surroundings faster
Why does the desert fox have much larger ears than the arctic fox
Blood flowing through the ears transfers energy from inside the body to the surface of the ears.
Big ears have a much larger surface area than little ears so they transfer energy to the surroundings more quickly than little ears, helping them keep cool
Why do fluids rise when heated
Most fluids expand when heated.
This is because the particles move about more and they move apart, taking up more space.
Therefore heating part of a fluid makes that part less dense and therefore it rises
What can be fitted into homes to reduce energy loss
-cavity wall insulation
-aluminium foil behind radiators
How does aluminium foil behind radiators reduce energy loss
Putting aluminium foil between a radiator panel and the wall reflects radiation away from the wall
How does lost insulation reduce energy loss
Loft insulation such as fibreglass reduces the rate of energy transfer through the roof.
Fibreglass is a good insulator. The air between he fibres also helps to reduce the rate of energy transfer by conduction
How does cavity wall insulation reduce energy loss
Cavity wall insulation reduces energy loss through the outer walls of the house.
The 'cavity' of an outer wall is the space between the two layers of brick that make up the wall. The insulation is pumped into the cavity. It is a better insulator than the air it replaces. It traps the air in small pockets, reducing convection