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Flashcards in Energy Transfer By Heating COPY Deck (49):

Energy 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

Sankey diagrams


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

39 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

-loft insulation
-cavity wall insulation
-double glazing
-draught proofing
-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


How do double glazed windows reduce energy loss

Double-glazed windows have two glass panes with dry air or a vacuum between the panes.
Dry air is a good insulator so it reduces the rate of energy transfer by conduction.
A vacuum cuts out energy transfer by convection as well.